Metallurgy Analysis of 4kW Nd:YAG Butt Laser Welded Copper and its Alloys

摘要

Copper, bronze and brass wrought sheets (0.5-2 mm in thickness) were butt-welded using a 4 kW Nd:YAG CW laser focused down to 0.35+/-0.05 mm at speeds ranging from 1-7 m/min. The high energy density deposited in these materials, creates a narrow superheated molten bead having an initial thermal gradient higher than the critical value for constitutional undercooling, thus resulting in planar growth into the sine of the superheated melt, as observed by optical microscopy. For copper, as the solidifying front advances in the form of long and thick columnar grains of planar morphology, its thermal conductivity draws heat away from the melt into the solid, decreasing the thermal gradient and thus allowing cellular and dendritic growth instead. At the center of the bead, equiaxed grains results as the melt undercools and segregated impurities act as inoculants. In the case of bronze samples, the initial planar-epitaxial growth shifts into thin columnar crystals of dendritic morphology, soon followed by the nucleation of multiple size equiaxed-dendritic grains. For brasses, the sudden vaporization of zinc from the melt causes considerable loss of material and turbulence in the fusion zone impeding the development of a stable columnar solidifying front. All welds showed parallel walls indicating that a keyhole type of fusion welding took place. Microhardness profiles for copper, bronze and brass measured across the weld bead and along its central axis were interpreted in the light of segregation phenomena and under the effects of grain size.