Science and Technology of Welding and Joining

In order to develop a new welding consumable for the ultrahigh strength HT980 steel, the microstructural characteristics, tensile properties, low temperature toughness and weld cracking susceptibility were evaluated for triplex stainless steel weld metals. Steels of 13Cr–0·5Mo–Ni with varying contents of Ni and C were investigated. The amount of retained austenite increased with increasing Ni and C contents. The tensile strengths of the weld metals decreased while their impact absorbed energies increased with increasing amount of retained austenite. Hot cracking susceptibility of the weld metals minimised in that metal containing 40–50% δ-ferrite at the completion of solidification. The triplex stainless steel weld metals, except for those containing lower amounts of retained austenite, were negligibly susceptible to cold cracking. It may be possible to design a preheat free welding consumable from a triplex stainless steel in the compositional range containing 20–40% of retained austenite.

Transient liquid phase bonding between single crystal superalloy DD98 and polycrystal superalloy M963 has been characterised in terms of microstructure, crystallographic orientation analysis and stress rupture test. Result shows that during isothermal solidification (IS), with increasing bonding time, the liquid interlayer thickness decreases. As a result of incomplete IS, many compounds formed in the centreline. After 1190°C/4 h bonding, the IS process has been complete. It is notable that an interface formed in the bonding zone due to crystallographic orientation difference between parent metals. Result of stress rupture test shows that the rupture life of the bonded joint is more than 140 h at 800°C/350 MPa, which is comparable to that of the M963 alloy. Failure of the joint occurs in the segment of M963 alloy.

One of the most relevant aspects of friction stir welding is the possibility to weld different materials. In the present paper, the authors present an improved continuum finite element model for the simulation of friction stir welding processes aimed to obtain T joints, made of a stringer in AA7175-T73511 and of a skin in AA2024-T4. The model, taking into account the thermomechanical behaviours of the two different materials, is utilised to study the occurring material flow and residual stress state. Numerical results are compared with experimental observations: the model is able to predict the material flow, obtaining important information on the joint failure mode.

The stirring of the molten material during CO₂ laser welding with a dynamic polarised laser beam (DP) was studied. The key element of the set-up is a high power interference laser beam modulator for CW power rating up to 4 kW. Metallographic cross-sections and EDX analyses of the realised welds in St37 using the contrast material nickel proved the enhancement of melt pool stirring due to DP. Moreover, roughness measurements confirmed a smoothing of the top bead surface as well as a correlation between melt pool stirring and the formation of the top bead ripple structure.

Intermetallic layer of dissimilar tungsten inert gas welding–brazing butt joint of aluminium alloy/ stainless steel has been studied. A visible unequal thickness intermetallic layer has formed in welded seam/steel interface, and the thickness of the whole layer is <10 μm. The interface with Al–12Si filler metal consists of τ ₅-Al₈Fe₂Si layer in welded seam side and θ-(Al,Si)₁₃Fe₄ layer in steel side with the hardness values of 1025 and 835 HV respectively, while the interface with Al–6Cu filler metal consists of θ-Al₁₃(Fe,Cu)₄ layer with the hardness of 645 HV. The average tensile strength of the joint with Al–12Si filler metal is 100–120 MPa, and the fracture occurs at θ-(Al,Si)₁₃Fe₄ layer, while the joint with Al–6%Cu filler metal presents high crack resistance with tensile strength of 155–175 MPa, which reaches more than 50% of aluminium base metal strength.

Welding of AZ31B magnesium alloy is carried out using alternating current pulsed metal inert gas (ac-PMIG) welding with 1·6 mm diameter of filler wire. Typical current waveform is used to make sure arc given an accurate energy input into filler wire. The arc characteristics, metal transfer forms, microstructure and mechanical property of ac-PMIG welding of AZ31B magnesium are investigated. The results show that a stable welding procedure and continuous joints can be obtained easily under a wide range of welding parameters. The most important factors for ac-PMIG welding are negative electrode (EN) ratio and pulse rework current, which give an accurate energy input into filler wire. The grain in fusion zone is much finer and more uniform, and grain size does not grow significantly in the heat affected zone compared with base metal. The average ultimate tensile strength of weld beads is 97·2% of base metal.

Time–frequency diagram is applied to estimating the stability of gas metal arc welding process. A methodology has been developed to obtain indexes based on image processing of the spectrogram in order to evaluate the relationship between the spectrogram and the stability of process. The acoustic emission (AE) signals have been processed and characterised to obtain the spectrogram of the acoustic signals generated by arc. Statistical analysis of image generated by spectrogram and temporal parameters of AE signals has been used as a tool to obtain a method for stability evaluation. As a main result of the research, it demonstrates the effectiveness of the application of image processing of the time–frequency diagram for evaluating the stability in the welding processes. The results demonstrate the validity of this method to characterise the stability using the image characterisation.

Microstructures, and hence mechanical properties, of high strength steel weld metals are affected by cooling rate. Weld metal microstructures for a nominal composition of Fe–0·05C–0·3Si–2Mn–3Ni–0·5Cr–0·6Mo (wt-%) were therefore characterised for a range of cooling rates using high resolution scanning electron microscopy, and transformation behaviour, assessed from cooling curves, is presented as a continuous cooling transformation diagram. As deposited last bead microstructure changes gradually from lower bainite and martensite interspersed with coalesced bainite, via a mixture of relatively fine upper and lower bainite, to coarse upper bainite as cooling rate decreases. The microstructure of reheated beads follows the as deposited structure closely and becomes coarse with slower cooling. Mechanical properties correlate with observed microstructure and transformation behaviour. Results suggest high strength and good toughness for cooling rates between 800 and 500°C of about 3–13 s. A fine microstructure will then form with varying proportions of martensite, lower bainite, coalesced bainite and fine upper bainite.

In this paper, an examination of dissimilar wrought and heat treated AZ31 spot welds (AZ31/AZ31_HT) was conducted to allow visualisation of material flow based on their differing etching characteristics. The results indicated that dissimilar AZ31/AZ31_HT welds could successfully mirror the material flow in friction stir spot welding. The upper sheet material moved downwards towards the rotational pin, whereas the lower sheet material displaced upwards and outwards during the penetrate stage, and then moved inwards towards the keyhole after reached the base of the shoulder.

Lap joints of dual phase steel sheets of 1·0 mm were prepared by adhesive bonding, spot welding and weld bonding processes using a one component epoxy base structural adhesive. Mechanical properties of the joints were evaluated by tensile shear and fatigue tests. The size of the weld nugget for both spot weld and weld bond was measured for different welding parameters (current, time) and compared. For identical welding parameters, weld bonded nuggets exhibit higher nugget diameter. Tensile shear strength of weld bonded joints is 40 and 58% higher than spot welded joints and 15 and 39% higher than adhesive bonded joints and for DP590 and DP780 steels respectively. Considering 10⁶ cycles, the endurance limit of weld bonded joint is much higher than spot welded joint but smaller than adhesive bonded joints. Overall the performance of weld bonded joints is superior to those of resistance spot welding.

High strength steels with good formability properties have been developed in recent years, especially for the automotive industry. Joining these metals is however increasingly difficult as the fusion joining processes destroy the carefully constructed microstructure of the steels, resulting in less favourable mechanical properties in and around the joint. A possible solution to this problem is the use of joining processes that require less heat input; laser brazing is one such process. In this work, the brazability of a dual phase steel sheet has been investigated by means of bead on plate brazes produced with two consumables, CuSi3 and CuAl8. Two brazability diagrams are reported and high speed video images are used to explain the differences in operating conditions for these two consumables. Temperature measurements in the steel provide an indication about the temperatures reached during the joining process, which in turn explain the changes observed in the hardness of the steel.

Similar and dissimilar friction stir welds made of aluminium alloys 2017-T6 and 6005A-T6 are compared in terms of heat inputs, temperatures, material flow distributions and resulting local and overall tensile properties. Similar welds are systematically hotter and weaker than the dissimilar welds. Predictions of a three-dimensional finite element model of the tensile test transverse to the weldline are assessed towards local deformation fields measured by digital image correlation. Deformation systematically localises on the weakest heat affected zone, which is on the 6005A side in the dissimilar welds.

This paper presents previously unavailable constants for the Sellars and Tegart constitutive model for hot metalworking. The materials considered are aluminium alloys 2024, 5083, 6061, 7050, 7075 and 356, carbon steel 1018, stainless steel 304, titanium alloy 6Al–4V, and magnesium alloys AZ31 and AZ61. These materials and their mechanical properties at high temperature are of great interest for latest generation manufacturing processes involving deformation to accomplish solid state joining, such as friction stir welding, cold spray and magnetic impulse welding. The results are also useful to model established processes, such as hot rolling, forging and creep. The methodology used to obtain the constants consists on non-linear regressions based on partial data sets as it was conducted previously. The input data were obtained from published values for hot compression experiments. All regressions presented here have a coefficient of determination R²>0·95. When possible, the results obtained were compared to previous published regressions.