Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6±0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 μm was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10–100 μm. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

Hardened austenite steel with columnar sub-grain structure formed by laser melting

Study of corrosion in biocompatible metals for implants: A review

The mechanical properties and corrosion resistance of 316 L stainless steel fabricated using the Laser Engineered Net Shaping (LENS) technique have been studied. The crack-free, full density samples made using SS316L alloy powder and the LENS technique are characterized by an unusual distinct dual-phase microstructure. STEM analysis revealed a significant increase of Cr and Mo content and a decrease of Ni in the grain boundaries. Based on the Cr and Ni content (austenite stabilizing elements), the Schaeffler diagram and the EBSD results, the existence of intercellular delta ferrite on subgrain boundaries and austenite in the fine-grains are observed. The XRD patterns, in addition to the FCC austenite phase, revealed the second BCC ferrite phase. Moreover, the sigma (FeCr) phases are present in the analyzed 316 L stainless steel. The occurrence of ferrite, which does not occur in regular stainless steel fabricated using conventional metallurgical methods, improves the mechanical and corrosion properties of the LENS-fabricated sample made using 316 L stainless steel powder. The obtained results prove that the microstructure of the SS316L fabricated using LENS is heterogeneous; its impact on the mechanical properties is visible. The analyzed samples are characterized by anisotropic mechanical properties that are favorable. For both the perpendicular and parallel directions of tensile tests, samples had a ductile fracture with many dimples inside of the larger dimples. The corrosion potential of SS316L LENS and classically manufactured steel is similar. The SS316L fabricated using LENS is characterized by a relatively low value of corrosion current density, which translates into much smaller corrosion rates.

The microstructure, mechanical properties and corrosion resistance of 316 L stainless steel fabricated using laser engineered net shaping

A review of powder additive manufacturing processes for metallic biomaterials

This work presents a comprehensive study on the influence of three different processing technologies (Selective Laser Melting, Hot Pressing and conventional casting) on the microstructure, mechanical and wear behavior of an austenitic 316L Stainless Steel. A correlation between the processing technologies, the obtained microstructure and the mechanical and wear behavior was achieved. The results showed that the highest mechanical properties and tribological performance were obtained for 316L SS specimens produced by Selective Laser Melting, when compared to Hot Pressing and conventional casting. The high wear and mechanical performance of 316L Stainless Steel fabricated by Selective Laser Melting are mainly due to the finer microstructure, induced by the process. In this sense, Selective Laser Melting seems a promising method to fabricate customized 316L SS implants with improved mechanical and wear performance.

316L stainless steel mechanical and tribological behavior—A comparison between selective laser melting, hot pressing and conventional casting

Effect of porosity and density on the mechanical and microstructural properties of sintered 316L stainless steel implant materials

Mechanical properties of P/M 316L stainless steel materials

Effects of sintering atmosphere on microstructure and mechanical property of sintered powder metallurgy 316L stainless steel

In this study, the mechanical and wear properties of AISI 316L stainless steel implant materials, produced by powder metallurgy (P/M), were investigated. AISI 316L stainless steel powder was cold-pressed with 800 MPa of pressure and then sintered at 1200, 1250 and 1300°C for 30 min as three sample groups. The microstructure, and mechanical and wear properties of the resulting steels were investigated. Light optical and scanning electron microscopiese were used to characterize the microstructure of the steels. Room temperature mechanical properties of the steels were determined by hardness measurements and impact tests. Wear was determined using the pin-on-disc wear test, and the results were evaluated according to weight loss. The results indicate that the sintering temperature, time and atmosphere are important parameters that affect the porous ratio of materials produced by P/M. Sintering at high temperature can eliminate small pores and make the residual pores spherical. The wear tests showed that the wear of the AISI 316L stainless steel implants changed depending on the sintering temperature and load. Spherical pores in the samples increase the wear resistance. Moreover, decreasing the porosity ratio of these materials improves all of their mechanical properties.

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Production of 316L stainless steel implant materials by powder metallurgy and investigation of their wear properties

In the present study, diffusion bonding of aluminium alloy (AA7075) sheet materials which are used especially in the automobile and aerospace industry has been investigated at temperatures of 425 and 450 °C and pressures of 2 and 3 MPa for 180 min in argon atmosphere. The microstructural and mechanical properties of bonding have been characterized with different welding parameters such as bonding temperature and pressure. The microstructure was characterized by light optical microscope, scanning electron microscope and energy dispersive spectroscopy, while the mechanical properties were determined by tensile-shear tests and microhardness tests. The results obtained are discussed from both the microstructural and mechanical points of view. It was observed in the microstructural investigations that the interfacial oxide layer decreased with increasing of the bonding temperature and pressure. The maximum shear strength was found to be 131 MPa for the Al 7075 sample bonded at 450 °C and 3 MPa for 180 min. It is shown that in certain extent, the bonding temperature and bonding pressure have great effect on the joint shear strength. With the increasing of bonding temperature and pressure, the shear strength of the joints increases due to diffusion of atoms in the interface. The strength achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. The maximum hardness value of the Al 7075 sample bonded at 450 °C, 3 MPa for 180 min is 92.5 HV0.2. Increasing hardness with increasing temperature can be attributed to the formation of metallic bond at high temperatures and pressures.

Naci Kurgan

Papers

Effect of porosity and density on the mechanical and microstructural properties of sintered 316L stainless steel implant materials

Mechanical properties of P/M 316L stainless steel materials

Effects of sintering atmosphere on microstructure and mechanical property of sintered powder metallurgy 316L stainless steel

Production of 316L stainless steel implant materials by powder metallurgy and investigation of their wear properties

Investigation of the effect of diffusion bonding parameters on microstructure and mechanical properties of 7075 aluminium alloy