Abstract
In the automotive industry resistance, spot welding is the dominant technology in sheet metal joining of advanced high strength steels (AHSS). In order to improve the mechanical performance of AHSS welds, in-process tempering via a second pulse is a possible approach. In this work, two different double pulse welding schemes were applied to a 1200 MPa transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel. The different microstructures in the welds were characterized via light optical and scanning electron microscopy. Additionally, hardness mappings with several hundred indents were performed. It is shown that the second pulse, following a low first pulse which is high enough to produce a weld nugget that fulfills the quality criterion of a minimum spot weld diameter of 4*√t, leads to partial reaustenitization and consequently to a ferritic/martensitic microstructure after final quenching. Hardness mappings revealed that this inner FZ is harder than the surrounding FZ consisting of tempered martensite. In contrast, if the highest current without splashing is chosen for the first pulse, the same second pulse does not reaustenitize the FZ but only temper the martensite.
Highlights
In order to reduce the car body weight and subsequently minimize the amount of emissions, further development of advanced high strength steels (AHSS) for automotive applications is required
This etching clearly visualizes the transition from the fusion zone (FZ) to the heat-affected zone (HAZ), since phosphorus in the FZ is mainly segregated between the dendrites, while it preferentially segregates at the grain boundaries in the upper critical heat-affected zone (UCHAZ)
The different zones were characterized by light optical microscopy (LOM), scanning electron microscopy (SEM), and hardness mappings, and the following conclusions can be drawn: 1) If the first pulse is low and the weld nugget small, an inner FZ with a different microstructure than the residual FZ forms after the second pulse
Summary
In order to reduce the car body weight and subsequently minimize the amount of emissions, further development of advanced high strength steels (AHSS) for automotive applications is required. Transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steels are promising representatives of this generation. Their superior mechanical properties are achieved by a multiphase microstructure containing bainite and a sufficient amount of retained austenite stabilized by carbon enrichment during an isothermal holding step [2,3,4]. The heat-affected zone (HAZ) does not melt but is influenced by the heat input. It can be subdivided into the zones described in the following. The temperature in the intercritical heataffected zone (ICHAZ) rises to a level between A1 and A3, and its microstructure after quenching consists of ferrite and martensite.
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