Investigation on the microstructure and machinability of ASTM A131 steel manufactured by directed energy deposition

This paper investigates the microstructure and machinability of ASTM A131 steel parts manufactured by directed energy deposition (DED). The surface finish of A131 steel parts is improved by a combination of additive manufacturing (AM) and subtractive manufacturing technologies. Microstructures of the DED and hot-rolled (HR) samples are studied. A large amount of acicular martensite appears on the top face (DED-top) and side face (DED-side) of the DED samples which are substantially different as compared to the ferrite + pearlite microstructure commonly observed in HR samples. The measured microhardness on the DED-top face is over 30% higher than that of the HR samples. As a major post-processing method for AM, milling operation was conducted at varying cutting speeds. Cutting force, tool wear, chip formation, surface roughness, and microhardness before/after milling were investigated to evaluate the machinability of additively manufactured parts. It is evident that the microhardness of both DED-top and DED-side samples change slightly after milling. The surface roughness (Ra) can be greatly modified from >20 μm to <1 μm by the post-processing. The DED-side sample yields the highest cutting forces due to the interference between the cutting tool and a large number of melt-pool boundaries that restrict material flow. Tool wear tends to escalate in the machining of the DED samples. Moreover, the DED samples present a lower chip curvature than HR samples with much less burr formation. In addition, the chip morphology analysis indicates that DED samples have shorter chip serration spacing and continuous chip formation. Comprehensive analysis indicates that DED enhances the machinability of the work material ASTM A131 steel.