Investigation on the paint baking effect in resistance spot welds of advanced high strength steels, The

Marshall, David V.

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Investigation on the paint baking effect in resistance spot welds of advanced high strength steels, The

Marshall, David V.

Advisor

Speer, J. G.

Date Issued

2024

Abstract

One approach automakers take to improve vehicle fuel efficiency is to reduce vehicle mass, which requires steels with improved strength and toughness compared to conventional steels. Over the past few decades, steel researchers have engineered the third generation (Gen3) of advanced high strength steels (AHSSs) that exhibit improved strength and toughness relative to first generation (Gen1) AHSSs at a decreased cost relative to second generation AHSSs. Though Gen3 steels have been successfully developed, their implementation in automotive assembly has been limited by challenges in resistance spot welding, the most commonly used joining technique in the automotive industry. Gen3 spot weld mechanical properties are often insufficient in comparison with those of Gen1 spot welds, though it has been observed that the automotive paint-baking cycle can mitigate this insufficiency. The microstructural evolution associated with these baking-induced improvements is not fully understood, which highlights the area of research this thesis seeks to address. The purpose of this study was to investigate the effects of the paint-baking cycle (180 °C, 20 min) on Gen1 and Gen3 spot weld microstructures and mechanical properties and to understand the origin of the differences in their baking sensitivities. A new specimen geometry coupled with digital image correlation was implemented and revealed greater plastic strains were measured within the heat-affected zone (HAZ) of baked welds compared with unbaked welds. Subsequent experiments were developed to isolate the different HAZ microstructures to quantify the baking sensitivities of each by implementing the small punch test method. Small punch test results identified a trough in energy absorption within the intercritical HAZ of the Gen3 steel that was mitigated after paint baking. The Gen1 HAZ microstructures were characterized by a minor sensitivity to baking and the absence of an intercritical trough in energy absorption. Thermocouple measurements taken during Gleeble®-simulation of HAZ microstructures confirmed depression of the martensite start (MS) temperature with decreasing intercritical temperature, and this effect was more prominent in the Gen3 steel. To elucidate the differences in Gen1 and Gen3 spot weld baking sensitivities, a carbon diffusion distance model was developed that predicted the greatest fraction of baking-sensitive martensite would form in the intercritical HAZ of the Gen3 spot welds. 3D atom probe tomography conducted on Gen3 spot weld HAZ microstructures identified carbon redistribution in twinned martensite regions in the intercritical HAZ and carbon segregation to martensite lath boundaries in the supercritical HAZ of baked specimens. The intercritical HAZ was concluded to be a baking critical microstructure regarding Gen3 spot weld baking sensitivity, and this represents a new finding in the field of research on AHSS spot welds. The insensitivity of the DP spot welds to baking is proposed to be due to the higher MS temperature through the intercritical HAZ, enabling carbon redistribution during cooling of the weld that is not possible in Gen3 spot welds.

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Investigation on the paint baking effect in resistance spot welds of advanced high strength steels, The

Marshall, David V.

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