Experimental Investigation on the Laser Welding Characteristics of 6061-T6 Aluminum Alloy Sheets
2018 Volume 59 Issue 9 Pages 1446-1451
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2018 Volume 59 Issue 9 Pages 1446-1451
Generally, welding is an indispensable process in the assembly of automobile body parts, while resistance spot welding is applied in many cases. However, an efficient welding technique is necessary because an aluminum alloy has lower resistance and higher thermal conductivity than steel. Based on this point of view, laser welding has been studied as a new method to obtain stable welding quality and ensure deep penetration with low heat input, regardless of the type of material used. In this study, the lap joint weldability of the Al 6061-T6 thin plate, which is a heat-treated aluminum alloy, was investigated by varying the focus position, including the laser power and welding speed. The main result showed that the sound bead was formed when the power was 2 kW, the welding speed was 2 mpm (meter per minute), the focus position was at −0.8 mm, and the N2 shielding gas flow rate was at 10 l/min. Also, the strength of the welds under optimum welding condition was 52% compared to the base material. However, the welded part was subjected to ageing heat treatment (170°C, 12 hours), and demonstrated a strength of 295 MPa, which is about 90% of the base metal.
It is increasingly becoming an imperative, rather than an option, to develop the technology in reducing the weight of a vehicle by applying aluminum, which is excellent in terms of strength and economy. Also, it is easy to modify, as the application of automobile parts made of various lightweight materials, such as aluminum and magnesium, in securing competitive fuel economy is attracting growing public attention in the middle of strengthening environmental regulations.1) In fact, many automobile manufacturers are reducing vehicle weight by applying aluminum alloy to hoods, trunks, doors, roofs, and the chassis of a number of premium cars.
Aluminum alloy that is used in the automobile chassis is applied in the form of casting material, rolled plate material, extruded material, or other materials, depending on the processing method, whereas welding is widely applied in rolled plate and extruded material rather than in casting material, which has poor weldability.2–5) Since its specific gravity is one third of iron, aluminum is very light, and is excellent in corrosion resistance, thermoelectric conductivity, and recyclability. Unlike steel, aluminum has high thermal conductivity; however, it requires high current, and it is very difficult to use in electric resistance spot welding. Therefore, the TIG (Tungsten Inert Gas) arc welding is applied to aluminum instead, though it is also urgently necessary to develop practical welding techniques that can be applied in mass production.
On the other hand, it is possible to improve production efficiency by reducing the overall number of processes to improve productivity, and secure stable welding quality by replacing or minimizing the adhesive bonding, which is used as an alternative in addressing the water tightness problem of the electric resistance spot welding part. Laser welding is a so-called keyhole welding method that uses a point heat source with high energy density, forcing the welding portion to form a mechanism that is markedly different from conventional fusion welding methods, such as arc welding. Therefore, it is possible to obtain full penetration at high speed with relatively low input heat energy since it transfers the energy required for welding directly along the depth, rather than incrementally from the surface of the material.6,7) The aluminum alloy has a low absorption rate in relation to the laser, since the reflectivity of the laser beams on the surface of the aluminum alloy is over 80%. Since the shorter the laser wavelength, the better the absorption rate, welding by using a YAG laser system with a large wavelength has been widely applied in recent years.
In this study, laser lap-joint welding was applied to a thin plate of 6061 (hereinafter referred to as Al 6061-T6), which was T6 heat-treated with a lightweight aluminum alloy. Then welding experiments were carried out according to the process conditions of the main welding variables, such as laser power, welding speed, and focus position, by using 5 kW high-powered laser welding equipment to optimize laser welding conditions, which were then followed by an observation of the cross-section of the laser welded part and an evaluation of the mechanical property. Furthermore, the type of shielding gas and the flow rate were examined to evaluate the quality of the welded part after investigating the formation of weld beads, such as under-fill and the presence of porosities.
The target material is Al 6061-T6, which is a typical heat-treated alloy of the Al–Mg–Si system, as shown in Table 1. The test specimens for laser welding were cut in advance according to the following specifications, as shown in Fig. 1: lap width: 30 mm, and welding length: 200 mm, so the width should be 150 mm, the length at 200 mm, and thickness at 1.0 mm.
Schematic diagram of the laser lap welding specimen.
Figure 2 also shows a 5 kW fiber laser welding system that was used in this experiment, which allows welding by a teaching robot arm even in the case of a three-dimensional form. The specimens were fixed with a specially designed clamp before it was welded. Meanwhile, aluminum should be treated with care so as not to directly damage the focal lens by slightly tilting the beam, rather than firing the beam vertically, because the reflectance of aluminum is much higher than that of steel during laser welding. Experiments were carried out by changing the focus position and the shielding gas flow rate, in addition to the power and welding speed, which are the main factors in influencing heat input. For the pulse welding mode, the pulse time was adjusted by selecting a pattern with generally good bead appearance through a preliminary bead-on-plate experiment. The peak power was 3 kW and the pulse time was set at 10 ms. Also, the number of cycles was set at 60 pps (pulse per second), and the number of laser shots was set at 500.
Appearance of the Fiber laser welding system for the experiment.
In an attempt to inspect the quality of the welded part, appearance survey and post-polishing etching were performed to observe the cross-section of the welded part, while the cross-sectional area was investigated by using an image analysis program (iSolution DT). Then, after obtaining and cutting the specimens through discharge processing, as shown in Fig. 3, a tensile test was performed on the test specimens that have good as-welded conditions and went through ageing heat treatment to evaluate the mechanical properties of the welded joints. Using a tensile testing machine (SHIMADZU, AG-IS, 5 tons) at room temperature, the tensile properties of the joints were evaluated at a cross-head speed of 1 mm/min. The tensile test specimens were measured vertically to the weld line. A metallurgical inspection was also performed on the cross-sections of the welds. Furthermore, EBSD (Electron Backscattered Diffraction) was used to observe the microstructure of the welds. A Vickers hardness tester (Mithutoyo, AAV-502) was used to measure the hardness of the welds with a 300 g load applied for 10 sec.
Dimensions of the tensile test specimen of the laser welded joint.
Next, another test specimen of the same size as that of the tensile test specimen, but with a shear at both ends, was used to calculate the fatigue limit according to the test load (40 to 100 MPa) by using a fatigue tester (MTS810) under the conditions of a stress ratio of 0.1 and a frequency of 15 Hz.
To determine the optimum welding condition of the Al 6061-T6 alloy, a preliminary test of the bead-on-plate was carried out. The proper welding speed was fixed at 2 mpm, as shown in Table 2, with the beam tilting at 15° to prevent damage of the focal lens, the laser power varying between 2.0∼2.5 kW, the focus position varying between −0.5∼−0.8 mm, and the N2 gas flow rate varying between 5∼10 l/min, respectively. Also, the appearance of the laser lap welding after the previously described test was shown in Fig. 4.
Laser welding experiment results for the comparison of the joint qualities.
An observation of the laser weld bead showed that the weld bead width increased when the laser power was high under the same conditions, while the bead width tended to expand relatively, even when the focus depth increased in the thickness direction.
On the other hand, when the periodic pulse control mode was applied to reduce porosity and under-fill through regular vibration and stirring of molten metal during the welding, the effect was less significant than that of the continuous welding mode, and there was no significant change in the shielding gas flow rate. It increased the power and relatively generated noticeable under-fill in the pulse laser welding mode which has a large heat input, but the porosities decreased compared to the full penetration condition (No. 1, No. 3) of the continuous welding mode.
Figure 5 shows the observed results of the cross-section under each welding condition when examining the shape of the welded part. Continuous welding mode (No. 1∼No. 3) conditions were all good with full penetration. However, the No. 4 condition of the pulse control mode showed incomplete penetration, with only a penetration rate of about 5%, and an inner defect was observed locally. It has been generally known that the porosity of the welded part of the aluminum material is mainly due to the following causes: the difference in the solubility of hydrogen in the solid phase, and in the liquid phase during the solidification of the bubbles and molten aluminum as a result of the instability of the keyhole; the presence of the alloy element with a low vaporization point; and the transition from complete penetration to incomplete penetration.8,9)
Cross-section view of all welding conditions for the selection of the sound condition.
In condition No. 5, it was fully penetrated with the increase of the heat input as power increased to 2.5 kW in the pulse control mode, but the under-fill phenomenon, in which the weld metal was filled less than the height of the base material, occurred significantly. This is presumably due to the bead being formed that was slightly lowered below the surface of the base material, as the molten metal is mainly affected by gravity during the laser welding of soft aluminum.
3.2 Investigation of inner defect in laser weldsFigure 6 is a schematic view that shows the cross-sectional observation method in investigating an inner defect in a laser welded part. Next, Fig. 7 shows the porosity distribution observed in the cross-section at each condition. In the figure, the area represented the black circle is the part where porosities occurred. It can be seen that a relatively large amount of porosities occurred under the incomplete penetration condition, in comparison to those under the full penetration condition. In general, porosity is a common defect type in laser welding and the main cause is known to be the hydrogen included in the weld zone. The amount of hydrogen adsorbed on the surface depends not only on the welding heat input, but also on other process conditions and environmental factors such as the focus position, shielding gas flow rate, the gap of the lap jointed specimens, and surface contamination condition. Porosities can also occur under the full penetration condition. However, in aluminum-based laser welding which requires deep penetration characteristic, partial penetration condition tends to generates porosities very noticeably compared to full penetration condition, and the porosities tend to be unevenly distributed in the length direction.
Schematic drawings of the investigation method for the inner defects.
Cross-section analysis that is parallel to the welding line; the black circle represents porosity in the visual image.
Then, Fig. 8 shows a graph that quantitatively measures the amount of porosity generation on the cross-section, as observed under each condition (see Fig. 7). Compared with other welding conditions, Conditions No. 2 and No. 5 show that the porosity generation rates are remarkably low at about 0.8%. Thus, considering it in this experiment, the cross-section was observed in parallel to the weld line as shown in Fig. 7 for each condition. The result confirmed that No. 4 under the partial penetration condition generated more porosities in the weld area at the same position compared to No. 1 and No. 3 under the full penetration condition.
Measurement of the porosity fractions at each welding condition.
Partial penetration, porosities, and the under-fill mentioned above are considered the main factors that affect the mechanical properties of the welded part.10) Particularly, it is necessary to pay close attention to them since welded porosity is a defect that can easily occur in the laser welding of aluminum. Finally, the power of 2 kW, the focus position at −0.8 mm, and the shielding gas (N2) flow rate of 10 l/min were obtained under the conditions of continuous welding mode and a welding speed of 2 mpm.
3.3 Evaluation of laser welding characteristicsThe tensile and the fatigue test were performed to evaluate the mechanical properties of the laser welded parts. First, Fig. 9 shows the results of the tensile test of the laser welds, while Fig. 10 shows the tensile test results before and after heat treatment. In condition No. 2, the test specimen was broken at the heat affected zone, while the greatest strength was obtained at 172 MPa. Still, it was about 51% lower than the 340 MPa of the base metal. The weakness of the welded part can be explained by the strengthened phases that exist in the structure of the T6 heat-treated alloy that were dissolved due to melting during the welding. Strength can be restored by precipitating the strengthened phase again through the post-heat treatment.11) Therefore, the solid-solution was heat-treated (170°C, 12 hours) after the solution treatment to increase strength. As a result, the weld strength of 295 MPa was observed, an 87% increase over the base metal.
Tensile failure position and the strength of the welded joints.
Comparison of the tensile test result before and after heat treatment of the welded joints.
For reference, the tensile strength of the laser welds is expressed in terms of the stress unit (MPa) divided by the cross-sectional area of the tensile test specimen (1 mm thickness × 25 mm width) when comparing the shear strength (N) of the lap joints with the base metal. The ageing heat treatment conditions in this experiment are generally identified as peak ageing heat treatment conditions in which the maximum strength can be obtained in the Al 6061 alloy. On the other hand, when the ageing heat treatment is performed under different conditions, it may be difficult to secure the desired tensile strength. For example, under the heat treatment conditions of 170°C and 20 minutes, sufficient age hardening effect cannot be obtained due to short retention time, thereby earning only limited strengthening effect. For the aluminum alloy 6061, the solution treatment requires 90 minutes at approximately 530°C considering the material thickness, and the ageing heat treatment requires long heat treatment for about 7 hours at approximately 170°C. We reasoned that shortening the heat treatment and time could contribute to the improvement of productivity. This study was conducted to investigate the strength increase effect on the aluminum laser weld zone of the condition of 20 minutes at 170°C, which is similar to the bake hardening condition of the painting process in automobile body assembly. After comparing the hardness test results before and after the heat treatment, it can be seen that the hardness of the heat-treated condition (170°C, 5 hours) is about 20HV higher than that of the as-welded condition, as shown in Fig. 11. It is also thought that the slight increase in hardness due to the effect of the ageing heat treatment was similar to the tensile test results. Also, Fig. 12 shows the microstructure of the base metal and the weld metal, as observed by EBSD. It can be seen that a coarse solidification structure is formed in the weld metal with a lower hardness value than the base metal. Especially, an aluminum 6000 series has a hardening mechanism that allows heat treatment by precipitation hardening. Therefore, to find out the reason that the weld hardness is lower than that of the base metal, we also investigated the precipitation condition through a microstructure analysis of the weld zone as well as the difference in the grain size. Figure 13 shows comparison of photographs by a transmission electron microscope according to the ageing heat treatment conditions. The results showed that unlike the typical base metal structure that was ageing heat treated in the related references,12) almost no needle-shaped precipitates existed in the weld structure that had been ageing heat-treated for 20 minutes at 170°C. Thus, the weld metal had very lower hardness and strength than the base metal. The weld metal that had been ageing heat-treated for 12 hours at 170°C showed a higher strength by approximately 80% than that of the base metal due to the precipitation hardening effect.
Comparison of the hardness test result before and after heat treatment of the welded joints.
EBSD photomicrograph showing the base metal (left side) and the weld metal (right side).
TEM images showing the microstructure of the 6061-T6 laser weld zone according to the ageing heat treatment conditions.
Figure 14 shows the results of the fatigue test for each load to achieve sound welding conditions. At 100 MPa, which is the highest test load, fatigue life was 3 × 104 cycles due to weld failure. At the lowest load of 40 MPa, the test specimen did not break until 1 × 106 cycles. As mentioned above, the tensile strength of the welds is relatively lower than that of the base metal because of the dendritic microstructure formation of the weld metal, as well as the geometric shape of the welding bead, such as under-fill. Therefore, countermeasures that would prevent the under-fill of the welds in the laser welding of aluminum alloys should be actively examined.
Fatigue test result of the laser welded joints at various stresses.
In this experiment, laser welding was performed on an Al 6061-T6 alloy thin plate with a thickness of 1 mm. The result showed that the bead appearance looked well when the power was 2 kW, the welding speed was 2 mpm, the focus position was at −0.8 mm, and the shielding gas flow rate (N2) was at 10 l/min. Meanwhile, the rate of porosity irradiation to the cross-section was remarkably low with full penetration. The strength of the welds under optimum welding conditions was 52% compared to the base material. However, the welded part was subjected to ageing heat treatment (170°C, 12 hours), and demonstrated a strength of 295 MPa, which is about 90% of the base metal. The result of the fatigue test on those samples with appropriate welding conditions showed that the fatigue strength at the highest test load condition was 3 × 104 cycles at 100 MPa.
Therefore, the result of the above-mentioned study showed that it could be partially applied to the method that was designed to improve the mechanical properties of the welds by reducing porosity generation and by applying ageing heat treatment when laser welding is used in a heat-treatment type of aluminum alloy in the future, through an optimization of the laser welding condition and an evaluation of the mechanical properties of the welds.
This research was the result of funding from the Korean government (Ministry of Trade, Industry, and Energy), and was supported by the R&D project of the economic cooperation industry.
