Analysis of the metallurgical phase formation and their influence on the joint properties and the failure mechanisms of laser welded hard metal to steel joints

Laser beam welding of hard metal to steel offers multiple advantages regarding resource saving, mechanical strength of the joint and automation capability. The present work focuses on the fundamental research and development of the laser based welding process of tungsten carbide-cobalt hard metals with a tempering steel. Metallurgical analysis of the welding process showed that the formation of intermetallic and/or intermediate phases has a significant influence on the properties and mechanical strength of the dissimilar joint. The amount of molten hard metal in the steel melt bath plays a key role for the formation of the different phases. Therefore, a new parameter dy was defined, which correlates with the hard metal content in the melt pool. It is shown that for hard metals with 12 wt.% of cobalt binder, the phase transformation in the weld seam starts with a relative hard metal content of 10 vol.%. This threshold is dependent on the relative cobalt concentration in the hard metal. The tungsten carbide grain size has a low influence on the phase transformation in the weld seam. Steel melt pools with hard metal content lower than 10 vol.% show under metallographic observation a martensitic/bainitic microstructure. Simulation of the stress formation in the joint showed that due to the volume expansion of martensite during the transformation, tensile stress in the hard metal part was formed. Under shear load, these tensile stresses compensate with the induced compressive stresses and results an almost stress free interface. High shear strengths of the dissimilar joints are possible. A higher percentage of hard metal melting during the welding process increases the carbon and tungsten content in the melt bath. Consequently, the martensite start temperature decreases significantly. When the initiating temperature for martensite transformation falls under room temperature, the weld seam transforms into an austenitic microstructure. Because of the missing volume expansion during cooling of the weld seam volume, low stresses in the hard metal are generated. Under shear load of the joint area, high tensile stresses appear in the sintered part. These stress concentration decreases the shear strength of the weld and lead to premature failure. For the industrial use case, high mechanical strength and a robust manufacturing process is needed. Therefore, the laser welding process of hard metal to steel was optimized. The joint properties strongly depend on the weld bead geometry. Weld seams with x- or v-shaped profiles enable local concentrated metallurgical bonding of the sintered part to the steel sheet. Reduction of the horizontal focal distance of the laser beam to the interface increases the bonding ratio, but also intensifies the melting of the hard metal part and lead to the metallurgical transformation. By tilting a v-shape weld seam, it was possible to optimize the bonding behavior and to minimize the amount of liquefied hard metal in the melt bath. Hard metal with low amounts of binder showed a high temperature sensitivity. After laser welding of these grades, hot cracks were found in the sinter material. These cracks were formed due to the high stresses, which are generate during cooling of the dissimilar joint. Therefore, a laser based heat treatment process was developed and applied. With a defined pre- and post-heating of the joint area, the cooling rate was reduced significantly and the stresses in the hard metal part minimized. High shear strengths were the result