Sansan Ao

Weld Growth Mechanisms and Failure Behavior of Three-Sheet Resistance Spot Welds Made of 5052 Aluminum Alloy

This paper investigates the weld nugget formation in three-sheet aluminum alloy resistance spot welding. The nugget formation process in three equal thickness sheets and three unequal thickness sheets of 5052 aluminum alloy were studied. The results showed that the nugget was initially formed at the workpiece/workpiece interfaces (i.e., both upper interface and lower interface). The two small nuggets then grew along the radial direction and axial direction (welding direction) as the welding time increased. Eventually, the two nuggets fused into one large nugget. During the welding process, the Peltier effect between the Cu-Al caused the shift of the nugget in the welding direction. In addition, the mechanical strength and fracture mode of the weld nuggets at the upper and lower interfaces were also studied using tensile shear specimen configuration. Three failure modes were identified, namely interfacial, mixed, and pullout. The critical welding time and critical nugget diameter corresponding to the transitions of these modes were investigated. Finally, an empirical failure load formula for three-sheet weld similar to two-sheet spot weld was developed.

Application of Pre-heating to Improve the Consistency and Quality in AA5052 Resistance Spot Welding

Making consistent resistance spot welds of aluminum alloy with good quality and at high volume has several obstacles in automotive industry. One of the difficult issues arises from the presence of a tough non-conducting oxide film on the aluminum sheet surface. The oxide film develops over time and often is non-uniform across the surface of the aluminum alloy sheet, which makes the contact resistance characteristics irregular at the faying interface during welding. The consistency in quality of the final spot welds is therefore problematic to control. To suppress the effect of the irregular oxide film on the spot weld quality, application of a pre-heating treatment in the welding schedule for aluminum alloy 5052 is investigated in this present work. The current level of the pre-heating required to reduce the scatter of the contact resistance at the W/W (workpiece-to-workpiece) faying interface is quantified experimentally. The results indicate that the contact resistance at the W/W faying interface with a pre-heating treatment becomes much consistent and can be reduced by two orders of magnitude. Having the uncertain variation of the contact resistance at the W/W faying surface virtually reduced or removed, the quality of the spot welds in terms of the peak load and nugget diameter is examined and shows a great improvement. The proposed method may provide a robust method for high-volume spot welding of aluminum alloy sheets in auto industry.

Simulation of cutting process in the cable recycling system

The utilization of Waste Electrical and Electronic Equipment is a hot spot in environmental protection field presently and the resource utilization of cable wastes is an important subject. An enormous amount of electrical cable is disposed of as scrap each year. In order to recycle the valuable copper wires, cable granulator technique is used widely. However, one of the shortcomings of this technique is it has serious tool wear. In order to better understand the reason for tool wear, this paper simulates the stress and strain distribution in the cutting tool and copper during the cutting process in cable granulator by finite element method. The result shows that a tensile stress region, which is the main reason for blade tipping, appeared in the upper blade. Besides, the tensile stress in the right side of upper blade is higher than that in the left side. Therefore, in order to extend the life of cutter, we suggest using different materials in different stress zone to manufacture cutting tool. After the cutter was worn out, the right side of the blade can be renovated using material with well tensile performance through overlaying welding, as well as the left side of the blade can be renovated by material with high red hardness, high wear resistance, and high compression strength through overlaying welding. This method can reduce the consumption of precious metals and raise the utilization rate of materials.

Microstructural Characterization and Mechanical Behavior of NiTi Shape Memory Alloys Ultrasonic Joints Using Cu Interlayer

NiTi shape memory alloys (SMAs) are a class of functional materials which can be significantly deformed and recover their original shape via a reversible martensitic phase transformation. Developing effective joining techniques can expand the application of SMAs in the medical and engineering fields. In this study, ultrasonic spot welding (USW), a solid-state joining technique, was used to join NiTi sheets using a Cu interlayer in between the two joining sheets. The influence of USW process on the microstructural characteristics and mechanical behavior of the NiTi joints was investigated. Compared with conventional fusion welding techniques, no intermetallic compounds formed in the joints, which is extreme importance for this particular class of alloys. The joining mechanisms involve a combination of shear plastic deformation, mechanical interlocking and formation of micro-welds. A better bonding interface was obtained with higher welding energy levels, which contributed to a higher tensile load. An interfacial fracture mode occurred and the fracture surfaces exhibited both brittle and ductile-like characteristics with the existence of tear ridges and dimples. The fracture initiated at the weak region of the joint border and then propagated through it, leading to tearing of Cu foil at the fracture interface.

Effect of specimen width on the failure behavior in resistance spot weld tensile shear testing

Specimen dimensions have significant effect on the tensile shear strength (TSS) of resistance spot welds (RSW) in the commonly employed tensile shear testing. Minimum required dimensions specified in different industrial test standards vary somewhat however. In this paper, the most influential dimension, the tensile shear test specimen width, is investigated for a low-strength steel (galvanized steel Q235), a high-strength steel (TRIP980), and aluminum alloy 5052. The relationships between the specimen width and weld properties, such as TSS, energy absorption, and failure modes, were investigated experimentally. The results show that there is a critical width of the specimen beyond which the TSS of the RSW maintains a constant value and the failure is at the spot weld. This constant TSS is a saturated value. The perceived TSS from the tensile shear testing is lower than the saturated TSS when the width is smaller than the critical value. The displacement at the TSS and the energy up to failure from the tensile shear testing, however, shows opposite trend due to the large plastic deformation from the base sheet metal in narrower specimens. From the results, a minimum width of tensile shear test specimen for determining saturated TSS is recommended.