Tensile and fatigue fracture behaviour of a spot welded Al-Mg alloy

Abstract

The influence of microstructure on the mechanical properties and fracture behaviour of Al alloys has been the subject of many investigations. The microstructure of the alloys can be modified by varying the processing and heat treatment. Welding can also change the microstructure of the alloys. As one of the most practical joint methods for aluminium alloy sheets, the application of spot welding is restricted by a low heat affected zone (HAZ) strength level due to softening reactions during welding, in certain cases. In other cases the cracking resistance or the fatigue strength becomes the limiting factor. The investigation [1, 2] of spot welding on tensile and fatigue properties of the AlMg alloys showed that the tensile shear fatigue strength of spot welded joints increased in proportion to the nugget diameter and the acid cleaning of the base metal was very useful as a countermeasure to prevent the degradation of the mechanical property of the joints. However, for the tensile and fatigue fracture behaviour of the spot welded Al-Mg alloys, no research results are yet available. The aim of this work is to study the influence of the loading mode on the fracture behaviour of a spot welded AlMg alloy. The Al-Mg alloy (Al-5.48 Mg-0.32 Cu-0.07 Fe0.03 Si-0.02 Ti-0.01 Zn; all wt %) used was from a cold rolled sheet with the thickness of 1 mm in T4 condition. The sheet was spot welded at a pressure of 1960 N and current of 24 kA by using a 3-phase a.c. pedestal type spot welder. Tensile shear specimens with widths of 30 mm and lengths of 160 mm were baked at 180 8C for 30 min after spot welding. Tensile tests were carried out with a cross-head speed of 2.5 mm miny1. Tension-tension fatigue tests were performed at room temperature in a relative humidity of about 50%. The specimens were cycled sinusoidally at 30 Hz, using a stress ratio of 0.01. The microstructure of the alloy after spot welding was observed in an optical microscope. Tensile and fatigue fractures were examined in a JSM-35CF scanning electron microscope. Optical observation revealed that the alloy contains equiaxial grains with a similar size both in the HAZ and in the unaffected area far from the nugget. Grain coarsening did not occur in the HAZ during spot welding. Grain boundary melting occurred in the HAZ and coarse grain boundaries can be seen clearly (Fig. 1). The tensile shear load and tensile shear fatigue load of the alloy are shown in Table I. The ratio of fatigue load=tensile load (also the ratio of fatigue strength=tensile strength) for the spot welded alloy is very low. The crack developed around the edge of the nugget for the tensile shear specimens and buttonhole failure occurred. The fatigue specimens showed about the same failure as the ordinary tensile specimens without welding and a crack initiated at the edge of the nugget (in the HAZ). The tensile specimen shows mixed rupture with dimples and intergranular fracture. Dimples and intergranular fracture can be seen clearly in Fig. 2. At high magnification, very fine dimples can also be seen on the intergranular fracture surface. The chemical compositions on the fracture surface, as indicated by A and B in Fig. 2b, were measured by EDS (energy dispersive spectroscopy) and the result is given in Table II. High contents of Mg, Cu and Fe have been detected on the intergranular fracture surface, whereas the dimpled area shows about the same composition as the bulk composition given by the chemical method. For the specimens tested at different fatigue loads, the proportion of the fatigue crack propagation and