13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Ultrasonic Spot Welding of Lightweight Alloys V.K. Patel1, S.D. Bhole1, D.L. Chen1,* 1Department of Mechanical and Industrial Engineering, Ryerson University, M5B 2K3, Canada *Corresponding author: dchen@ryerson.ca Abstract The structural applications of lightweight alloys in the automotive and aerospace industries inevitably involve welding and joining of challenging dissimilar Mg-to-Al and Mg-to-steel while guaranteeing safety and structural integrity. Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW) – an environment-friendly solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle Al12Mg17 intermetallic compound in the Mg-to-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure in both Mg-to-Al and Mg-to-HSLA (high strength low alloy) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-to-Al dissimilar joints and in the mode of partial nugget pull-out in the Mg-to-HSLA steel dissimilar joints. In particular, the addition of Sn interlayer resulted in energy saving since the welding energy required to achieve the maximum strength decreased from 1250 J to 1000 J in the Mg-to-Al joints and from 1750 J to 1500 J in the Mg-to-HSLA steel joints. Keywords Magnesium alloy; Ultrasonic spot welding; Intermetallic compounds; Tin interlayer 1. Introduction Various industries, especially automotive and aerospace sectors, have a pressing need for structural components that are lighter and stronger, aiming to improve energy efficiencies and reduce anthropogenic environment-damaging emissions and pollution while guaranteeing safety and reliability. Aluminum (Al) and steel have already a wide variety of structural applications in the transportation industry due to their excellent properties, such as good ductility, formability and thermal conductivity. To reduce pollution and save energy [1], ultra-lightweight magnesium (Mg) alloy has increasingly been used in the vehicle fabrication due to its lower density, higher specific strength and stiffness, excellent size stability and acceptable process ability [2]. The structural application of Mg alloys inevitably involves welding and joining of similar Mg-to-Mg alloys and dissimilar Mg-to-Al and Mg-to-steel. In the auto body manufacturing resistance spot welding (RSW) has been a predominant process [3,4]. Since the differences in properties among these metals are large, like melting point, electric conductivity and thermal physical properties, etc., it is fairly challenging to join Mg-to-Al and Mg-to-steel [5,6]. Also, the high-energy consumption and the requirement for frequent electrode maintenance have limited its prevalent application to the Mg-to-Al alloys. Furthermore, in the welding of dissimilar metals a rapid formation of brittle intermetallic compounds (IMCs) occurs, which can seriously degrade the mechanical properties of welded joints [3]. Recently attention has been paid to two solid-state welding processes, namely friction stir spot welding (FSSW) and ultrasonic spot welding (USW), because the liquid phase reaction in the fusion zone during RSW can be avoided. Although FSSW has the potential to produce effective welds between dissimilar materials, the relatively long welding cycle (or time) would be a limiting factor for its widespread adoption in automotive manufacturing [4]. Another solid-state welding technology of USW is able to produce coalescence via a simultaneous application of localized high-frequency vibratory energy and moderate clamping forces [7,8]. In comparison with FSSW, USW has been shown to have a shorter weld cycle (typically <0.4 s) and produce high quality joints that are stronger than FSSW when compared on basis of the same nugget area [9,10]. Besides, the
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