Microstructure and corrosion performance of 316L stainless steel fabricated by Selective Laser Melting and processed through high-pressure torsion

Abstract

For the first time, the novel combination of severe plastic deformation (SPD) and Additive Manufacturing (AM) in a single process sequence was explored. 316L stainless steel (316L SS) alloy was firstly fabricated by Selective Laser Melting (SLM) AM process and subsequently processed by high-pressure torsion (HPT) SPD technique under a constant pressure of 6 GPa for different torsional revolutions. All the processed samples were subjected to electrochemical testing in a 3.5 wt % NaCl aqueous solution using open-circuit potential, potentiodynamic polarisation, and electrochemical impedance spectroscopy techniques, and characterised with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microscopic measurement results revealed that the melt pools and cellular structures obtained via SLM become increasingly refined through increased HPT revolutions, accompanied by significant porosity reduction and significant increase in microhardness. TEM observations revealed a homogeneously distributed nanoscale grains after 10 turns. Moreover, the results demonstrated that HPT processing significantly enhances corrosion performance of the 316L SS alloy in NaCl solution, due to

Effect of sample orientation on the microstructure and microhardness of additively manufactured AlSi10Mg processed by high-pressure torsion

“…The results indicate a much narrower distribution of pore diameter ranging from 0 to 40 μm and Only small amounts of pores in the sizes of 15-40 μm (< 10 counts) were observed for both 1/4-V and 1/4-H disks. Such dramatic reduction in porosity level was also observed by Yusuf et al [20] in their study for SLM-fabricated and then HPT-processed 316L SS. These observations indicate that the HPT-imposed shear strain is able to effectively 'close' the pores, significantly reducing pore sizes for larger pores (40-90 μm) and possibly eliminate smaller pores (0-20 μm).…”

“…The porosity content in the as-received and HPT-processed disks was analysed using ImageJ analysis software based on images taken from OM and SEM observations following the procedures described in Ref. [20]. The microstructures of the as-received and the HPT-processed disks were characterized via OM and SEM after being ground using different grits of SiC paper, polished using 3 and 1 μm Al 2 O 3 suspensions to a mirror-like surface finish and finally etched with Keller's reagent.…”

“…In fact, it is only very recently that the combination of SLM and HPT for 316L stainless steel was studied by the current authors [20]. It was found that HPT processing is able to substantially reduce porosity even at low strain levels, achieving nano-scale grain sizes and resulting in enhanced microhardness and corrosion performance.…”

“…To date, a wide range of materials have been processed by SLM, including stainless steels [5][6][7], Ni-based alloys [8][9][10], Ti-based alloys [11][12][13] and Al-based alloys [14][15][16]. Various reports have suggested improvements in mechanical properties for AM-fabricated parts compared to that of their traditional counterparts, including higher yield and tensile strengths [17,18], better corrosion resistance [19][20][21] and enhanced fatigue life [22]. Such improvements are attributed to the unique and very fine microstructure as a result of rapid solidification due to the short laser-material interaction time and high cooling rates of AM processes (10 3 -10 8 K s −1 ) [23][24][25].…”

Corrosion behaviour of 316L stainless steel manufactured by selective laser melting

“…Porosity is reported to be a possible site for the initiation of localized corrosion in SLM 316L stainless steel . The corrosion behaviour has been systematically investigated as a function of the main laser parameters (laser power and scan speed), which mainly determine the amount and size of pores in the material.…”