不同焊接速度下Cu/Al/Cu纳秒激光焊接接头组织及性能研究

Microstructures and Properties of Nanosecond Laser Welded Cu/Al/Cu Joints Under Different Welding Speeds

采用纳秒脉冲激光对铜/铝/铜三层薄片结构进行焊接,分析焊接速度对焊缝成形、微观组织与接头力学性能的影响。结果表明:较低的焊接速度容易引起气孔和裂纹等接头缺陷,较高的焊接速度造成焊缝中的气孔聚集于界面,合适的焊接速度有利于获得良好的熔透深度和较少的缺陷;焊缝中存在Cu Al_(2)、Cu_(9)Al_(4)与Cu_(4)Al_(3)等铝/铜二元化合物,且焊缝中生成的化合物的种类与焊接速度有关;当焊接速度为21 mm·s^(-1)时,界面较少的缺陷和钉状焊缝使得接头具有的最大剪切力为95.1 N。接头发生了三种断裂:焊点撕裂、焊缝剪切+熔核拔出复合断裂以及熔核拔出失效,焊点边缘的裂纹及脆性相是焊点撕裂失效的原因,界面处焊缝边缘的铜/铝化合物和气孔缺陷是复合断裂的原因,较浅的熔深和界面化合物导致了熔核拔出失效。

Objective In electronics and precision devices,achieving reliable connections for composite joints of thin metal sheets such as Al/Cu is critical,as this helps reduce Cu usage costs.Conventional welding methods such as brazing and stir friction welding are limited by the sample size and tend to introduce excessive heat input into the joint.Thus,they are not suitable for joining thin metals.Suitable methods for joining thin metal sheets include ultrasonic and laser welding.Laser welding,particularly short-pulse laser welding,has precise heat input,high energy density,and good controllability,making it suitable for connecting thin sheets.Accordingly,in this study,a short-pulse laser is used to weld Cu/Al/Cu laminar joints.The effects of scanning speed on the joint microstructures and properties are investigated to provide guidance in joining laminar metal sheets.Methods A nanosecond-pulsed laser is used to join Cu/Al/Cu laminates.The laser scanning speed is used as a major parameter that affects the joint quality.First,the effect of scanning speed on weld formation is studied.The microstructural evolution of typical joints is then analyzed.Finally,the effects of welding speed on the mechanical properties are investigated,and the fracture mechanism of the joints is elucidated.Results and Discussions In terms of weld formation,the heat-affected zone at the joint edge and the penetration depth both decrease as the welding speed increases(Fig.3).The pores in the weld migrate upward as the melting depth decreases,and the joint quality improves when the welding speed is 21 mm/s(Fig.4).In terms of microstructures,the interface produces numerous bulk CuAl_(2)phases at a low welding speed(15 mm/s)(Fig.6).As the welding speed increases,the number of interfacial compounds decreases.In addition,the compound Cu_(9)Al_(4)phase appears near the copper base material,whereas the part near the aluminum is mainly subeutectic,eutectic,and perieutectic structures composed ofα-Al and CuAl_(2)phases.When the welding speed reaches 27 mm/s,the Cu_(4)Al_(3)phase appears in the Cu clusters(Fig.6).In tensile tests,the maximum shear force of 95.1 N is obtained in the joint at a welding speed of 21 mm/s(Fig.9).Following fracture analysis,three fracture modes are observed.When the welding speed is low,the joint experiences the tearing failure,which is caused by deep longitudinal cracks in the outer ring of the welded joint due to considerable heat input,and brittle compounds such as CuAl_(2)are produced at the weld edges(Figs.10 and 11).As the welding speed increases(21 mm/s),the cracks and porosity defects in the weld decrease,and the joint has a certain melting depth,resulting in a combined failure mode(weld shear and nugget pullout failure)in the joint(Fig.11).As the welding speed becomes excessively high,the molten nucleus has difficulty resisting the tensile shear force because of the smaller penetration depth,resulting in a button pullout fracture mode.Conclusions A nanosecond-pulsed laser is used to weld three-layer Cu/Al/Cu lamellar structures,where defects such as porosity and cracking at the joint interface can be reduced by adjusting the welding speed.The joint formation quality is found to be excellent at a welding speed of 21 mm/s.The diffusion of Al into the upper and lower Cu base materials to produce nail-shaped areas improves joint strength.The Al/Cu binary compound type is found to be related to the welding speed.The CuAl_(2)phase is prevalent in the Cu welds,whereas Cu_(9)Al_(4)and Cu_(4)Al_(3)are present only near the Cu base material or Cu clusters.The maximum joint shear force of 95.1 N is obtained at a welding speed of 21 mm/s.Three types of fractures occur in the Cu/Al/Cu joints at different welding speeds,namely,joint tearing,mixed-mode fracture(weld shear and nugget pullout failure),and button pullout failure.The nail-shaped structure has a hindering effect on joint fracture.