Hongmei Li

The microstructures and fatigue behavior of dissimilar resistance spot welded joint between 5083 aluminum alloy and DP980 high strength steel are studied. Al/steel joint has fusion‐brazing characteristics composed of Al nugget and intermetallic compound (IMC) layer. The microstructure, microhardness, static tensile/shear property, and fatigue test of welded joints are carried out. The number of fatigue cycles increases with the decreasing of load level, and the fatigue limit load is 1.08 kN about 21% of static tensile/shear force. The fatigue test results reveal that different fatigue load levels lead to different fracture mode. It can be summarized as nugget pullout failure (fatigue load > 3.5 kN), interfacial failure (1.5 kN < fatigue load < 3.5 kN), and aluminum alloy base metal fracture (fatigue load < 1.5 kN). Furthermore, the fatigue fracture mechanism under different load levels is analyzed.

This review aimed to provide a comprehensive overview of the recent progress in fusion welding of steel and heterogeneous metals and to introduce current research and application. Amongst numerous metals, TiNi shape memory alloys and steel have been most heavily investigated for producing excellent joints. However, the base materials on both sides of the traditional fusion welding must melt and the large melting point difference between the two seriously affects the weld formation. The metal compounds formed by the base materials on both sides also hinder the improvement of the mechanical properties of the joint. This review focused on the fundamental understanding of the microstructural characteristics, processing and property relationships in the welding and joining of heterogeneous joints. The primary problem finding intermediate metals with enhanced plasticity and toughness to improve the formation of joints. Welding of light metals and high specific strength metals has a wide range of applications in the aerospace industry, it is committed to reducing the quality of flight equipment.

A butt welding-brazing joint of 5A06 aluminum alloy and DP1180 ultrahigh-strength steel was carried out by using plasma arc welding (PAW) with Al-Si welding wire. Interface characteristics, formation mechanism and mechanical properties of the joint were investigated. The results showed that the dissimilar joint contained bond zone, weld zone and Al/steel interface zone. During PAW, the inter-diffusion of Fe and Al and the interfacial reaction occurred, and the double-layer structure intemtallics (IMCs) composed of Fe₄Al₁₃ layer and Fe₂Al₅ layer were produced in the interface zone. The thickness and morphology of both IMC layers depended on different positions in the interface zone. The Fe₂Al₅ layer thickness decreased obviously and its morphology changed from continuous layer to discontinuous layer with the decrease of welding heat input. The average tensile strength of the joint was 88 MPa and the joint fractured at the Al/steel interface zone with the highest hardness (524 HV). The interface zone IMCs were the main factor of affecting the mechanical properties of aluminum alloy and ultrahigh-strength steel PAW joint.

Ti₃Al-Nb alloy (Ti-24Al-15Nb) was welded by a pulsed laser welding system without and with pure Nb filler metal. The results indicated that pure Nb filler metal had profound effects on the microstructures and mechanical properties of the laser-welded joints. The joint without filler metal consisted of the weld zone (α’₂ + B2), heat affected zone HAZ1 (α₂ + B2), HAZ2 (α₂ + O + B2) and base metal (α₂ + O + B2), and gas pores were generated in the weld resulting in the deterioration of the joint strength (330 MPa) and elongation (1.9%). When the Nb filler metal was used, the weld microstructure (NbTi solid solution + O + B2) was obtained, and the joint properties were significantly improved, which was associated with the strengthening effect of the NbTi solid solution, O phase precipitation and the slip transmission between O and B2 phases, and the restraining of the formation of martensite (α’₂) and gas pores in the weld. The strength (724 MPa) and elongation (5.1%) of the joint increased by 119.4% and 168.4% compared with those of the joint without filler metal, and the joint strength was able to reach 81.7% of the base metal strength (886 MPa). It is favorable to use pure Nb filler metal for improving the mechanical properties of laser-welded Ti₃Al-Nb alloy joints.