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Abstract :
[en] 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.
