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Paint baking effects on resistance spot welds in third generation and dual phase advanced high strength steels
Marshall, David V.
Marshall, David V.
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2021
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Abstract
The effects of a 20 min paint baking cycle at 180 °C on the performance of resistance spot welds are characterized in this thesis for welds made in three third-generation (Gen3) advanced high-strength steels (AHSSs) and three dual-phase (DP) steels. The strength and energy absorption measured in cross-tension and tensile shear testing of welds was characterized for as-welded and baked conditions with two different weld schedules. One schedule generated smaller, expulsion-prone welds, and the other generated larger, expulsion-free welds that satisfy the industry recommended minimum size to exhibit the favorable plug failure mode. The improvements in mechanical performance were correlated with changes in the fracture locations and characteristics of larger welds after baking, to identify the possible locations of the weld structure responsible for the baking sensitivity.
In cross-tension testing, the effects of baking were most significant in the smaller welds which exhibited low strength, low energy absorption, and an interfacial mode of failure prior to baking. The largest increase in strength and energy absorption after baking occurred in two of the Gen3 steels, the TRIP1180 and QP1000 steels. Significant effects of baking were also observed in larger welds made in these two steels which exhibited plug failure in the as-welded condition. Welds in the DP steels were less sensitive to baking, though a measurable effect was identified, particularly in the DP steel with the highest amount of Si. The force-displacement curves were observed in as-welded and baked specimens which revealed that changes after baking occurred at early stages of testing for smaller welds and later stages of testing in larger welds. Tensile shear testing of welds made in a QP1000 and DP980 steel revealed an opposite trend to that observed in cross-tension testing, in that larger welds exhibiting plug failure in the as-welded condition exhibited greater baking sensitivity than smaller welds failing at the interface. Following this observation, specimens with reduced sample width were used to enable weld rotation and bending of the base metal. In this modified geometry, welds that exhibited minor baking sensitivity (and failed in the weld interface) when tested with the standard geometry were now shown to be highly sensitive to baking (failing in the heat-affected zone (HAZ)). The change in failure mode of the as-welded condition in association with the increased baking sensitivity suggested that important microstructural changes must occur in the HAZ during baking.
Fractography of larger welds tested in the as-welded and baked conditions revealed the occurrence of low energy fracture in various regions of the weld. In a TRIP1180 steel, intergranular fracture in the upper critical HAZ and quasi-cleavage fracture in the intercritical HAZ were observed in the as-welded condition, but not after baking. Intergranular fracture was also observed for the as-welded QP1200 steel, but was still active after baking, albeit with a slight increase in ductile nature. Quasi cleavage fracture was observed along the fusion zone periphery in many of the DP steels both before and after baking, with varying degrees of mixed mode characteristics evident. A band like fracture morphology was observed in the as-welded and baked QP1200 and QP1000 welds, involving ridges of ductile microvoid coalescence separated by flatter regions. Of the weld regions examined using scanning electron microscopy, only the intercritical HAZ of the QP1000 and TRIP1180, the most baking-sensitive steels, revealed baking induced changes visible using the magnification available. The martensite-austenite constituent present in the intercritical HAZ of these steels appeared to undergo tempering during baking, which may serve to mitigate strain localization and subsequent low energy fracture in this weld region.
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