Microstructure and Mechanical Properties of an Ultrasonic Spot Welded Aluminum Alloy: The Effect of Welding Energy

Abstract

The aim of this study is to evaluate the microstructures, tensile lap shear strength, and fatigue resistance of 6022-T43 aluminum alloy joints welded via a solid-state welding technique–ultrasonic spot welding (USW)–at different energy levels. An ultra-fine necklace-like equiaxed grain structure is observed along the weld line due to the occurrence of dynamic crystallization, with smaller grain sizes at lower levels of welding energy. The tensile lap shear strength, failure energy, and critical stress intensity of the welded joints first increase, reach their maximum values, and then decrease with increasing welding energy. The tensile lap shear failure mode changes from interfacial fracture at lower energy levels, to nugget pull-out at intermediate optimal energy levels, and to transverse through-thickness (TTT) crack growth at higher energy levels. The fatigue life is longer for the joints welded at an energy of 1400 J than 2000 J at higher cyclic loading levels. The fatigue failure mode changes from nugget pull-out to TTT crack growth with decreasing cyclic loading for the joints welded at 1400 J, while TTT crack growth mode remains at all cyclic loading levels for the joints welded at 2000 J. Fatigue crack basically initiates from the nugget edge, and propagates with “river-flow” patterns and characteristic fatigue striations.

Microstructure and Mechanical Properties of Ultrasonic Spot Welded Mg/Al Alloy Dissimilar Joints

“…Figure 3 shows the peak tensile lap shear load and critical stress intensity factor (K c ) of USWed ZEK100/Al6022 dissimilar joints as a function of welding energy at a constant power of 2 kW and clamping pressure of 0.4 MPa. The critical stress intensity (K c ) in Figure 3b, which was used to better normalize the effect of energy input, was calculated based on Zhang's solution [16,47,48]:…”

“…In the present study, t = (t 1 + t 2 )/2, where t 1 and t 2 are the thicknesses of ZEK100 and Al6022, respectively. Using the same algorithm as other investigators [15][16][17][18][19][20][21][22][23][24], a nugget zone area of A = 8 × 5 mm 2 was used to assess the tensile lap shear strength with an equivalent nugget diameter of d calculated from:…”

“…At a relatively low welding energy of 500 J, the interface diffusion layer was discontinuous and thin (0.2~0.5 µm in thickness, Figure 2a) due to the low temperature and strain rate. In this case the material was not softened enough (i.e., the yield strength of the base material still remained high) owning to the relatively low temperature, the flowability of metal was thus limited, leading to a limited coalescence of the bonding surfaces [16,24,25]. As a result, some isolated islands could be formed with the development of microwelds, and a lower extent of diffusion and discontinuous coalescence at the weld interface resulted in poor welding strength [22,24].…”

“…Such applications inevitably entail the welding and joining of Al and Mg alloys. However, due to the rapid formation of brittle intermetallic compounds (IMCs) at the weld interface, caused by the high mutual diffusivities in these two materials even at relatively low temperatures, welding of Al to Mg is tremendously challenging, especially in the fusion welding processes [14][15][16][17].…”

“…Three main approaches involving: (1) a lower welding temperature; (2) a shorter welding time; and (3) a filler material addition could be used to reduce the IMC thickness and minimize the defects [20][21][22][23][24]. Hence, solid-state joining methods such as friction stir welding (FSW), friction stir spot welding (FSSW), linear friction welding (LFW) and ultrasonic spot welding (USW) that could avoid liquid phase reactions are increasingly attracting attention because of their lower welding temperature, shorter welding time and lower energy consumption or lower cost, compared with fusion welding techniques, while other solutions like clinching, self-piercing rivets (SPR), and adhesive bonding need additional costs of consumables and surface treatment [16,18,22,23]. USW is more effective than FSSW (where an exit hole is normally present) and is considered to be one of the most interesting and emerging joining techniques for making dissimilar Mg/Al joints [15,[21][22][23][24][25][26][27][28].…”