Composition change of stainless steel during microjoining with short laser pulse

Weld metal composition change in 200 mum deep, 304 stainless steel microjoints fabricated using millisecond long Nd-YAG laser pulses was investigated experimentally and theoretically. The variables studied were pulse duration and power density. After welding, concentrations of iron, manganese, chromium, and nickel were determined at various locations of the microjoint using the electron microprobe analysis. The temperature field was simulated as a function of time from a well-tested three-dimensional transient heat transfer and fluid flow model. Using the computed temperature fields, vaporization rates of various alloying elements resulting from both concentration and pressure driven transport of vapors and the resultant composition change of the alloy were calculated. The calculations showed that the vaporization took place mainly from a small region near the center of the beam-workpiece interaction zone, where the temperatures were very high. Furthermore, the alloying element vaporization was most pronounced toward the end of the pulse. After the laser spot welding, the concentrations of manganese and chromium in the weld pool decreased, whereas the concentrations of iron and nickel increased. The composition changes predicted by the model were in fair agreement with the corresponding experimental results for various conditions of microjoining with short duration pulses. (C) 2004 American Institute of Physics.