Grain structure and solidification cracking in oscillated arc welds of 5052 aluminum alloy
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I.
INTRODUCTION
I N a recent study, Kou et al. l have discovered a new, effective mechanism for reducing solidification cracking in the autogenous GTA welds (i.e., gas-tungsten arc welds made without filler metals) of 2014 aluminum alloy. The effectiveness of this mechanism, i.e., alternating grain orientation, was most pronounced in welds made with transverse arc oscillation of low frequency and high amplitude, and solidification cracking was reduced significantly in these welds. This mechanism was effective due to the fact that solidification cracking is intergranular and that columnar grains which reverse their orientation at regular intervals force the crack to change its direction periodically, thus making crack propagation difficult, as illustrated in Figure 1. In the present study, we shall investigate the effect of magnetic arc oscillation on the grain structure and solidification cracking of the autogenous GTA welds of 5052 aluminum alloy. This alloy is susceptible to solidification cracking, but to a significantly lesser degree than alloy 2014 and is, therefore, much more widely used for welding. 2 The purpose of the present study is to find out if alternating grain orientation can also be produced in alloy 5052, and how the response of this material to magnetic arc oscillation compares with that of alloy 2014. Other relevant previous studies are also summarized as follows. Tseng et al. 3 observed that the subgrain size and solidification cracking of the autogenous GTA welds of HY-80 steel were reduced by magnetic arc oscillation during welding. Garland 4 reported grain refining and reduction in solidification cracking in the autogenous GTA welds of an A1/1.7 to 2.8 pct Mg alloy by vibrating the welding torch. Sharir et al. 5 obtained fine equiaxed grains in the GTA welds of tantalum by magnetic arc oscillation. David et al.6'7 and Scarbrough et al.S reduced the size of the columnar grains in the autogenous GTA welds of iridium
S. KOU, Associate Professor, and Y, LE, Graduate Student, are with the Department of Metallurgical and Mineral Engineering, University of Wisconsin, Madison, WI 53706. Manuscript submitted November 20, 1984.
METALLURGICAL TRANSACTIONS A
alloys by magnetic arc oscillation; the latter investigators reported reduction in the rejection rate of the welded parts due to cracking.
II.
EXPERIMENTAL PROCEDURE
The material used for welding was 5052 aluminum sheets of 1.6 mm (%6 inch) thickness. The chemical analysis of the material is given in Table I. The experimental procedure was similar to that in the previous study 1 and will be described briefly here. The design of the fish-bone test specimens is shown in Figure 2. This design is similar to that originally proposed by Houldcroft, 9 except that a 25.4 mm long slot was used in the present study for better control over crack initiation. As shown, welding was started at the root of the slot and a longitudinal crack was initiated and propagated along the centerline of the weld, due to the thermal strains acting around and behind the
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