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Abstract

A systematic work was studied to illustrate the influence of laser power on the forming quality of Al6061 alloy by selective laser melting (SLM). The relationship between laser power and molten pool was simulated by finite element analysis (FEA). Phase composition, defects, and microhardness were also measured and analyzed. The results show that, with the increase of laser power, the molten pool gradually changes from rectangular shape to droplet shape. And the cooling rate gradually increases from 3.282 × 10⁴°C/s to 5.189 × 10⁴°C/s. Higher laser power (400 W) is accompanied by higher molten pool maximum temperature (2012.73°C). This may lead to larger temperature gradient inside the sample causing evaporation and spatter of powder. On the contrary, lower laser power leads to unmelt of some powders, which increases the number of pore defects and influences the forming quality of samples. X-ray diffractogram (XRD) displays the Al6061 alloy characterized by the obvious preferred orientation under different laser powers and the grain size increased from 32.57 nm to 35.38 nm. With the increase of laser power, the number of defects, especially holes and microcracks, was first decreased and then increased. However, the microhardness of the sample decreased almost linearly from 98.6 HV_0.05 to 88.86 HV_0.05. All changes are the result of the comprehensive action of laser power and molten pool state. Besides, the action mechanism of laser power on the forming quality was also clarified in this work.

Keywords

Laser power forming quality Al6061 alloy selective laser melting FEA

Introduction

In recent years, additive manufacturing (AM) is considered one of the leading sectors of the upcoming Industrial Revolution “Industry 4.0” due to its various advantages over traditional manufacturing technologies.^1,2 Selective laser melting (SLM), one of AM techniques, overcomes the obstacles of slow cooling speed and long process chain in the production and application of traditional methods, and shows unique advantages for processing complex structures and thin-walled parts.³ During the whole SLM process, high-energy laser beam is applied to scan the metal powder layer according to predetermined path in an inert shielding gas atmosphere, and the powder undergoes rapid melting and cooling until the three-dimensional part is formed.^4,5 Obviously, in order to fully fuse between metal powder layers, the laser needs to provide enough heat energy to ensure the forming quality.^6,7 It has been reported in the literature that the forming quality goes together with laser power, scanning speed, scanning spacing, layer thickness, scanning strategy, and so on. Therefore, current study mainly focuses on the adjustment of the above parameters and multi-objective optimization.^8,9

To date, the SLM forming of metal powder such as titanium alloy, nickel alloy, stainless steel alloy, and otherwise has been studied quite extensive and in-depth research.^10–12 For aluminum alloys, AlSi10Mg is well studied, however, relatively few studies on Al6061 alloy due to the hot crack problem during SLM process, though it is widely used in aerospace and automotive fields.^13,14 In order to solve this problem, researchers have made different attempts. Opprecht et al.¹⁵ eliminated the hot crack phenomenon of Al6061 by adding yttrium-stabilized zirconia (YSZ) to induce grain refinement and found adding 2 vol% YSZ can completely avoid cracks caused by continuous equiaxed belt at the boundary of molten pool. Loh et al.¹⁶ using laser power and scan speed as variables simulated the SLM process of Al6061 alloy and discussed the progression of the melt pool and the rate of temperature change. The simulation results show that the melt penetration and width increased with the increase of laser power and reduced scan speed. Besides, it is also found that the total volume of powder molten and volume of molten powder evaporated enlarged by increasing the laser power. Jerrard et al.¹⁷ investigated the consolidation behavior and microstructure of Al6061 alloy and its mixture with copper powder under different SLM processing parameters. The results show that SLM parameter variation and the addition of copper powder significantly changed the consolidation behavior of Al6061 and formed a denser and much finer microstructure. Carluccio et al.¹⁸ studied the effectiveness of scandium and TiBor(R) grain refiners by controlling the microstructure of Al6061 during SLM process and found that the microstructure of molten pool was successfully controlled by adding scandium and Tibor grain refiner. Louvis et al.¹⁹ considered modifying the SLM process to reduce the required laser power and improve the laser scanning rate, while still producing relatively dense Al6061 components. The results show that the major confounding factor in processing aluminum and its alloys was found to be oxidation due to the presence of oxygen within the build chamber. In addition, the results indicate that if 100% dense aluminium components are to be produced by SLM, using low laser powers, methods need to be developed that can either disrupt these oxide films or avoid their formation. Liu et al.²⁰ surveyed the influence of the different substrates on the thermal behavior, microstructure, and microhardness during SLM of Al6061 thin-wall parts and clarified the influence of substrate material on SLM machining of Al6061, which has important theoretical significance and engineering application value. Maamoun et al.^21,22 conducted the influence of SLM process parameters on the forming quality of Al6061 and analyzed the relative density, porosity, surface roughness, and dimensional accuracy of formed parts. It turned out that to achieve the optimum values of each performance, and a series of energy density and SLM process parameters are required. Zhang et al.²³ fabricated Al6061 alloy by SLM and studied the densification, microstructure, and properties, and found that the processing parameters such as laser power and scanning speed have a great influence on the forming quality.

Based on the above research, the influence of SLM process parameters on the forming quality of Al6061 alloy has been reported in the literature, but most of them are separated by simulation and experiment, and the simulation analysis needs to be further studied. On this basis, the effect of laser power on the forming quality of Al6061 alloy formed by SLM was studied by combining theoretical simulation and experimental verification. Moreover, the trends of molten pool, phase composition, defects, and microhardness under different laser powers were also analyzed so as to further improve the forming quality of Al6061 via SLM processing.

Materials and experimental process

Material

The Al6061 powder with a particle size distribution of 15–53 μm was provided by Avimetal Powder Metallurgy Technology Co., Ltd China. The chemical composition and morphology of powder were listed and observed, as shown in Table 1 and Supplementary Figure 1, respectively.

Table 1.

Chemical composition of Al6061 powder.

Element	Fe	Mn	O	Zn	Cr	Mg	Cu	Si	Al
Mass fraction ( $%$ )	0.68	0.028	0.075	0.035	0.16	1.13	0.24	0.55	Bal.

SLM equipment and processing parameters

SLM equipment (XDM120) was produced by Suzhou XDM 3D Printing Technology Co., Ltd China. It was equipped with a 200 W/500 W IPG fiber laser, and the laser wavelength is 1070 nm. A sample with a size of 10 mm × 10 mm × 7 mm was designed for this study. In order to study the influence of laser power, it was set to 200, 250, 300, 350, and 400 W, respectively, and other SLM processing parameters were listed in Table 2.

Table 2.

Selective laser melting (SLM) processing parameters.

Parameters	Scanning strategy	Protective gas	Scanning speed	Layer thickness	Hatch spacing
Value	One direction	Argon	1000 mm/s	0.03 mm	0.12 mm

Finite element simulation

To explore the influence of laser power on the molten pool, the whole forming process was simulated by ANSYS. The structure model was composed of lower substrate with a size of 0.7 mm × 0.5 mm × 0.1 mm and upper powder bed with 0.4 mm × 0.4 mm × 0.06 mm size, as shown in Supplementary Figure 2. To ensure the accuracy of calculation, a hexahedron was selected as the mesh shape. The mesh size was divided into smaller parts by the mapping mesh partition method, and the cell size was set to 0.01 mm × 0.01 mm × 0.03 mm. The lower substrate was a non-machining area which is divided by 0.05 mm free mesh with a tetrahedral shape.

During this simulation, the initial condition is defined as follows^24,25:

T (x, y, x, 0) = T_{0} (x, t, x, 0)

(1)where T₀ is the initial temperature, 298 K.

Besides, the boundary condition is defined as

- k {(\frac{\partial T}{\partial Z})}_{z = 0} = Q - h (T_{a} - T_{s}) - τ δ (T_{α}^{4} - T_{s}^{4})

(2)where, k is the thermal conductivity, Q is the rate of heat flow, h is the heat convection coefficient, T_a is the initial temperature of the powder bed, T_s is the surface temperature of Al6061 alloy, τ is the Stefan–Boltzman constant, δ is the heat radiation coefficient.

Gaussian heat source is applied as the thermal source and is defined as

q (r) = \frac{2 A P}{π R^{2}} \exp (- \frac{2 r^{2}}{R^{2}})

(3)where A is the laser absorption rate of the material, P is the laser power and the values are 200, 250, 300, 350, and 400 W, respectively, r is the distance from a point in the spot to the center of the heat source, and R is the radius of the laser spot, 50 μm.

Measurement methods

X-ray diffractogram (XRD), Bruker D8 Advance XRD machine, Bruker, Germany was used at a 2θ range with a scan rate of 2.0°/min to characterize the phase composition. The Al6061 sample was grounded and polished by abrasive paper (grade varied from #200 to #2000) and YMP-2 grinding machine until the metal mirror surface appeared. Optical microscope (OM) DM-2700M, Leica, Germany was used to observe the pore distribution along the forming direction. Microhardness was tested by HVS-1000ZCM-XY digital microhardness tester, Shanghai Suoyan Testing Instrument Co., Ltd China. The load was 50 g and the retention time was 15 seconds, randomly measured 5 positions and calculated the average value as the data in this study.

Results and discussion

Finite element simulation

The temperature contours of Al6061 alloy formed by SLM under different laser powers are shown in Figure 1. As shown, at this point, the laser heat source center is located at the midpoint of the second scanning path, and the gray area in the figure represents the molten pool. From Figure 1(a) to (e), it can be seen that with the increase of laser power, the molten pool shape changes as follows. Based on the rectangle, the back end is gradually stretched downward, the front end is gradually transformed into an arc, and the whole is transformed from a rectangle to a droplet shape. The temperature of the molten pool decreases from the center of the laser heat source to the edge of the molten pool, indicating that the temperature distribution of the molten pool gradually approaches the Gaussian distribution with the increase of laser power.

Figure 1.

Temperature contours of Al6061 alloy under different laser powers: (a) 200 W, (b) 250 W, (c) 300 W, (d) 350 W, and (e) 400 W.

In addition, the isotherms at the front and the left end of the laser heat source are denser and the temperature gradient is larger. This is because the Al6061 alloy powder at the right and the back end of the laser point, after rapid melting, cooling, and solidification, has become a solid state. However, the front end and left end are still powders to be melted. Because the thermal conductivity of solid materials is greater than that of powder materials, so the heat transfer at the right end and back end of the laser heat source point is faster, and the temperature gradient is much smaller than that at the front end and left end, and the isotherms are more sparse. This is also the main reason for local concave in the upper right corner and convex in the lower-left corner of the molten pool area in temperature field cloud picture.

As can be seen from Supplementary Figure 3, under the five groups of laser power from 200 W to 400 W, the temperature gradient after cooling 442 μs is 249.87°C, 319.99°C, 340.53°C, 383.47°C, and 416.37°C, respectively. As shown in Figure 2, the calculated cooling rates are 3.282 × 10⁴°C/s, 3.682 × 10⁴°C/s, 4.530 × 10⁴°C/s, 4.955×10⁴°C/s, and 5.189 × 10⁴°C/s, respectively. It can be seen from the simulation results that under the premise of the same cooling time, the higher laser power will lead to a faster cooling rate of the molten pool. The actual situation should be the same, SLM process parameters will jointly determine the formation of the powder bed molten pool in the processing process, and the cooling rate of the molten pool is closely related to the evolution process of the molten pool. When only the laser power increases, the depth of the molten pool increases, and the fluidity of the liquid phase in the molten pool is strengthened, leading to the increase of the contact area between the molten pool and the surrounding solid. All these obviously increase the heat dissipation efficiency and cooling rate of the molten pool.

Figure 2.

Effect of different laser powers on the cooling rate.

To further explore the influence of laser power on the temperature change at the monitoring point over time, the temperature curve at midpoint of the first scanning path of five groups of laser power with time was analyzed, as shown in Figure 3. It is clear from the simulated curve that the increase of laser power leads to the overall temperature rise at this point. When the laser power is 200 W, the transient peak temperature of the molten pool is 979.18°C when the laser heat source directly acts on this point. When the laser power is 250 W, the temperature increases to 1599.94°C. When the laser power increases to 300 W, 350 W, and 400 W, the transient peak temperature of the molten pool increase to 1766.53°C, 1905.37°C, and 2012.73°C, respectively. The reason for this phenomenon may be that with the increase of laser power, other conditions remain the same, and the energy that laser transfers to the powder material is also more. It can be found that the same laser power variation range leads to different degrees of molten pool temperature variation, which may be related to the laser energy loss caused by the reflectivity of materials and thermal properties of alloy materials such as thermal conductivity.

Figure 3.

The variation trend of temperature with time under different laser powers.

XRD analysis

To analyze the effect of laser power on the phase composition, the XRD pattern of Al6061 alloy powder and samples were tested, as shown in Figure 4. It can be seen from that Al alloy have obvious diffraction peaks of {111}, {200}, {220}, {311}, and {222} crystal planes, which are all Al phases after calibration. Weak diffraction peaks of {111} and {220} crystal planes were also observed, which was identified as the Si phase. The diffraction peaks were weak due to the precipitation of nano-sized Si particles in the microstructure. In addition, the diffraction peak of the {111} crystal plane was also observed, which was labeled as the Mg₂Si phase. This peak indicated the formation of new substances, but due to the relatively small amount of precipitation of the metal compound, the intensity of the diffraction peak was weak. It also can be seen that the relative strength of the {111} and {200} crystal planes is opposite to the standard powder diffraction pattern of Al, indicating that the Al6061 alloy prepared under different laser powers has a preferred orientation along the {200} crystal plane, which may be related to the addition of trace elements in Al6061 alloy.

Figure 4.

X-ray diffractogram (XRD) patterns of the aluminum alloy at different laser powers.

Based on above XRD patterns, the average half-peak width full width at half maxima (FWHM) of the Al6061 alloy at {200} crystal plane under different laser powers was analyzed by Jade and Origin software, and the grain size was calculated by the Scherrer formula^21,22:

D = \frac{k λ}{β \cos θ}

(4)where D is the grain size, nm, K is Scherrer's constant, 0.9.

λ

is the wavelength of X-ray, 0.15406 nm,

β

is FWHM, and

θ

is the peak position, radian.

FWHM and grain size of the Al6061 alloy at different laser powers are shown in Table 3. It can be seen that FWHM decreases by increasing the laser power, indicating that low laser power leads to wider FWHM and confirming that the grain size increases by increasing the laser power. In addition, some studies show that the sharpness of XRD peaks is positively correlated with the grain size of samples.^24–26 As shown, with the increase of laser power, the peaks in the XRD pattern become increasingly sharp, which also confirms the change process of grain size from fine to coarse. It can be seen that the change in laser power affects the grain size. When high laser power is applied to aluminum alloy powder, more energy is absorbed and the temperature of the molten pool is higher. Even if the cooling rate of the molten pool is slightly increased, the molten pool with the higher temperature still needs more solidification time, resulting in longer crystal core growth time. Therefore, the grain size is larger at high laser power.

Table 3.

FWHM and grain size at the crystal plane of Al6061 alloy (200) at different laser powers.

Power/W	200	250	300	350	400
FWHM/°	0.2439	0.2389	0.2333	0.2295	0.2245
Grain size/nm	32.57	33.25	34.05	34.62	35.38

Pore distribution and surface morphology

The optical topography with 50x magnification of polished Al6061 alloy manufactured under different laser powers was shown in Figure 5. It can be seen that the surface defects are mainly microcracks and holes. Due to the different initiation reasons of holes and cracks, the growth trends of the two types of defects with the change of laser power are analyzed in two parts.

Figure 5.

Metallographic microscope observation results of Al6061 sample manufactured under different laser powers: (a) 200 W, (b) 250 W, (c) 300 W, (d) 350 W, and (e) 400 W.

As shown in Figure 5(a), there are many unevenly distributed large holes in part of the upper surface at the laser power of 200 W, and the shape of them is irregular. Due to a shortage of the energy of the laser radiation is passed to the powder particles, some powder receives the heat is not enough to make it completely melt. Irregular shaped small blocks of molten powder is formed within the powder bed. The space occupied by the blocks of molten powder will directly lead to the initiation of the hole, resulting in incomplete melting of the final solidified layer and poor metallurgical bonding of the printed parts. Unmelted alloy powder can also be seen in larger holes as shown in Figure 5(a). As the laser power increases from 200W to 300W, the number of holes gradually decreases with the increase of laser power. As shown in Figure 5(d) and (e), when the laser power reaches 300 W, the number of holes decreases significantly and the radius of holes decreases. Obviously, this is because the powder can be completely melted with the increase of laser power, and the holes caused by unmelted defects are reduced or even disappear. However, more laser power is not always better. When increasing to 350 W, irregular holes reappear, even worse at 400 W laser power. This is because too much laser energy input leads to powder splashing, splashing on the powder will form large metal particles, and then produce unmelted and porosity defects, resulting in the decline of molding quality.

Different amount of microcracks can also be observed on the upper surface of the sample. In general, the rapid melting and solidification during SLM manufacturing result in large temperature gradients and high cooling rates, which makes the molding process inevitably accompanied by high residual stress levels. High residual stress zones around the molten pool can lead to cracking, delamination, thermal deformation, and other phenomena.^27,28 It is worth mentioning that most of these microcracks on the upper surface of the sample appear as semi-closed rings. The shape is similar to equiaxed grain. As shown from Figure 5(a) to (c), when the laser power is 200 W and 250 W, there are obvious microcracks on the upper surface, but there are almost no cracks on the surface when the laser power is 300 W. As shown in Figure 5(d) and (e), when the laser power continues to rise to 350 W and 400 W, the number of surface cracks begins to increase again. This trend is caused by the fact that low laser power will lead to incomplete melting while high laser power will cause splashing. Both of these conditions will directly lead to an increase in the number of holes. And the existence of holes will cause stress concentration, which will lead to an obvious increase in the number of cracks at low or high laser power. In addition, the stress concentration phenomenon in the hole is also the reason why most cracks extend from the hole in the figure. In short, with the increase of laser power, the number of holes and cracks in the sample decreases first and then increases when other laser process parameters are unchanged.

Microhardness

Figure 6 showed the microhardness of Al6061 alloy under different laser powers. It is clear, with the increase of laser power in the range of 200 W to 300 W, the average microhardness of samples decreased. Generally, metal powders are quickly melted into liquid after being scanned by a high-energy laser, which facilitates the distribution of alloying elements. However, if the laser power increases, the liquid phase residence time of molten pool becomes longer. On the one hand, the crystal nucleus will grow in the liquid phase of molten pool after formation, which leads to a coarser grain. On the other hand, alloying elements will continue to segregate, resulting in Si particles in the metal matrix that cannot be uniformly distributed in the matrix, resulting in a coarse structure with uneven grains,²⁹ which eventually leads to a decrease in the average microhardness of the samples. This explains the decreasing trend of microhardness by increasing the laser power in the figure. However, the increase of laser power will lead to a faster cooling rate, and rapid cooling can slow down the grain growth trend and hinder the segregation of alloying elements to a certain extent.³⁰ That is to say, when the laser power increases to a critical point, the decreasing trend of microhardness will be alleviated considering the influence of cooling rate. This explains that when the laser power reaches 300 W and continues to increase in the figure, the slope of the curve gradually decreases and the curve gradually becomes flat. In conclusion, with the increase of laser power, the microhardness of the samples gradually decreases, and the decreasing trend gradually becomes slow.

Figure 6.

Effect of different laser powers on the microhardness of Al6061 alloy.

Action mechanism

The action mechanism of laser power on the forming quality of Al6061 alloy during SLM process is shown in Figure 7. The lower laser power leads to the failure of the powder particles to fuse in the SLM manufacturing process, the continuity between the molten pools will be affected and reduced, and the metal molten liquid flow will be blocked greatly, and the feeding ability will be weak. All of these are not conducive to liquid flow expansion in the molten pool, resulting in a serious increase in the porosity and a decrease in the density. The suitable laser power makes the lower layer remelt, promotes the metallurgical bonding between layers, reduces the internal cavity phenomenon, and is beneficial to obtain samples with lower porosity and higher density. However, when it is too high, the laser radiation to the bottom of the molten pool heat is more, and in the printing process will appear the phenomenon of over melting. After heat accumulation, the metal powder absorbs enough heat to vaporize instantly, and the liquid metal splashes in the molten pool under multiple factors of airflow protection and laser impact. Moreover, the metal vapor produced strongly impacts the bottom of the molten pool, resulting in the formation of some porosity defects in the molten pool, resulting in a decrease in density. This explains the phenomenon that the number of holes will increase when the laser power is too high, which is consistent with the existing research results.

Figure 7.

Action mechanism of laser power on the forming quality of selective laser melting (SLM) processing Al6061 alloy.

In addition, the lower laser power causes severe spheroidization, resulting in a rough surface of the solidified layer during SLM, accompanied by a large number of pores, reducing the density and quality of parts. Although in the forming process, the higher laser power ensures the formation of sufficient liquid metal melt in the lower molten pool, avoids the splash of droplets, improves the wettability of melt and solidified layer, and alleviates the spheroidization phenomenon. However, high laser energy leads to excessive spheroidization of liquid metal, resulting in large residual stress and parts deformation. Therefore, the selection of appropriate laser power is the key to obtain higher-forming quality samples.

Conclusion

This research aimed to investigate the effect of laser power on the forming quality of Al6061 alloy manufactured by SLM, and the following conclusions can be drawn:

With the increase of laser power from 200 W to 400 W, the shape of molten pool gradually changes from rectangular shape to droplet shape. And the cooling rate gradually increases from 3.282 × 10⁴°C/s to 5.189 × 10⁴°C/s. The instantaneous peak temperature of molten pool is 979.18°C at laser power of 200 W, then increases to 1599.94°C, 1766.53°C, 1905.37°C, and 2012.733°C, respectively, as the laser power increases to 250 W, 300 W, 350 W, and 400 W.

XRD analysis reveals a significant preferred orientation along the (200) crystal plane. Besides, the grain size increased from 32.57 nm to 35.38 nm as the laser power increases from 200 W to 400 W.

With the increase of laser power, the overall number of defects, including holes and microcracks, first decreases and then increases with the increase of laser power. It is affected by maximum temperature and temperature gradient distribution of molten pool.

Within the appropriate laser power range (200 W–300 W), the microhardness of the sample decreased almost linearly from 98.6 HV_0.05 to 88.86 HV_0.05. With the laser power increasing further, the decreasing trend of microhardness gradually decreased, and finally decreased to 83.7 HV_0.05 when the laser power was 400 W.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221149002 - Supplemental material for Effect of laser power on the forming quality of Al6061 alloy manufactured via selective laser melting

Supplemental material, sj-docx-1-pie-10.1177_09544089221149002 for Effect of laser power on the forming quality of Al6061 alloy manufactured via selective laser melting by Chenglong Li, Meiping Wu, Weipeng Duan and Yiqing Ma, Huijun Liu, Xiaojin Miao in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

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 disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities and the Project of Team of National Defense Science and Technology Innovation of China (grant numbers JUSRP121041 and 2016300TS009).

ORCID iDs

Chenglong Li

Meiping Wu

Weipeng Duan

Yiqing Ma

Xiaojin Miao

Supplemental material

Supplemental material for this article is available online.

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