Research On Microstructure And Mechanical Properties Of Laser Welding Of6061Aluminum Alloy

Laser welding of aluminum alloy has a lot of advantages such as high energy density, low heat input, high welding efficiency and ease of automation, so it has a broad prospect in the field of aerospace and machinery manufacturing. In this paper,6061aluminum alloy was laser welded by using of Rofin-FLO10Laser System with low (≤1kW) power and YLS-10000Laser System with high (>1kW) power. The absorption capacity, microstructure, mechanical properties and influence of post-weld artificial aging on properties of the welding joint of6061aluminum alloy were re-searched by means of optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM), micro-hardness tester, transmission electron microscope (TEM) and differential scanning calorimetry (DSC), respectively.When laser welding with low power was performed, surface blasting, plasma spraying aluminum powder, dyeing with nigrosine and activating flux could improve the laser absorption of aluminum alloy in varying degrees. Although these surface treatments increased the penetration of weld, they also brought welding defects such as porosities, inclusions, cracks, and so on. Welding parameters also had a significant effect on the welding penetration and defects. Compared with welding speed, the in-fluence of laser power on welding penetration was more obvious. Pulse laser could decrease porosity of the weld but increase cracking tendency, the cracking tendency increased with the decrease of pulse frequency.When laser welding with high power was performed, microstructure in the welding zone (WZ) and fusion zone (FZ) was equiaxed grain and columnar grain, re-spectively. The microstructure of the WZ was more refined than that of the base metal (BM). Microhardness of WZ was lower than that of BM, which was about60HV, this result was because of the dissolution of the strengthening phase β" during the weld process. However, the TEM observation results indicated that a small amount of β" phase and β’phase were observed in the WZ, which is different from the conventional arc welding in which almost no β’phase and β" phase can be formed in WZ. The for-mation of β" and β’phase was attributed to the laser welding characteristics which behaved as rapid heating and rapid cooling. The high cooling rate at high temperature stage limited the precipitation of the Mg2Si phase and drove the Mg and Si atomics to dissolve in the Al matrix which was similar to solution treatment of aging aluminum alloy. The properties of the welding thermal cycle provided a relatively slow cooling rate between the temperature100℃～300℃, then the metastable phase (3" and β’could be precipitated in this stage. However, the amount of solid solution atomics was lim-ited, so the tensile strength and plasticity of WZ were lower than that of BM with arti-ficial aging significantly.Post-weld artificial aging of weld joint was performed in different solution tem-perature and aging temperature after being laser welded with high power. The results showed that the microhardness of WZ could reach70-80HV which was equal to the value of BM when the parameter530℃x6h+180℃x3h was adopted, the microhard-ness obtained using other parameters was lower than that of BM. The solution tem-perature and aging temperature had a significant effect on the result of the post-weld artificial aging, lower solution temperature could not drive all of Mg and Si atomics to dissolve in the a-Al matrix completely, then the amount of β" and β’phase in the post-weld artificial aging was less. Lower aging temperature provided lower driving force which could not produce enough strengthening phases. However, the higher ag-ing temperature maybe caused overage and reduced the mechanical properties of the weld. The TEM observation results indicated that a large amount of P" phase could be observed in the WZ with the parameter530℃x6h+180℃x3h in the post-weld artifi-cial aging, and less P" phase could be observed in the WZ under the condition of other post-weld artificial aging parameters. These results indicated that530℃x6h+180℃x3h was a suitable post-weld artificial aging parameter.