Selective Laser Melting 316L Stainless Steel Research Articles

The passivity of selective laser melted 316L stainless steel

The passive film properties of as-received selective laser-melted 316L stainless steel (SLMed 316L SS) without obvious pores were studied and compared with those of wrought and solution-annealed (SA) SLMed 316L SSs. Among the samples, the as-received SLMed 316L SS had the lowest passive current density, which was attributed to two factors: the super high sub-grain boundary density increased the number of nucleation sites for passive film formation, and the micro-galvanic effect between the sub-grain boundary and the inner sub-grains also accelerated the rapid formation of a thick protective layer. The poor passivation and re-passivation abilities of the SA SLMed 316L SS were ascribed to the extensively precipitated nanosized oxide and sulfide particles, especially those at the grain boundary.

Ultrasonic Measurement of Stress in SLM 316L Stainless Steel Forming Parts Manufactured Using Different Scanning Strategies.

Selective Laser Melting (SLM) technology is a new kind of additive manufacturing technology developed in in the last decade. Measurement and control of stress in metal forming layer is the basic problem of SLM forming parts. Critical Refraction Longitudinal (LCR) wave method was used to measure stress. The acoustic-elastic formulas for measuring stresses in SLM 316L stainless steel forming parts manufactured using meander, stripe, and chess board scanning strategies, respectively, were established based on static load tensile test. The experimental results show that the acoustic time difference of LCR wave in SLM specimen manufactured with 316L stainless steel increases linearly with the increase of stress when the tensile stress is less than critical stress (372 MPa, 465 MPa, and 494 MPa). Due to the inhomogeneous deformation of the anisotropic SLM forming layer and the dimple-micropore aggregation fracture mechanism, the acousto-elastic curve fluctuates up and down along the irregular curve when the tensile stress is larger than critical stress. The results of corroboration experiments show that nondestructive measurement of stress in SLM forming specimen can be realized by using LCR wave method. The scanning strategy can significantly affect the tensile strength and yield strength of SLM forming specimen. The stresses were all in tension stress state at the edge of the specimens, whatever scanning strategy was used. Sub-area scanning and scanning sequence of alternate and intersect were adopted, which can effectively reduce the stress in the SLM forming specimen. The overall stress values of SLM forming specimen manufactured using chess board scanning strategy were smaller than that using meander and stripe strategies. The distribution of stress were more uniform.

Tribological Property of Selective Laser Melting–Processed 316L Stainless Steel against Filled PEEK under Water Lubrication

As a 3D printing technology, selective laser melting has remarkable advantages such as high processing flexibility, high material utilization, and short production cycle. The applications of selective laser melting technology in industry have become quite extensive. There are many tribological studies on selective laser melting materials, but few based on water lubrication (Zhu, et al., Journal of Zhejiang University-Science A, 19(2), pp 95–110). In this article, the tribological properties of 316L stainless steel processed by selective laser melting and traditional methods have been studied under water lubrication. Polyether ether ketone (PEEK) filled with carbon fiber (CF)/polytetrafluoroethylene (PTFE)/graphite was selected as the counterpart. 316L stainless steel and PEEK are a tribopair commonly used in water hydraulics. This study is of great significance to the application of selective laser melting material of tribopairs in water hydraulics. Friction and wear tests were carried out on a pin-on-disc contact test apparatus under different operating conditions. The friction coefficient, specific wear coefficient, scanning electron microscopy (SEM) of the worn surface, and energy-dispersive spectroscopy (EDS) of the surface adhesions of the three tribopairs were measured and compared. The results revealed that the friction coefficient of the selective laser melting (SLM) 316L stainless steel was significantly higher than that of traditionally processed (TP) 316L stainless steel, which might be caused by the pores on the surface of SLM 316L stainless steel. Adhesion and cutting on the surface of SLM 316L stainless steel were also more serious, resulting in a higher specific wear coefficient of its counterpart PEEK composite compared to PEEK composite against TP 316L stainless steel.

Microstructure and Corrosion Characteristics of Selective Laser-Melted 316L Stainless Steel: The Impact of Process-Induced Porosities

The microstructure and corrosion susceptibility of selective laser-melted (SLM) 316L stainless steel on planes aligned and perpendicular to the building direction in aqueous 3.5 wt.% NaCl solution were investigated and compared with their conventionally wrought counterpart. Cyclic potentiodynamic polarization results confirmed a superior pitting resistance and a reduced rate of metastable pitting for the SLM-fabricated samples regardless of the sample’s orientation compared to the wrought alloy. Although the process-induced porosities in the SLM samples did not affect the corrosion and pitting potentials of the alloy at the initial immersion time, they likely contributed to the reduced re-passivation potential of the surface. Furthermore, at longer immersion times, the electrochemical impedance spectroscopy and immersion testing results confirmed a more detrimental impact of the process-induced porosities on lowering the electrochemical stability of the SLM-316L surface perpendicular to the building direction, where larger size and higher density of crevice-like porosities were detected.

Hybrid additive manufacture of 316L stainless steel with cold spray and selective laser melting: Microstructure and mechanical properties

Fusion based metal additive manufacturing (AM) techniques such as selective laser melting (SLM) offer many advantages when compared to traditional manufacturing techniques, however, are often limited by the low build rates achievable, particularly for the manufacture of large components at scale. This may be alleviated by combining SLM with other more rapid AM processes such as cold spray (CS). Therefore, in this work, a hybrid AM process combining SLM and CS was utilised for the production of CS-SLM hybrid components. The proposed hybrid process allows the fabrication of complex geometries with SLM and the rapid manufacture of simple geometries using CS. The hybrid parts were manufactured by depositing a thick 316L stainless steel structure onto an SLM 316L stainless steel part via CS, followed by heat treatment to modify the microstructure and improve the CS-SLM adhesion. The microstructure, phase composition and mechanical properties of the as-fabricated and heat-treated part were studied using various materials characterisation methods. Based on the experimental results and analysis, it was found that the CS part had a grain structure similar to the feedstock in the as-fabricated state, while the SLM part was characterised by cellular subgrains confined in coarse grain structures. Due to the ‘fusion’ nature of the process, the SLM part delivered improved mechanical properties when compared to the CS part; however, this difference was reduced after heat treatment through the improvement in the tensile strength of the CS part by over 200% for both helium and nitrogen. Also, heat treatment improved the CS-SLM adhesive strength due to enhanced interface diffusion. Overall, this study demonstrates that the proposed hybrid AM is a promising technique for the manufacture of free-standing CS-SLM components.

Study of the effect of heat treatment on fatigue crack growth behaviour of 316L stainless steel produced by selective laser melting

AbstractThis study is focused on stainless steel type 316L produced by selective laser melting (SLM). This steel is very resistant to corrosion in acidic environments and has extremely good strength properties at elevated temperatures. It is also characterized by a very good weldability. These properties allow for various applications of 316L in different fields. The widespread application of 316L opens up various possibilities for production of parts using SLM. Therefore, it is important to characterize the fatigue crack growth behaviour. In the present paper, the crack growth behaviour of SLM 316L stainless steel has been investigated in its as‐built condition and in different heat treatment conditions. The effect of build orientation on the crack growth path is also studied by performing fatigue crack growth tests on compact tension specimens built at 0° and 45° orientations relative to the build direction. A heat treatment above the recrystallization temperature followed by quenching is shown to create compressive residual stresses that improve the resistance against crack propagation considerably. The 45° build orientation shows crack propagation at an angle to the initial notch plane, which reveals that anisotropy still persists after heat treatment.

Sliding Wear Characteristics and Corrosion Behaviour of Selective Laser Melted 316L Stainless Steel

Stainless steel is one of the most popular materials used for selective laser melting (SLM) processing to produce nearly fully dense components from 3D CAD models. The tribological and corrosion properties of stainless steel components are important in many engineering applications. In this work, the wear behaviour of SLM 316L stainless steel was investigated under dry sliding conditions, and the corrosion properties were measured electrochemically in a chloride containing solution. The results show that as compared to the standard bulk 316L steel, the SLM 316L steel exhibits deteriorated dry sliding wear resistance. The wear rate of SLM steel is dependent on the vol.% porosity in the steel and by obtaining full density it is possible achieve wear resistance similar to that of the standard bulk 316L steel. In the tested chloride containing solution, the general corrosion behaviour of the SLM steel is similar to that of the standard bulk 316L steel, but the SLM steel suffers from a reduced breakdown potential and is more susceptible to pitting corrosion. Efforts have been made to correlate the obtained results with porosity in the SLM steel.