5 Laser welding of advanced high-strength steels (AHSS)

Laser welding of advanced high-strength steels (AHSS) related to automotive industry applications is described in this chapter. There are many advantages of using laser welding for body-in-white applications. For instance, the use of laser welding in laser-welded blanks and for reducing flange widths during assembly welding allows automotive designers to reduce the weight of parts, which leads to increased fuel economy and reduced carbon dioxide emissions. Because of the unique microstructure and high hardenability of AHSS, the microstructure and mechanical properties across the weld zone are highly nonuniform. The microstructure of the fusion zone and heat-affected zone (HAZ) are described in detail, including the effects of composition and cooling rate. The hardness profile across the weld in AHSS typically reveals HAZ softening, which occurs because martensite in the base metal is tempered. HAZ softening with the effects of heat input, martensite fraction in the base metal and steel composition is discussed in detail. These changes in the microstructure and local mechanical properties are then correlated to the performance of laser-welded blanks such as tensile properties, fatigue resistance and formability. The chapter provides information about the metallurgy and performance of AHSS laser welds coupled with issues and potential solutions.