Binder Jetting Additive Manufacturing of High Porosity 316L Stainless Steel Metal Foams.

Ganesh Kumar Meenashisundaram,Shenglu Lu,Jun Wei,Zhengkai Xu,Jyi Sheuan Ten,Mui Ling Sharon Nai

doi:10.3390/ma13173744

Ganesh Kumar Meenashisundaram, Shenglu Lu + Show 4 more

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https://doi.org/10.3390/ma13173744

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Abstract

High porosity (40% to 60%) 316L stainless steel containing well-interconnected open-cell porous structures with pore openness index of 0.87 to 1 were successfully fabricated by binder jetting and subsequent sintering processes coupled with a powder space holder technique. Mono-sized (30 µm) and 30% (by volume) spherically shaped poly(methyl methacrylate) (PMMA) powder was used as the space holder material. The effects of processing conditions such as: (1) binder saturation rates (55%, 100% and 150%), and (2) isothermal sintering temperatures (1000 °C to 1200 °C) on the porosity of 316L stainless steel parts were studied. By varying the processing conditions, porosity of 40% to 45% were achieved. To further increase the porosity values of 316L stainless steel parts, 30 vol. % (or 6 wt. %) of PMMA space holder particles were added to the 3D printing feedstock and porosity values of 57% to 61% were achieved. Mercury porosimetry results indicated pore sizes less than 40 µm for all the binder jetting processed 316L stainless steel parts. Anisotropy in linear shrinkage after the sintering process was observed for the SS316L parts with the largest linear shrinkage in the Z direction. The Young’s modulus and compression properties of 316L stainless steel parts decreased with increasing porosity and low Young’s modulus values in the range of 2 GPa to 29 GPa were able to be achieved. The parts fabricated by using pure 316L stainless steel feedstock sintered at 1200 °C with porosity of ~40% exhibited the maximum overall compressive properties with 0.2% compressive yield strength of 52.7 MPa, ultimate compressive strength of 520 MPa, fracture strain of 36.4%, and energy absorption of 116.7 MJ/m3, respectively. The Young’s modulus and compression properties of the binder jetting processed 316L stainless steel parts were found to be on par with that of the conventionally processed porous 316L stainless steel parts and even surpassed those having similar porosities, and matched to that of the cancellous bone types.

Highlights

316L stainless steel (SS316L), a quotidian austenitic steel, offers a wide range of applications in the marine, energy, aerospace, semiconductor and medical industries due to its high strength and corrosion resistance [1]
The apparent density of powders drop along with the growth of interparticle friction forces and this is due to the prevailing high resistance of SS316L particles containing poly(methyl methacrylate) (PMMA) to re-arrange during their apparent flow leading to poor powder packing and flowabilty characteristics [40]
Keeping the carbon (C), hydrogen (H) and oxygen (O) contents to the lowest levels throughout the binder jetting and subsequent sintering processes is of paramount importance especially for the successful processing of low carbon austenitic stainless steel SS316L to ensure its superior corrosion and mechanical properties

Summary

Introduction

316L stainless steel (SS316L), a quotidian austenitic steel, offers a wide range of applications in the marine, energy, aerospace, semiconductor and medical industries due to its high strength and corrosion resistance [1]. Low modulus biomaterials with high porosity and open-cell porous structures are of particular interests targeting orthopedic implant applications favoring bone in-growth [3]. High corrosion resistant and sintered porous SS316L parts containing open-cell porous structures are the most preferred materials for filtration applications where resistance to high pressures and temperatures are essential especially in the presence of oxidizing acids or high chlorides [5]. The simplest liquid-state process for fabricating closed-cell porous metal parts is by adding foaming agent or injecting inert gas to the melts which are later cooled-down in a casting process. Solid-state processes such as powder metallurgy technique can be used to fabricate porous metal parts at much lower processing temperatures with: (a) low compaction pressure or loose powders sintered at lower temperatures, and (b) by using powder space holder technique (PSH) [7]. Some of the most commonly used powder space holder materials are:

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