Laser powder bed fusion of light-weight, high-strength steels

Abstract

Ongoing efforts to reduce carbon footprint, have increased interest in the manufacturing of light-weight steels, especially in industries such as automative, aerospace and construction where weight savings are critical. Lightweight steels with Aluminium as an alloying element show high specific strength and stiffness and show great potential for weight saving with densities as low as 6.7 g/cm3. Additional alloying elements like manganese help stabilize the austenite phase that is more ductile than ferrite and has greater strain hardenability. Additive manufacturing methods have shown rapid growth in recent decades. Processes like the laser powder bed fusion (LPBF) offer considerable design freedom as well as new options for processing such as in-situ alloying and site-specific property control. However, the rapid solidification rates and repeated thermal cycling, both of which are inherent to LPBF can lead to defects such as cracking and porosities as well as undesirable microstructures. Aluminium when used as an alloying element in steel further exacerbates these issues due to its low melting point and high thermal expansion coefficient, which leads to thermal shrinkage between solidified dendrites and facilitates hot cracking. This study investigates the processing of FeMnAl steel alloys by LPBF and identifies optimal processing parameters particularly the scan speed. These optimal parameters are then used to investigate the effect of increasing aluminium concentration on the properties of steel samples through a high throughput compositional grading method. Physical properties like density and tensile properties are studied using experimental methods. Finally, the optimal percentage of aluminium in the alloy is discussed.