No abstract

No abstract

Solute segregation on a macroscopic scale in a weld between two dissimilar metals or alloys has long been recognized, but fundamental understanding of macrosegregation in dissimilar-metal welding is still lacking. Two mechanisms for macrosegregation were proposed based on the liquidus temperature of the bulk weld metal, T LW , relative to the liquidus temperature of metal 1, T L1 , and the liquidus temperature of metal 2, T L2. According to the mechanisms, two distinctly different macrosegregation features can form. A "peninsula" of an unmixed metal 1 can form if T LW < T L1. On the other hand, a "beach" of unmixed metal 2 irregular in shape can form if T LW > T L2. To verify the mechanisms, a pure Cu sheet was butt welded to a low carbon steel sheet by gas-tungsten arc welding without a filler metal. Composition measurements were conducted inside and across the weld metal. A peninsula of unmixed steel and an irregular-shaped beach of unmixed Cu were observed, which verified the mechanisms. In addition, the bulk weld metal exhibited a layered structure caused by undercooling of the bulk weld pool into a metastable miscibility gap in the Cu-Fe phase diagram. Macrosegregation in previous studies on laser-and electron-beam welding of Cu to steel or stainless steel was discussed in light of the findings in the present study.

The solidification cracking susceptibility of aluminium alloys 2024, 2219, 6061 and 7075 was evaluated recently by the transverse motion weldability test (the lower sheet moved in the transverse direction of lap welding to cause cracking). The tested welds were analysed in the present study. Their transverse cross-sections showed wide backfilled cracks in 2219 (least susceptible) and narrow open cracks in 6061 (most susceptible). Their fracture surfaces showed continuous eutectic in 2219 and discontinuous eutectic in 6061, consistent with the less backfilling in 6061. T was plotted against (f S ) 1/2 (T: temperature; f S : fraction solid) to explain the differences. The critical local strain rate near the crack was estimated, significantly lower in 6061 than 2219 and consistent with the higher crack susceptibility of 6061.

The susceptibility to solidification cracking was predicted for aluminium welds of 2024 Al made with filler metals 2319, 4043 and 4145 Al and of 6061 Al made with filler metals 4043 (traditional) and 4943 Al (new, higher strength). The maximum |dT/d(f S ) 1/2 | (T: temperature; f S : fraction of solid) was used as the crack susceptibility index. In each case, the index was calculated using the weld metal composition based on the measured dilution level. The predicted crack susceptibility decreased in the order of 2319 Al, 4043 Al and 4145 Al for 2024 Al, and was similar between 4043 Al and 4943 Al for 6061 Al. These predictions agreed well with the experimental results of recent crack susceptibility tests of these welds.

No abstract

Aluminium alloys are susceptible to solidification cracking. A criterion was derived to determine whether an aluminium alloy is susceptible to cracking utilising a recently proposed cracking index, maximum dT/d(f S ) 1/2 near (f S ) 1/2 = 1 where T is the temperature and f s is the fraction of solid, and aluminium-binary phase diagrams under limited solid diffusion conditions. If the maximum dT/d(f S ) 1/2 of an aluminium alloy is equal or less than 1423°C, it has good crack resistance to solidification cracking. The criterion was used to determine the critical amount of the filler metal 4043 Al to reduce the crack susceptibility of 6061 Al with the help of commercial thermodynamic software and database.

Various tests have been proposed to assess solidification cracking susceptibility. A new solidification cracking test, called the stationary weld pool deformation test, is introduced in the present study. The test employs a stationary weld pool during testing, and it is conducted by joining two square small workpiece with spot gas tungsten arc welding and moving one piece away from the other one to cause cracking. The test has been applied on aluminium alloys 2024, 5083, 6061 and 7075, and the susceptibility of them was found consistent with literature. The fracture surface of some of these aluminium alloys were examined, and typical characteristics of solidification cracks were seen. Advantages and disadvantages of the test method were discussed.