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Abstract

High-density polyethylene (HDPE) has become a preferred material for modern pipeline systems due to its exceptional advantages. Among various joining techniques, butt fusion welding is the most widely employed and cost-effective method for connecting HDPE pipes. In response to growing demands for improved weld quality and long-term reliability, this review presents a comprehensive synthesis of recent advances aimed at enhancing the overall performance of welded joints in HDPE pipelines. It begins with an examination of the intrinsic material properties of HDPE in relation to weldability, followed by an overview of the principles and procedures of butt fusion welding, and an in-depth analysis of the mechanisms and implications of welding defect formation. The review then turns to recent progress in both destructive testing (DT) and non-destructive evaluation (NDE) techniques for weld quality assessment, as well as the optimization of key welding process parameters. In addition, emerging modification strategies of HDPE material are highlighted for their potential to improve weld strength, structural stability, and overall joint integrity. Building upon these thematic areas, the review identifies existing technical barriers and unresolved knowledge gaps, and outlines future research directions aimed at advancing both fundamental understanding and practical solutions for the next generation of high-performance HDPE pipeline systems.

Keywords

High-density polyethylene (HDPE) pipes butt fusion welding welding defects process parameter optimization destructive testing and non-destructive estimation (DT/NDE)material modification and emerging directions

Introduction

Polyethylene (PE) was first identified in 1898 by German chemist Hans von Pechmann, who inadvertently synthesized a waxy white substance during the thermal decomposition of diazomethane.¹ In 1933, Reginald Gibson and Eric Fawcett at Imperial Chemical Industries (ICI) achieved the first practical synthesis of PE by polymerizing ethylene under high-pressure conditions—an accidental breakthrough that laid the foundation for industrial-scale production.^2,3 A major advancement occurred in 1953, when Karl Ziegler and Giulio Natta developed the Ziegler–Natta catalyst system (TiCl₄–Al(C₂H₅)₃), enabling ethylene polymerization under low-pressure and moderate-temperature conditions.^4,5 This innovation gave rise to high-density polyethylene, distinguished by its superior crystallinity, tensile strength, stiffness, and chemical resistance. In 1955, Farbwerke Hoechst AG in Germany commenced the first large-scale production and in-plant application of HDPE pipes, marking the beginning of their use in engineered infrastructure.⁶ Recognizing the rapid growth and technical significance of HDPE pipe systems, the German Institute for Standardization issued the first formal standard for PE pipelines—75/8074DIN—in January 1959.⁷ Since the early 1960s, HDPE pipes have been gradually introduced and tested in municipal water distribution and gas supply networks, paving the way for their global adoption. Over the decades, various grades of PE have been developed, including low-density polyethylene (LDPE),⁸ linear low-density polyethylene (LLDPE),⁹ medium-density polyethylene (MDPE),¹⁰ high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWPE),¹¹ cross-linked polyethylene (PEX),¹² and others. Each category is distinguished by differences in crystallinity, branching, and density, which in turn govern mechanical, thermal, and chemical performance.

With the rapid advancement of urban infrastructure and industrial systems, modern pipeline networks are required to deliver high mechanical performance,¹³ long-term durability,¹⁴ excellent chemical resistance to corrosive media,¹⁵ and robust stability under cyclic mechanical and thermal loads,¹⁶ along with adaptability to harsh environments and scalability for large-scale deployment. Among all PE variants, HDPE pipes have emerged as a preferred material of choice to meet these demands due to its exceptional mechanical properties,¹⁷ high strength-to-weight ratio,¹⁸ superior chemical resistance to corrosion,¹⁹ excellent thermal weldability, and cost-efficiency.²⁰ Building on aforementioned advantages, HDPE pipes have been widely adopted in a variety of sectors,²¹ including water supply and gas distribution,²² agricultural irrigation,²³ mining and oil transmission,²⁴ marine engineering,^25,26 nuclear power plants,^27,28 stormwater management,²⁹ landfill leachate collection,³⁰ and district heating systems.³¹

Owing to the inherent limitations in manufacturing pipe length, the complexity of pipeline routing, and the need for modular installation on-site, HDPE pipes must be reliably joined to form a continuous and structurally sound network by fusion welding methods, including butt fusion welding,²⁸ electrofusion welding,³² socket fusion welding,³³ and saddle fusion welding, as specified in ASTM-F2620-11,³⁴ laser welding,³⁵ friction stir welding,^36,37 and ultrasonic welding.³⁸ Among various fusion welding methods, butt fusion welding remains the most widely adopted and cost-effective method for joining HDPE pipes.^39,40 The long-term durability of HDPE pipeline systems in service is critically dependent on the quality of welded joints, which is primarily governed by the intrinsic properties of HDPE and the control of welding process parameters. The effectiveness of destructive testing (DT) and non-destructive evaluation (NDE) techniques plays a crucial role in accurately assessing and verifying weld integrity. Welded joints in HDPE pipes are widely recognized as the most failure-prone regions, primarily due to the occurrence of weld defects. These defects arise from a combination of factors, including improper welding process parameters,⁴¹ the intrinsic thermophysical limitations of HDPE, interfacial contamination by moisture, dust, or oxidized layers,⁴² misaligned or insufficient clamping during butt fusion welding,⁴³ and issues related to the selection, calibration, maintenance, or overall quality of welding equipment. In addition, external environmental conditions—such as ambient temperature fluctuations, wind exposure, and humidity levels—as well as operator skill level and training, play critical roles in governing weld quality and defect formation. Common welded defects in welded joints of HDPE pipe include, for example, uneven weld bead,⁴⁴ lack of fusion,⁴⁵ thermal degradation,⁴⁶ and others. The formation mechanisms and performance implications of these defects will be comprehensively discussed in the subsequent section. These issues significantly compromise the structural integrity and long-term durability of welded joints of HDPE pipes, often leading to premature failure,⁴⁷ fluid leakages,⁴⁴ and reduced service life, ultimately jeopardizing the overall reliability of pipeline systems. In real-world applications, such defects have been directly implicated in major safety incidents involving gas explosions, property damage, and even fatalities. Documented failures associated with defective welded joints include an explosion at Cadia Mine in New South Wales, Australia (July 22, 2024),⁴⁸ associated with recently completed HDPE pipe connections and resulting in significant equipment damage; a candy factory explosion in West Reading, Pennsylvania (March 24, 2023),⁴⁹ caused by gas leakage from a welded HDPE pipeline segment, leading to seven fatalities and extensive structural destruction; a fatal pressure test accident in Wuhan, China (May 15, 2017),⁵⁰ where improper use of an SHY160 welding machine on a 250 mm HDPE pipe led to joint failure and the death of a pedestrian; a residential gas explosion in Yuba City, California (January 12, 2017),⁵¹ triggered by leakage at a defective plastic pipe joint and causing injuries and major property loss; and a natural gas–fueled building explosion in East Harlem, New York City (March 12, 2014),⁵² resulting from a failed fusion joint at a service tee, which caused eight deaths and injured over 50 people.

To minimize weld defects occurred and reduce the risk of in-service failures in HDPE pipes, considerable research has focused on enhancing the structural integrity and long-term durability of welded joints during butt fusion welding. Ongoing research efforts are primarily concentrated on three complementary areas: (1) the optimization of key welding process parameters to ensure consistent welded joint formation; (2) the enhancement of both destructive (DT) and non-destructive evaluation (NDE) methods for accurate quality assessment; and (3) the development of material modification strategies aimed at improving interfacial bonding and overall welded joint performance. Despite significant progress in improving the quality of welded joints of HDPE pipes during butt fusion welding, a comprehensive review that systematically synthesizes the diverse multidisciplinary contributions in this field is still lacking. Most existing studies remain limited to isolated aspects of the welding process—such as thermal control, defect morphology, or single-parameter effects—without integrating materials science, processing technology, and evaluation methodologies into a unified framework. To bridge this gap, the present review offers an integrated and in-depth synthesis encompassing the intrinsic properties of HDPE; the fundamental principles and procedures of butt fusion welding; the formation mechanisms and mechanical implications of welding defects; recent advances in DT/NDE techniques; optimization of welding process parameters; and material modification strategies aimed at enhancing weld performance. In addition, the review identifies key technical challenges and outlines future research directions to support the development of more reliable and intelligent HDPE pipeline systems.

To achieve this objective, the paper is structured as follows. The first part introduces the fundamental material properties of HDPE relevant to butt fusion welding, followed by a section outlining the principles and procedural stages of the butt fusion welding process. The subsequent part analyzes the formation mechanisms of welded defects and their influence on the mechanical performance and long-term reliability of HDPE pipeline systems. Further sections review both conventional and emerging approaches to weld quality evaluation, including DT/NDE techniques, and discuss recent advances in the optimization of welding process parameters. The paper then explores material modification strategies designed to improve interfacial bonding strength and structural compatibility. Finally, the concluding part summarizes the key barriers and proposes future research directions for developing high-integrity, defect-tolerant welded joints in HDPE pipelines.

Intrinsic material properties of HDPE pipes relevant to weldability

Molecular structure and crystallinity

HDPE is a semicrystalline, non-polar polyolefin characterized by high linearity, low branching density, and well-ordered molecular architecture,⁵³ all of which fundamentally originate from its coordination polymerization mechanism. In addition, HDPE possesses a chemically inert backbone and relatively low chain entanglement, both of which influence its melt behavior and interfacial diffusion characteristics during butt fusion welding. HDPE is synthesized from ethylene monomers (CH₂ = CH₂) via coordination polymerization under relatively low-pressure (∼1–2 MPa) and moderate-temperature (∼70 °C–110°C) conditions, using Ziegler–Natta or metallocene catalysts.^4,5 These catalysts enable stereo- and regio-selective head-to-tail insertion of ethylene units while suppressing chain transfer and branching, resulting in highly linear polymer chains.^54,55 As a result, this molecular configuration gives rise to a two-phase morphology, characterized by crystalline lamellae embedded within an amorphous matrix. HDPE typically exhibits a crystallinity of 70–80% and a material density exceeding 0.94 g/cm³.⁵⁶ Within this microstructure, disordered and flexible chain segments in the amorphous regions facilitate molecular interdiffusion and chain entanglement at the weld interface—both of which are essential for establishing strong interfacial bonding in welded joints of HDPE pipes. Conversely, the highly ordered crystalline domains improve thermal conductivity and promote localized recrystallization during the cooling phase, contributing to enhanced weld strength and structural integrity. However, the elevated crystallinity and high molecular linearity of HDPE significantly hinder chain mobility and slow down interfacial diffusion, thereby limiting the effectiveness of molecular entanglement and fusion across the weld interface.

Thermal, rheological, and viscoelastic characteristics

The thermal, rheological, viscoelastic properties of HDPE critically govern melting zone formation and interfacial molecular behavior, thereby directly influencing the weldability of HDPE pipes during butt fusion welding. HDPE exhibits a notably narrow melting-crystallization window (125-135°C melting, 100-115°C recrystallization) and low, temperature-dependent thermal conductivity (∼0.45 W/m·K), primarily due to its disordered, loosely packed amorphous regions. These characteristics impede heat transfer and precise thermal control during welding, causing steep temperature gradients, incomplete fusion, and structural inhomogeneities, especially in thick-walled or large-diameter pipes. During butt fusion welding, the molten zone of HDPE pipe exhibits non-Newtonian, shear-thinning rheological behavior, with melt viscosity highly dependent on both temperature and shear rate. This pronounced shear-thinning behavior facilitates uniform melt flow and effective interfacial wetting during the bead-up and fusion stages, thereby promoting the formation of high-integrity welded joints. To accurately capture the temperature- and shear-rate-dependent rheological behavior of HDPE, various constitutive models have been developed, including the Power Law model,^57,58 the Cross model,⁵⁹ the Carreau-Yasuda model,⁶⁰ the Arrhenius equation,⁶¹ and Williams–Landel–Ferry (WLF) equation.⁶² In addition to the non-Newtonian, shear-thinning rheological behavior, HDPE also demonstrates significant nonlinear viscoelastic behavior, characterized by time-dependent stress relaxation and strain recovery, which further affects its deformation behavior and interfacial dynamics during the welding process. To model this temperature-dependent viscoelastic response, a range of viscoelastic constitutive frameworks have been employed, such as the Maxwell model, Generalized Maxwell model (Prony coefficients), and the Kelvin-Voigt model, Standard Linear Solid (SLS), among others.^63,64 Together, these rheological and viscoelastic constitutive frameworks not only provide valuable insights into the thermo-mechanical behavior of HDPE under dynamic welding conditions, but also serve as foundational tools for developing predictive numerical models. Their integration into simulation workflows significantly enhances the ability to evaluate and optimize joint quality in butt fusion welding applications.

Overview of the butt fusion welding process

Butt fusion welding is the most widely adopted and cost-effective method for connecting HDPE pipes to produce homogeneous welded joints with excellent mechanical integrity and long-term durability. According to ISO 21,307:2017,⁶⁵ butt fusion welding procedures are classified into three main categories based on heating temperature and fusion pressure: Single Low Pressure (SLP), Single High Pressure (SHP), and Dual Low Pressure (DLP).^28,66 Each welding standard defines a specific set of process parameters to ensure consistent and reliable joint quality across varying pipe sizes, material grades, and application conditions. The SLP procedure is primarily adopted in most countries such as Germany under DVS 2207-1,⁶⁷ the SHP procedure is widely utilized in the United States following ASTM F2620,³⁴ and the DLP procedure is mainly applied for large-diameter water pipes in regions such as the United Kingdom. A standard butt fusion welding cycle comprises five sequential stages: initial bead-up (pre-heating),⁶⁸ heat soak,⁶⁹ heater plate removal,⁷⁰ and fusion jointing,⁷¹ and cooling.⁷² As illustrated in Figure 1, the process begins with the bead-up stage, during which a bead-up pressure is applied to ensure intimate contact between the pipe ends and the heater plate, thereby facilitating uniform heat transfer across the interface. The adequacy of this stage is typically assessed by the size of the initial bead formed on the heater plate, which, according to ISO 21,307:2017, should reach at least 0.5 + 0.1× e_n, where e_n represents the nominal wall thickness of the HDPE pipe, mm. This is immediately followed by the heat soak stage, during which the fusion pressure is reduced to the designated heat soak pressure and maintained for a specified duration. In this phase, the pipe ends remain in firm contact with the heater plate, allowing sufficient thermal conduction to form a uniform molten layer across the interface. The development of this melt layer is accompanied by the formation of a bead width that meets the target specifications based on the nominal outside diameter (DN) and its standard dimension ratio (SDR). Upon completion of the heat soak period, the heater plate is swiftly removed within a few seconds to minimize heat dissipation. Upon completion of the heat soak period, the two molten pipe ends are immediately brought into contact under a fusion jointing pressure during the joining stage. The final stage of the welding cycle is the cooling stage, where the welded joint remains under constant pressure to promote molecular diffusion and chain entanglement across the interface, thereby consolidating the weld structure.

Figure 1.

Schematic profiles of fusion pressure and temperature during butt fusion welding of HDPE pipes.

These observations collectively underscore the critical importance of precise and coordinated control over all key process parameters throughout the butt fusion welding cycle. Each stage governs distinct aspects of thermal input, melt behavior, and pressure response, and deviations in any parameter—such as heating temperature, fusion pressure, processing time, cooling rate—can adversely affect interface quality. Moreover, given the nonlinear and interdependent nature of thermal and viscoelastic behavior in HDPE pipe, optimal weld quality cannot be achieved through isolated parameter adjustment alone. Instead, an integrated, multi-parameter control strategy—tailored to specific pipe geometries, SDR classifications, and ambient operating conditions—is essential for producing defect-free, structurally sound HDPE joints with reliable long-term performance.

Although Figure 1 delineates five temporal intervals (t1–t5) and three pressure levels (P1–P3), practical butt fusion welding protocols primarily regulate the heat soak time (t2), cooling time in machine under pressure (t5), bead-up pressure (P1), and heat soak pressure (P2). The fusion jointing pressure (P3) is typically set equal to the bead-up pressure (P1), reflecting standardized machine programming. The bead-up time (t1) is not explicitly specified but is generally inferred from the formation of a consistent bead height, typically 1.5–2.0 mm per side. The heater plate removal time (t3) is minimized to mitigate thermal losses and prevent surface oxidation at the molten interface, while the time to achieve fusion jointing pressure (t4) is often implicitly embedded within t5 and not independently controlled. As a result, only selected parameters are quantitatively prescribed, whereas others—such as t1, t3, and t4—are determined based on operator expertise, visual inspection, and equipment responsiveness. However, in finite element method (FEM) of HDPE butt fusion welding, all five temporal and three pressure variables can be explicitly incorporated to systematically investigate their individual and combined influences on weld formation and joint quality.

Analysis of the mechanisms and implications of weld defect formation

Most welded defects in HDPE pipe joints arise from the combined influence of the intrinsic thermophysical behavior and inadequate control of process parameters during butt fusion welding. Additionally, interfacial contamination, end-end misalignment or insufficient clamping, and surface oxidation may lead to localized imperfections that compromise weld integrity. These issues can be further exacerbated by improper selection, calibration, maintenance, or quality deficiencies of welding equipment; adverse environmental conditions—including ambient temperature fluctuations, wind exposure, and humidity; and insufficient operator skill or training. Among these factors, the sensitivity of weld quality to key process conditions—including heating temperature, fusion pressure, cooling rate, and processing time—is particularly pronounced. Figure 1 schematically outlines the standard butt fusion welding cycle and its corresponding pressure–time profile, highlighting the precise temporal coordination required at each stage to achieve uniform interfacial bonding. Deviations from optimal welding process parameters—such as underheating, overheating, excessive or insufficient pressure, or uncontrolled cooling rate—can disrupt interfacial melt homogeneity, hinder molecular interdiffusion, and promote the formation of voids, cold weld lines, or incomplete fusion zones. Furthermore, external disturbances such as misalignment and surface contamination exacerbate these issues, especially at the weld interface where chain mobility and diffusion are most critical. Figure 2 provides an overview of common weld defects associated with these issues. For comparison, Figure 2(a) depicts an ideal welded joint from butt fusion welding, characterized by a well-developed weld seam and symmetric, uniform beads on both sides—indicative of adequate thermal input and proper process control.

Figure 2.

Defects in welded joints induced by deviations in welding parameters.

Underheating, typically caused by insufficient heating temperature or a shortened heating time, restricts interfacial melting and limits molecular diffusion and chain entanglement, leading to cold weld lines or incomplete fusion zones, as illustrated in Figure 2(b) and (c). In contrast, overheating may result in thermal degradation or oxidative embrittlement near the pipe surface, producing brittle interfacial layers that compromise mechanical performance, as shown in Figure 2(h). In addition to the temperature-related defects, improper fusion pressure at the HDPE pipe ends is regarded as another of the most critical contributors to weld defects at welded joints of HDPE pipes. Insufficient pressure during fusion stage can lead to inadequate interfacial contact, void formation, and undersized weld seams, as demonstrated in Figure 2(e), (f), and (j). Compared with insufficient pressure, excessive pressure may displace molten material, reduce molten zone thickness, and inhibit chain interpenetration—resulting in narrow welds and oversized beads with poor strength, as shown in Figure 2(g). The cooling phase plays a critical role in the development of weld microstructure. Excessively fast cooling limits polymer chain mobility and crystallization, increasing residual stresses and reducing interfacial entanglement, which exacerbates susceptibility to slow crack growth (SCG), as illustrated in Figure 2(l). On the other hand, overly slow cooling promotes lamellar thickening and the formation of large, brittle spherulites, while prolonged exposure to elevated temperatures accelerates oxidative degradation. Apart from the effects of above, Misalignment between pipe ends, as shown in Figure 2(d) and (i), introduces asymmetry and stress concentration zones, while surface contamination or oxidation may lead to inclusion-type defects and poor wettability, further weakening the fusion interface. Additionally, irregular bead morphologies, as shown in Figure 2(k) have been observed in aged HDPE materials, suggesting a degradation of rheological properties prior to welding.

These welding defects not only compromise the short-term mechanical performance of HDPE joints—manifested by reductions in tensile strength, flexural strength, elongation at break, and impact resistance⁷³—but also severely threaten their long-term structural reliability. Cold weld lines and entrapped voids serve as stress concentrators and crack initiation sites under internal pressure, impact loads, and fluctuating service conditions. Even joints that pass standard hydraulic pressure tests may exhibit premature failure when exposed to cyclic mechanical stresses or harsh operational environments.⁷⁴ Over the long term, the presence of thermal gradients and non-uniform crystallinity in the weld zone induces residual stresses and anisotropic mechanical behavior. These factors accelerate the onset of environmental stress cracking (ESC) and slow crack growth (SCG), particularly in buried pipelines or chemically aggressive environments where oxidative aging and sustained loading coexist.^75,76 Additionally, excessive crystallinity or the formation of large, imperfect spherulites due to suboptimal cooling may lower fracture toughness and increase the likelihood of brittle failure over prolonged service life. As such, welding defects pose a cumulative degradation risk that undermines both immediate performance and long-term durability of HDPE pipeline systems.

Destructive testing and non-destructive evaluation techniques

Given the inherent susceptibility of welded joints in HDPE pipes to various welding defects—and their pivotal role in maintaining the structural integrity of modern pipeline systems—rigorous weld quality evaluation is essential. Upon completion of the butt fusion process, joints must undergo comprehensive inspection to verify structural soundness and assess long-term performance. Common inspection protocols combine DT/NDE techniques, which offer complementary insights from macro-scale strength evaluation to micro-scale interfacial characterization.

DT techniques are widely employed to evaluate the mechanical and interfacial performance of welded joints in HDPE pipes by applying external loads or performing sectional analyses. Standard DT methods—such as tensile, flexural, impact, and creep testing—provide critical insights into the macroscopic strength and long-term durability of welded joints. To probe localized mechanical gradients and microstructural transitions of welded joints in HDPE pipe, high-resolution techniques like nanoindentation⁷⁷ and cross-sectional microscopy⁷⁸ are frequently utilized. Among these methods, creep testing is particularly essential for assessing the time-dependent deformation and service life of HDPE joints under sustained loading. Both conventional and accelerated creep tests are commonly adopted in research and certification contexts. Conventional methods, including hydrostatic pressure testing (HTP)⁷⁹ and tensile creep tests,⁸⁰ are valued for their reliability and realism but are limited by lengthy testing durations, making them inefficient for rapid evaluation. To address this, accelerated testing protocols—such as the Time-Temperature Superposition Principle (TTSP),⁸¹ Time-Stress Superposition Principle (TSSP),⁸² Time-Temperature-Stress Superposition Principle (TTSSP),⁸³ Stepped Isothermal Method (SIM),⁸⁴ Stepped Stress Method (SSM),⁸⁵ and the Methodology of Accelerated Characterization for long-term creep Prediction (MACcreeP)⁸⁶—utilize elevated temperatures, increased stress levels, and theoretical superposition to significantly reduce test times. These methods are often coupled with predictive modeling to extrapolate long-term creep behavior from short-term data, offering practical tools for service-relevant performance assessment. Despite their diagnostic value and quantitative rigor, DT techniques are inherently invasive, time-intensive, and unsuitable for in situ or operational pipelines, thereby limiting their real-world applicability in continuous monitoring scenarios.

In contrast, NDE techniques enable non-invasive, in situ assessment of weld integrity. Traditional methods like visual inspection and conventional ultrasonic testing (UT) offer rapid detection of surface and near-surface anomalies. Advanced techniques—including phased array ultrasonic testing (PAUT), infrared thermography (IRT), microwave and terahertz imaging, acoustic emission (AE), and guided wave ultrasonic testing (GWUT)—provide higher sensitivity, improved resolution, and real-time diagnostics. These methods enhance the detection of embedded flaws, lack-of-fusion zones, and early-stage cracks, although challenges such as signal attenuation, limited resolution, and equipment complexity remain. Radiographic testing offers high-resolution imaging of internal features but poses health and safety risks that constrain its use in field environments. A comparative summary of DT/NDE techniques, as outlined in Table 1, highlights their respective strengths and limitations. DT techniques are highly effective for evaluating mechanical strength, interfacial bonding, and failure behavior, but typically require specimen destruction, extended test durations, or strict geometric specifications.

Table 1.

Comparative summary of DT/NDE techniques: Key features and limitations.

Technique	Key features	Limitations	Literature
*Tensile testing	Measures tensile strength and elongation at break, while identifying failure location across the weld interface	Not applicable for in-service components due to its destructive nature	¹⁰¹
*Flexural testing	Evaluates flexural strength and identifies surface-initiated cracking under bending loads	Restricted to the detection of surface and near-surface defects only	¹⁰²
*Impact testing	Measures impact energy absorption and distinguish between brittle and ductile fracture behavior	Highly sensitive to temperature variations and dependent on notch geometry for accurate assessment	¹⁰³
*Fatigue testing	Evaluates resistance to cyclic loading and analyzes crack initiation and propagation behavior	Requires extended testing duration, limiting throughput in practical applications	¹⁰⁴
*Peel testing	Measures interfacial bonding strength and observes decohesion behavior along the welded interface	Not suitable for evaluating thick welded joints due to limited penetration or load applicability	¹⁰⁵
*Fracture toughness	Quantifies crack growth resistance and identifies critical flaw sizes influencing structural integrity	Requires complex specimen preparation, increasing testing time and variability	^106,107
*Creep test	Assesses long-term mechanical performance under sustained pressure and elevated temperature conditions	Time-consuming and inherently destructive, limiting applicability for routine inspections	^108,109
#Visual inspection	Enables rapid identification of surface defects, weld bead irregularities, and joint misalignment	Subjective and restricted to visually detectable surface flaws	^110,111
#Conventional UT	Detects subsurface inclusions and lack-of-fusion defects using acoustic wave reflection	Susceptible to signal attenuation in HDPE and highly dependent on operator expertise	¹¹²
#Phased array UT	Enables multi-angle scanning for internal defect imaging and evaluation, offering enhanced inspection coverage and spatial resolution	Reduced imaging sensitivity for small-sized defects, limited accuracy for out-of-focus anomaly detection, and increased system complexity due to specialized calibration requirements	⁹⁶
#Infrared thermography	Provides real-time, non-contact thermal imaging for detecting surface and near-surface defects and evaluating thermal behavior across the weld zone	High equipment cost, sensitivity to environmental conditions, limited resolution of infrared detectors, and inability to detect deep-seated defects	^113,114
#Radiographic testing	Examines internal structure and detects voids and inclusions using high-resolution X-ray or gamma-ray imaging	Poses health and safety risks, limiting its practicality for routine on-site inspection	⁹⁸
#Microwave/Terahertz imaging	Enables non-contact detection of internal defects through reflection-based imaging, sensitive to dielectric property variations across the weld zone	Distance-dependent resolution with limited sensitivity to small-sized defects, posing challenges in accurate defect characterization	^99,115,116
#Acoustic emission	Detects sudden events such as crack initiation through acoustic emission signals	Complex signal interpretation requiring advanced analysis and operator expertise	¹¹⁷
#Guided wave UT	Detecting the growth of internal cracks, weld defects, and corrosion	Limited resolution for detecting small or subtle defects	^100,118,119

“*” and “#” denote the DT and NDE techniques, respectively.

Beyond DT/NDE techniques, a range of analytical tools is employed to investigate structural, thermal, and chemical characteristics of welded joints under laboratory conditions. Techniques such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) offer insights into crystallinity, melting behavior, and thermal degradation. Scanning electron microscopy (SEM) and polarized optical microscopy (POM) reveal fracture morphology and spherulitic structure, while X-ray diffraction (XRD) and atomic force microscopy (AFM) provide crystallographic and nanoscale topographic data. Fourier transform infrared spectroscopy (FTIR) is effective for detecting oxidation and contamination at the weld interface. Although inherently ex-situ and unsuitable for field use, these methods are indispensable for failure analysis, weld validation, and material optimization.

Among the aforementioned characterization techniques, beyond DT/NDE methods, SEM has been extensively adopted to elucidate the microstructural features of welded joints in HDPE pipe during butt fusion welding.^87–89 Owing to its superior spatial resolution and diagnostic fidelity, SEM enables detailed visualization of fracture surfaces, weld beads, and interfacial zones, facilitating the identification of voids, cold weld lines, incomplete fusion, and oxidative degradation.⁹⁰ Moreover, SEM micrographs frequently reveal morphological indicators such as spherulitic texture, lamellar orientation, and phase continuity, thereby providing critical insights into the correlation between process parameters, crystalline morphology, and mechanical performance.^15,91–94 Thus, SEM is widely regarded as one of the most authoritative tools for assessing interfacial quality and weld integrity.

To address the limitations of conventional DT/NDE techniques —particularly regarding in situ applicability, resolution, and automation—emerging research has increasingly focused on intelligent, high-resolution, and data-driven inspection methods. These developments integrate advanced signal processing, artificial intelligence, and hybrid sensing technologies to achieve more accurate, real-time, and adaptive defect detection. The following section highlights representative studies that demonstrate the application and potential of such next-generation NDE solutions in HDPE pipeline systems. For example, the CNPC Tubular Goods Research Institute developed China’s first terahertz-based online detection system for HDPE pipelines,⁹⁵ integrating 3D reconstruction and layer-by-layer imaging, achieving >95% detection accuracy with <50 μm resolution. In ultrasonic imaging, Chen et al. proposed the D-CCF-TFM method,⁹⁶ which enhances PAUT performance via directionality and coherence compensation. Combined with a YOLOX-based deep learning model, the system enables high-accuracy, real-time defect classification. Building on this, Zhang et al. developed the STSVD-ITFM algorithm,⁹⁷ combining spatiotemporal singular value decomposition filtering with an improved TFM correction scheme. This approach effectively suppresses near-field clutter and improves defect localization and depth estimation. Experimental validation confirmed superior imaging performance over conventional methods. In support of field deployment, Gueugnaut et al.⁹⁸ applied PAUT to inspect saddle and socket joints with artificial defects. Although thin internal flaws in complex regions posed challenges, hydrostatic pressure testing (80°C, 5 MPa, 1000 h) confirmed structural reliability, validating PAUT’s effectiveness for weld quality assurance in PE systems. Beyond acoustics, Mohammed Saif ur Rahman et al.⁹⁹ explored microwave NDT, employing near-field wide-frequency probes for imaging embedded flaws in HDPE and GRE pipes. Results showed strong consistency with PAUT, highlighting its potential as a cost-effective, high-resolution tool for non-metallic pipe inspection. For long-range crack detection, Shah et al.¹⁰⁰ demonstrated the utility of GWUT using low-cost piezoceramic transducers. Third- and fourth-order longitudinal modes achieved reliable detection of cracks ≥1 wavelength. Finite element modeling validated its potential, despite directional limitations. Additionally, Muhammad Shaheer et al.²¹ applied nanoindentation to quantify MZ softening and HAZ gradients under varying welding procedures, revealing inconsistencies in joint quality not captured by macro tests. This reinforces the need for microscale assessment techniques in validating process parameters.

These developments exemplify the growing convergence of sensing, computation, and modeling in HDPE weld inspection, paving the way for intelligent, automated, and adaptive systems capable of overcoming the limitations of conventional DT/NDE techniques. The integration of multi-modal, real-time, and AI-assisted inspection strategies is increasingly essential for ensuring weld reliability across both laboratory research and industrial pipeline applications.

Optimization of welding process parameters of butt fusion welding in HDPE pipes

As outlined in the preceding sections, the majority of weld defects in HDPE butt fusion joints originate from improper control of critical welding parameters. These parameters play a decisive role in governing interfacial molecular diffusion, melt flow dynamics, and stress development throughout the welding process. Therefore, the precise optimization of welding conditions is imperative to minimize defect formation and to improve the structural integrity and long-term performance of welded joints in HDPE pipes.

In response to these challenges, a range of optimization methodologies have emerged in recent years. Experimental-based optimization is among the most commonly used strategies to determine optimal welding parameters for achieving high-quality welded joints in HDPE pipes during butt fusion welding. Traditional design of experiments (DoE) methods such as full factorial design (FFD), orthogonal array design (ORD), grey relational analysis (GRA), analysis of variance (ANOVA), response surface methodology (RSM), and Taguchi method have been extensively employed to systematically determine the optimal combination of key welding parameters. While these methods are effective in mapping parameter–performance relationships and identifying statistically significant factors, they often require substantial experimental effort and may not fully capture the complex thermal–rheological interactions inherent to the welding process. To address these limitations, modeling-based optimization approaches have been developed, drawing upon FEM, computational fluid dynamics (CFD), molecular dynamics (MD) simulations. These methods often incorporate coupled thermal, rheological, viscoelastic constitutive models. Such models enable the prediction of spatiotemporal evolution in temperature fields, residual stresses, and melt flow behavior across the welded region. By simulating interfacial fusion dynamics and capturing non-linear material responses under varying process conditions, such models offer mechanistic insight into weld defect formation and enable the identification of optimal process windows with reduced experimental effort. Complementing both experimental-based and modeling-based optimization, intelligent data-driven optimization has recently gained prominence with the integration of machine learning (ML), deep learning (DL), artificial neural networks (ANN), support vector machines (SVM), genetic algorithms (GA), particle swarm optimization (PSO), reinforcement learning (RL), and pattern search (PS). Altogether, these optimization strategies constitute a comprehensive and multi-layered toolbox for advancing the precision and reliability of welded joints in HDPE pipes butt fusion welding.

Saurabh Pathak et al.³⁹ optimized key welding process parameters including heating plate temperature, drag pressure, welding pressure using GRA method, aiming to enhance the tensile strength of welded joints in HDPE pipes with an inner diameter of 82 mm and a wall thickness of 8 mm wall. They found that the optimal parameters were 215°C, 20 bar, and 18 bar, respectively. Mahdi Saleh Mathkoor et al.⁶⁹ investigated the relationship between welding process parameters and joint profile geometry with a nominal diameter of 101.6 mm, a wall thickness of 7 mm, and a standard dimension ratio (SDR) of 17. The joint profiles considered included outer surface features (cap height and width) and internal surface characteristics (root height and width). The corresponding joint tensile strength was analyzed using ANOVA and an ANN. To determine the optimal welding process parameters, GA and PS approaches were subsequently employed. The optimal outputs yielded a strength of approximately 35 MPa with joint cap and root heights of 3.45 mm and 4.5 mm, and cap and root widths of 8 mm and 6.98 mm, respectively. Muhammad Shaheer¹²⁰ conducted a comprehensive study employing the FEM, calibrated against experimental tensile tests, to investigate the relationship between heat input and tensile failure behavior of welded joints with an outer diameter of 180 mm and a standard dimension ratio (SDR) of 11. The results revealed a stress concentration increase of approximately 30% at the outer weld notch, highlighting the critical influence of weld bead geometry on joint performance and overall tensile strength. Ihssan Srii et al.¹²¹ proposed a new approach to the analysis and prediction of mechanical properties of HDPE pipes using experimental tensile testing combined with deep learning (DL) and Bayesian-regularized ANN models. They concluded that artificial intelligence (AI)-based models provide highly accurate predictions (MSE ≈ 0.00,023, R² ≈ 0.99,934), offering strong potential for applications in modern pipeline networks. Furthermore, Ihssan Srii¹²² employed a hybrid approach combining FEM with ANN to predict the tensile strength of HDPE pipes used in water distribution systems. The results demonstrated strong consistency between the experimental trial outcomes and the predictions generated by the machine learning (ML) models, indicating the reliability and accuracy of the proposed hybrid technique. Walid Awadi et al.¹²³ employed the FFD method of DoE framework to evaluate the effect of key welding process parameters including heating temperature, heating duration and the applied strength on the performance of the butt fusion welding process. Based on experimental data, mathematical models were developed to describe the relationships between the input parameters (e.g. force and heating duration) and the corresponding outputs such as temperature distribution during the different welding phases and the thickness of the molten polymer. Under the specific conditions of fatigue loading. Lai et al.¹²⁴ found that a defect size of less than 15% of the MDPE pipe’s wall thickness did not affect the failure of joints welded by butt fusion. Jinesh Kumar Jain et al.¹²⁵ conducted a comprehensive performance analysis and process optimization study across multiple polymer processing approaches, employing techniques such as ANN, Taguchi method, ANOVA, and GRA. Boris Novakovic et al.¹²⁶ developed the mathematical models to describe material displacement and welding time using regression models and ML-based models (SVM), respectively. Additionally, this study presented a visual representation of HDPE melt displacement with the changes in heating temperature and fusion pressure. Lingchun Zhang et al.¹²⁷ selected and scored seven main factors affecting pipeline failure, and then established a pipeline failure model by using the particle swarm optimization (PSO) neural network. The model uses the neural network training of historical data to evaluate the failure of the water supply pipeline, and the PSO is used to optimize the neural network to effectively improve the training time and accuracy. The model error and correlation coefficient are 0.003 and 0.987, respectively.

The aforementioned studies have significantly advanced the understanding of butt fusion welding mechanisms and proposed a range of optimization strategies aimed at improving the structural integrity and long-term reliability of HDPE pipeline joints. These insights are highly valuable for the development of intelligent welding technologies and the efficient deployment of pipeline infrastructure. However, experimental approaches are often constrained by high costs, complex setups, and limited reproducibility. To address these limitations, FEM has emerged as a powerful and cost-effective computational tool, enabling virtual prototyping, multi-parameter sensitivity analysis, and in-depth exploration of the coupled thermal–mechanical phenomena during welding. When validated against experimental data, these numerical methods offer a robust framework for predicting defect formation, evaluating stress evolution, and optimizing process parameters—thereby accelerating the development of high-performance, defect-tolerant welding strategies for HDPE systems. Zhang Yi et al.¹²⁸ established a validated FEM model to investigate thermal and residual stress distributions in HDPE butt fusion joints. Optimal parameters (230°C, 100 s heating time, 2.5 MPa pressure) yielded the highest tensile strength, exceeding that of the base material. Simulation results closely matched experimental data, revealing that heating temperature and time significantly affect residual stress, whereas pressure has a negligible effect. Zeina Gerges¹²⁹ focused on optimizing process parameters for continuous drive rotary friction welding (RFW) of HDPE pipes. A 2D axisymmetric thermo-mechanical model incorporating temperature-, strain-, and strain-rate-dependent properties via the Zerilli–Armstrong equation was developed in DEFORM. Validated against published data and optimized using Taguchi and regression methods, the model identified 800 RPM, 20 mm/min feed rate, and 9 s friction time as optimal, minimizing energy consumption while ensuring weld quality. Sun et al.¹³⁰ explored residual stress distribution in HDPE welds using the hole-drilling strain method coupled with FEM. Results indicated that hoop residual stress was markedly higher than axial or radial components, increasing with pipe wall thickness. The maximum tensile stress was concentrated near the outer wall of the joint—highlighted as a potential failure zone. Yoo et al.⁶⁰ developed a two-dimensional axisymmetric finite element model to simulate the heat and fluid flow behavior during the butt fusion welding of HDPE pipes. By incorporating the Carreau-Yasuda viscosity model along with an Arrhenius-type temperature dependence, the simulation effectively captured key flow phenomena including thermal expansion during the heat soak stage, squeezing and fountain flows during welded joint formation, and bead curling at the weld periphery. Donghu Zeng et al.^131,132 developed a mathematical model (MM) based on Neumann’s solution to predict the average molten zone (AMZ) thickness in HDPE pipes during butt fusion welding, considering heating temperature (190-350°C), time (180 s), and heat convection. A 2D CFD model was constructed using finite element analysis for validation. Results showed a strong agreement, with relative errors between MM and CFD ranging from 0.280% to 10.830% (with convection) and −2.398% to 8.992% (without convection). Convection showed negligible influence, with MM model errors under 0.433%. This validated MM approach offers a low-cost and efficient alternative for AMZ prediction in large-diameter HDPE pipe welding.

In this review, the authors also performed four representative finite element (FE) simulations to evaluate the influence of key process parameters—including heating temperature, processing time, and fusion pressure—on the temperature evolution at different depths along the central axis of the welded joint throughout the butt fusion welding cycle. A two-dimensional (2D) axisymmetric HDPE pipe model was constructed, featuring a nominal outer diameter of ⌀110 mm, wall thickness of 6.6 mm (SDR 17), and axial length of 50 mm. The temperature-dependent, nonlinear thermophysical properties of HDPE were adopted from relevant literature sources.^133,134 The applied thermal and mechanical boundary conditions conformed to the butt fusion welding protocol illustrated in Figure 1 and detailed in Table 2. Accordingly, both the simulation setup and results presented in Table 2 and Figure 3 are derived entirely from the authors’ own modelling work.

Table 2.

Simulation conditions for analyzing temperature evolution in butt fusion welding of HDPE pipes.

Case	Heating temp. [°C]	t1 [s]	t2 [s]	t3 [s]	t4 + t5 [s]	P1 [bar]	P2 [bar]	P3 [bar]
1	220	50	79	5	600	6	1	6
2	220	50	40	5	600	6	1	6
3	220	50	79	5	600	9	1	9
4	280	50	79	5	600	6	1	6

Figure 3.

Temperature evolution at different depths (Z = 0–6.0 mm) of welded joints in HDPE pipelines during butt fusion welding under varying heating temperatures, processing times, and fusion pressures: (a) Case 1; (b) Case 2; (c) Case 3; and (d) Case 4.

Figure 3 presents the simulated temperature evolution at various depths (Z = 0–6.0 mm) from the weld interface under four representative butt fusion welding conditions, corresponding to Cases 1–4 in Table 2 and mapped respectively to Figure 3(a), (b), (c), and (d). In all cases, the temperature at the weld interface (Z = 0 mm) rises rapidly due to direct contact with the heater plate, reaching the target value within a short time. After heater removal, this location cools most quickly, as it is directly exposed to ambient conditions and lacks thermal buffering from adjacent material. Consequently, a brief plateau or inflection point typically appears before the onset of rapid cooling. In contrast, regions at Z >0 mm heat up more slowly through one-dimensional thermal conduction from the interface. These subsurface layers may continue warming briefly after heater removal due to thermal diffusion lag and subsequently cool at a slower rate, owing to their insulated position and reduced exposure to external heat loss. This depth-dependent thermal behavior—marked by spatial differences in heating and cooling rates—plays a critical role in governing interfacial crystallization, molecular diffusion, and residual stress development, all of which ultimately influence the structural quality and reliability of the welded joint. In Case 1, the standard condition with a heating temperature of 220°C and a heat soak time of 79 s establishes a well-defined thermal gradient across the pipe wall, as shown in Figure 3(a). When the heat soak time is reduced to 40 s in Case 2, thermal penetration is markedly reduced, particularly at intermediate depths such as Z = 1.5 mm and 3.0 mm, as illustrated in Figure 3(b). In Case 3, increasing the fusion pressure to 9 bar while keeping all other parameters identical to Case 1 results in a nearly unchanged temperature distribution, indicating that fusion pressure exerts minimal influence on the thermal field, as seen in Figure 3(c). In contrast, Case 4, which raises the heating temperature to 280°C, leads to substantially higher peak temperatures across all depths and significantly enhances thermal penetration, especially in the mid-wall region, as depicted in Figure 3(d). However, the elevated interfacial temperatures in this condition may also increase the risk of thermal degradation or oxidative damage within the weld zone.

These results collectively demonstrate that heating temperature and heat soak duration are the primary factors governing interfacial temperature distribution and melt uniformity, whereas fusion pressure exerts only a secondary influence. Accordingly, precise and coordinated control of thermal input parameters is essential for ensuring adequate interfacial melting and achieving a uniform temperature field—both of which are critical prerequisites for forming high-integrity, defect-free HDPE welded joints.

Material modification strategies for enhanced weldability of HDPE pipes

Although substantial progress has been achieved in improving the structural integrity and long-term performance of welded joints in HDPE pipes through the various process-based optimization strategies as outlined in Section 5, these approaches alone remain insufficient to fully address the intrinsic material limitations of HDPE as discussed in Section 1. To address these limitations at material level, extensive research has been devoted to developing diverse modification strategies of HDPE aimed at fundamentally improving the structural performance, interfacial adhesion, and functional adaptability of HDPE, particularly in the context of welded joints. Current mainstream modification strategies of HDPE encompass grafting, crosslinking, copolymerization, blending, filler incorporation, reinforcement enhancement, and nanoparticle-based modification.

Among these modification strategies, grafting provides a straightforward and effective means of functionalizing PE by covalently attaching polar monomers to the PE backbone without significantly disrupting the molecular architecture. This technique maintains the intrinsic mechanical integrity of PE while introducing surface polarity and improving interfacial compatibility with polar phases or functional additives. Another commonly used method is crosslinking, which is typically achieved by forming covalent bridges between polymer chains, thereby improving the resistance to environmental stress cracking (ESC), corrosion, creep and weathering of PE.¹³⁵ Although a variety of chemical and physical methods are available for inducing, the resulting three-dimensional (3D) network structure compromises recyclability, as the material can no longer be remelted or reprocessed. When compatibility with dissimilar polymers is a primary concern, copolymerization/blending emerges as a preferred strategy. Incorporating polar components through chemical copolymerization or physical blending enables precise tuning of interfacial adhesion, antistatic behavior, and heat sealing, while maintaining the inherent processability of PE. In some cases, blending is also used to balance the complementary properties of different types of PE. For instance, low-density, flexible, and low-strength LDPE can be combined with high-strength but less tough HDPE to yield materials with customized hardness and mechanical profiles suited to specific application needs. From the perspective of cost reduction, filler modification is often employed to enhance the overall performance of PE by improving its dimensional stability, stiffness, thermal resistance, and flame retardancy. When more pronounced mechanical reinforcement is needed, materials such as glass or carbon fibers can be introduced; these reinforcements, unlike general fillers, bring substantial improvements in rigidity, creep resistance, and shape retention, elevating PE to a level more typical of engineering plastics. In recent years, nanoparticle-based modification has opened new possibilities by leveraging the high surface-to-volume ratio and tunable interfacial activity of nanoscale additives. Table 3 provides a comprehensive comparison of these broadly applicable modification strategies in terms of their typical techniques, practical applications, and benefits for enhancing the fusion welding performance of HDPE pipes. Building upon this overview, the following section summarizes representative studies from the past decade that have employed these strategies to improve the overall performance of HDPE materials.

Table 3.

Comparative summary of material modification strategies for improving HDPE pipes weldability.

Modification strategy	Representative techniques	Representative cases	Benefits on welding	Literature
Grafting modification	Solution grafting, melt grafting, solid-state grafting, radiation-induced grafting	MAH-g-PE, AA-g-PE, PE-g-GMA	Enhanced surface polarity, improved weld bonding	^136,137
Crosslinking modification	Radiation crosslinking, chemical crosslinking, silane grafting and moisture curing	Radiation XLPE, DCP-XLPE (PE-Xa), VTMS-g-PE (PE-Xb)	Increased stress crack resistance, long-term durability	^{135,138–140}
Copolymerization modification	Coordination copolymerization, free radical copolymerization, Ionic copolymerization	EVA, EPR, EPDM, EGMA, EMAA	Improved melt compatibility, enhanced interfacial adhesion	–
Blending modification	HDPE/LDPE blends, PE/CPE, PE/EVA, PE/Rubber, PE/PA blends	HDPE/LLDPE, PE/CPE with compatibilizer, PE/EVA, HDPE/EPDM, PE/PA with MAH-g-PE	Improved toughness, adjustable crystallinity and barrier properties	–
Filler modification	General fillers (CaCO₃, Talc), functional fillers (carbon black, starch), organic fillers (wood fiber)	CaCO₃/PE, Talc/PE, Starch/PE, carbon black/PE, Al(OH)₃/PE, Mg(OH)₂/PE	Increased stiffness, thermal resistance, cost-effective performance	^141,142
Reinforcement modification	Glass fiber, synthetic fiber (PAN, PA, PVA, Aramid), whiskers (CaCO₃, K₂TiO₃), self-reinforcement	Glass fiber/PE, PAN fiber/PE, PA fiber/PE, self-reinforced PE	Significantly enhanced strength and rigidity	–
Nanoparticle modification	Layered silicates (MMT), metal oxides (ZnO, Al₂O₃), carbon-based nanofillers (CNTs, GO)	MMT/PE, SiO₂/PE, ZnO/PE, GO/PE, CNT/PE	Improved melt uniformity, enhanced crack and heat resistance	^143,144

The following content reviews several representative material modification strategies that have been systematically investigated to enhance the weldability and post-weld performance of HDPE pipes. These strategies aim to overcome the intrinsic limitations of HDPE—such as low interfacial polarity, restricted molecular mobility, and sluggish interdiffusion kinetics—which collectively hinder effective chain entanglement and interfacial fusion during the butt fusion welding process. By improving melt rheology, thermal resistance, interfacial adhesion, and structural compatibility, these modifications contribute to the formation of stronger, more reliable welded joints under both laboratory and field conditions.

For instance, Hassan and Koyama¹⁴⁵ employed in situ graft polymerization of styrene onto HDPE using lauroyl peroxide (LP) as a free-radical initiator. Their results showed that increasing the styrene and LP content led to reduced elongation at break and lower melting/crystallization temperatures, but significantly enhanced tensile strength—from 14.6 MPa to 20.6 MPa—indicating improved chain entanglement potential at the weld interface. In the context of crosslinking, Liu et al.¹⁴⁶ developed a two-step DTBP–TAIC system that increased the crosslinking degree to 82.1%, resulting in a 22% improvement in flexural strength and a 207% increase in impact strength, which is critical for maintaining structural integrity during and after welding. From a blending and filler reinforcement perspective, Wang et al.¹³⁶ fabricated a POE-g-MAH/CaCO₃/HDPE composite for recycled HDPE–PPR pipe applications. The inclusion of functionalized calcium carbonate and compatibilizers enhanced interfacial adhesion, tensile strength, and impact resistance, achieving mechanical performance comparable to virgin HDPE. Mao et al.¹³⁷ synthesized a carboxyl graphene-grafted HDPE nanocomposite (CG-g-MHDPE), and found that 8 wt% loading optimized dispersion, crystallinity, and thermal stability, while excessive content led to performance decline due to aggregation. Beyond mechanical enhancement, several studies have focused on long-term thermal aging and sustainability. Ahmad and Rodrigue¹³⁵ reviewed crosslinking techniques for PE and emphasized the trade-off between mechanical enhancement and recyclability. Al-Malaika et al.¹³⁸ introduced graftable antioxidants (g-AOs) that improved thermal-oxidative stability without interfering with peroxide-initiated crosslinking, enhancing the long-term reliability of welded joints. Additionally, Guo et al.¹⁴³ developed OMMT-modified UHMWPE pipes via screw extrusion using HDPE-based compatibilizers. The resulting nanocomposites exhibited significant improvements in Vicat softening point, deformation resistance, and mechanical strength—beneficial for ensuring weld zone integrity under thermal and mechanical loads.

Collectively, these representative studies demonstrate the effectiveness of material-level modifications in improving the weldability and performance of HDPE pipe joints. However, despite these advances, several critical challenges remain unaddressed, particularly concerning the predictive modeling of interfacial behavior and the integration of intelligent process control.

Current barriers and future research directions of butt fusion welding in HDPE pipes

Current barriers of butt fusion welding in HDPE pipes

Modern pipeline networks rely heavily on HDPE pipes in both daily life and industrial applications owing to their excellent mechanical strength, flexibility, corrosion resistance, and cost-efficiency. Over the past decades, substantial progress has been made in enhancing the performance and reliability of HDPE pipelines through various methods. However, there are still several critical technical challenges remaining unresolved.

One of the foremost challenges currently hindering the advancement of HDPE butt fusion welding lies in the absence of a unified and physically grounded mechanism that effectively correlates welding quality with critical process parameters such as heating temperature, fusion pressure, processing time, and cooling rate. Although numerous studies have investigated single-factor or limited multi-variable optimization strategies, these approaches often lack robustness and reproducibility across varying pipe dimensions, welding equipment, and field conditions, thereby limiting their generalizability and industrial relevance. Compounding this issue is the deficiency of comprehensive, high-quality databases that capture a wide range of welding conditions and corresponding joint performance outcomes. As a result, predictive modeling efforts remain constrained, and weld quality assessment continues to rely heavily on DT/NDE techniques. Equally critical is the limited understanding of the rheological and molecular-level phenomena governing the welding process. Specifically, the complex non-Newtonian behavior of HDPE—characterized by shear-thinning viscosity, viscoelasticity, and temperature-sensitive flow—along with the mechanisms of molecular diffusion and chain entanglement at the weld interface, remain inadequately captured in existing constitutive models. The lack of accurate, physics-informed viscoelastic formulations hinders the development of reliable FEM simulations capable of resolving the interfacial dynamics during the molten phase. Consequently, numerical results often diverge from experimental observations, particularly in predicting flow-induced deformation and interdiffusion behavior, which reduces the predictive power of simulation tools and necessitates time- and resource-intensive experimental iterations. Moreover, the translation of optimal welding parameters derived under controlled laboratory settings to real-world pipeline construction scenarios remains problematic. In large-scale field operations involving high-frequency, continuous, and multi-segment welding, the cooling phase frequently becomes a major bottleneck. This stage is particularly influential in determining the interfacial microstructure, as it governs the extent of polymer chain interdiffusion, entanglement, and crystallization. However, the cooling rate is highly sensitive to environmental factors such as ambient temperature, wind speed, and humidity, which introduces considerable variability in weld integrity and long-term performance. Lastly, a critical limitation in current industrial practices is the inadequate integration of intelligent sensing and control technologies into welding equipment. Most existing machines still depend on manual adjustments of process parameters, leading to operator-induced inconsistencies and reduced process repeatability. The absence of real-time sensing tools—such as infrared thermography for temperature profiling, displacement monitoring for interface tracking, and acoustic emission for stress evolution detection—prevents in situ characterization of fusion quality. Furthermore, the lack of data-driven analytics and closed-loop control mechanisms hampers the development of adaptive welding strategies, thereby limiting the ability to detect and mitigate welding defects such as cold welds, voids, or thermal degradation in real time.

To overcome these limitations, future efforts should focus on the development of multi-scale, experimentally validated constitutive models, the creation of comprehensive process-performance databases, and the integration of advanced sensing and AI-based control systems. Together, these advancements will be pivotal in transforming HDPE butt fusion welding into a more predictable, intelligent, and industrially scalable process.

Future research directions of butt fusion welding in HDPE pipes

Building on the critical challenges identified in this review, the following future research directions are proposed to advance the state of HDPE butt fusion welding:

(1) Develop integrated weld quality databases that systematically capture the relationships among process parameters, defect formation, and mechanical performance under various material types and environmental conditions. These databases will serve as essential foundations for predictive model training and intelligent process optimization.

(2) Design adaptive control algorithms that combine real-time sensor data, physics-informed predictive modelling, and reinforcement learning, thereby enabling closed-loop control systems suitable for field-scale welding operations with enhanced stability and repeatability.

(3) Introduce the concept of a quantifiable weldability index during polymer material development, allowing for the evaluation and ranking of HDPE grades based on their interfacial diffusion behavior, thermal compatibility, and viscoelastic properties.

(4) Standardize intelligent non-destructive evaluation (NDE) protocols by integrating conventional techniques with AI-driven pattern recognition and anomaly detection, facilitating fast, accurate, and scalable weld quality inspection across diverse applications.

(5) Establish real-time hybrid modelling frameworks that incorporate thermal conduction, shear-thinning rheology, interfacial flow dynamics, and crystallization kinetics, supported by AI surrogate models, to simulate and predict the evolution of weld structures under transient and heterogeneous field conditions.

(6) Explore sustainable welding strategies by evaluating the weldability and long-term performance of biodegradable or recycled PE materials, while incorporating life-cycle assessment approaches to address environmental and structural reliability concerns.

Collectively, these directions represent a multidisciplinary roadmap for transforming HDPE pipe welding into a predictive, intelligent, and sustainable process suitable for next-generation infrastructure.

In this context, the evolution toward “intelligent HDPE pipeline systems” will rely on the integration of advanced sensing, AI diagnostics, and autonomous control. Real-time techniques such as infrared thermography, acoustic emission monitoring, and strain field mapping can be combined with deep learning for defect detection and classification. Closed-loop machines equipped with adaptive parameter tuning will reduce operator variability and enhance reproducibility. Moreover, the incorporation of digital twins and data-driven maintenance frameworks will support proactive weld integrity management under dynamic service conditions. Notably, substantial progress has already been achieved in the field of weld defect inspection. For example, China has recently pioneered the development of the first terahertz-based online detection system for HDPE pipelines, enabling non-contact, real-time inspection of weld quality with sub-millimeter resolution, as detailed in the preceding section. Addressing the aforementioned technological barriers will collectively pave the way for a transformative shift toward predictive, intelligent, and resilient HDPE pipeline infrastructures—surpassing the limitations of conventional welding practices.

Conclusions

This review systematically synthesizes the state-of-the-art in the butt fusion welding of HDPE pipes, with a particular emphasis on enhancing weld quality, structural integrity, and long-term durability. The analysis highlights that the intrinsic material properties of HDPE—combined with critical welding process parameters such as heating temperature, processing time, fusion pressure, and cooling rate—collectively dictate the formation of robust welded joints, suppression of defects, and microstructural evolution at the fusion interface. Furthermore, the review outlines recent advances in DT/NDE techniques, optimization of welding parameters, and material modification strategies, all of which are pivotal to improving weld performance.

Despite the widespread adoption and demonstrated performance advantages of HDPE pipes, their butt fusion welding process remains hindered by several critical technical barriers, as outlined in the preceding section. These unresolved challenges often result in flawed welded joints, which compromise structural integrity and pose significant safety risks in long-term pipeline operation.

Looking forward, future efforts should prioritize the integration of multi-physics modelling, in situ monitoring, closed-loop control, and sustainable material strategies. Addressing these challenges is essential to advancing butt fusion welding of HDPE pipes into a predictive, intelligent, and scalable platform, capable of supporting the next generation of high-reliability modern pipeline infrastructure.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Donghu Zeng

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Case	Heating temp. [°C]	t1 [s]	t2 [s]	t3 [s]	t4 + t5 [s]	P1 [bar]	P2 [bar]	P3 [bar]
1	220	50	79	5	600	6	1	6
2	220	50	40	5	600	6	1	6
3	220	50	79	5	600	9	1	9
4	280	50	79	5	600	6	1	6

Case	Heating temp. [°C]	t1 [s]	t2 [s]	t3 [s]	t4 + t5 [s]	P1 [bar]	P2 [bar]	P3 [bar]
1	220	50	79	5	600	6	1	6
2	220	50	40	5	600	6	1	6
3	220	50	79	5	600	9	1	9
4	280	50	79	5	600	6	1	6

Analysis and optimization of welded joints of HDPE pipes during butt fusion welding: State-of-the-art and emerging directions

Abstract

Keywords

Introduction

Intrinsic material properties of HDPE pipes relevant to weldability

Molecular structure and crystallinity

Thermal, rheological, and viscoelastic characteristics

Overview of the butt fusion welding process

Analysis of the mechanisms and implications of weld defect formation

Destructive testing and non-destructive evaluation techniques

Optimization of welding process parameters of butt fusion welding in HDPE pipes

Material modification strategies for enhanced weldability of HDPE pipes

Current barriers and future research directions of butt fusion welding in HDPE pipes

Current barriers of butt fusion welding in HDPE pipes

Future research directions of butt fusion welding in HDPE pipes

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Case	Heating temp. [°C]	t1 [s]	t2 [s]	t3 [s]	t4 + t5 [s]	P1 [bar]	P2 [bar]	P3 [bar]
1	220	50	79	5	600	6	1	6
2	220	50	40	5	600	6	1	6
3	220	50	79	5	600	9	1	9
4	280	50	79	5	600	6	1	6