Study On Hot Cracking Of Overlap Joint In Laser Welding Dissimiliar Al Alloys

With the increasing requirement of environment protecting and energy saving,lightweight materials like Al alloys are widely used.As the different characteristic because of the different chemical compositions,Al alloys are used in various applications,so joining dissimilar Al alloys within overlap joint becomes inevitable.However,due to large solidification shrinkage,thermal tensions and deformations and the sensitive chemical compositions,hot cracking is a challenge issue in Al alloys welding.To provide guidance for the prevention of hot cracking in overlap laser welding of dissimilar Al alloys,the welding process of AA5754 and AA6013 was investigated in this paper.Sheets of dissimilar Al alloys were laser-welded in overlap joints.The effects of processing parameters such as laser power,orders of stacking,welding speed,overlap width and clamping condition on hot cracking behavior were studied.Weld joint's chemical composition,microstructure,fusion ratio and fracture morphology were further examined to understand hot cracking mechanism and provide guidance in hot cracking prevention.Besides,the numerical model of laser welding of Al alloy in overlap joint was established based on thermos-elastic-plastic theory.The temperature field and stress-strain field were developed to better understand the hot cracking in overlap joint.The conclusions are as following:(1)In overlap laser welding of dissimilar Al alloys,the participation of AA5754 will reduce hot cracking susceptibility,on the contrary,AA6013 will increase the hot cracking susceptibility.AA6013 is more sensitive to hot cracking than AA5754.Properly controlling the fusion ratio when overlap welding can avoid the hot cracking.(2)When overlap laser welding of dissimilar Al alloys,the effects of process parameters on hot cracking depend on the stackup.For the stackup of AA6013 up and AA5754 down,as the laser power increases,the hot cracking susceptibility reduces.Hot cracking can be effectively prevented as the laser power is over 3.85 k W;with the increase of welding speed,the hot cracking susceptibility increases;decreasing the overlap width can effectively increase the critical welding speed,to avoid the formation of hot cracking,welding speed should be less than these threshold.The critical welding speed was 3.0 m/min,3.3 m/min,3.9 m/min and 4.5 m/min when the overlap width was set as 12 mm,10 mm,8 mm and 6 mm,respectively.(3)The sound weld without hot cracking can be produced with proper process parameters.The tensile shear strength of the joints without hot cracking was more than 140 N/mm.(4)The numerical simulation indicates that when overlap laser welding of Al sheets,the maximum mechanical strain in the weld increases 17%,11% and 28% respectively when laser power increases from 3.5 k W to 4.5 k W,welding speed decreases from 3.6 m/min to 3.0 m/min,overlap width decreases from 15 mm to 5 mm,namely the increase of laser power,the decrease of welding speed and overlap width,will both increase the hot cracking susceptibility.