Effects of different surface modifications on the fatigue life of selective laser melted 15–5 PH stainless steel

Abstract

Effects of machining, electro-polishing and laser surface re-melting (LRM) on the fatigue life of Selective Laser Melted (SLM) 15–5 precipitation hardening (PH) stainless steel are reported. Electro-polished surface showed ~97.4% reduction of surface roughness (R_a) and hence improved the fatigue life drastically (~100%) as compared to as-built specimens. Synergic effect of both compressive residual stress and reduction in surface roughness caused drastic improvement (~138%) in fatigue life of machined SLM specimens when compared to its as-built counterpart. Laser Surface Re-melting is found to be an effective technique to reduce the surface roughness (~91.2%) as well as sub-surface defects of geometrically complex as-built SLM specimen and thus improved its fatigue life by ~119%. Fractography analysis showed surface roughness, sub-surface defects and micro notches as the primary crack initiating factors for SLM specimens. High surface roughness in as-built part causes multiple crack initiation sites for specimens failed under both low and high stress amplitude. However, for machined, electro-polished and LRM SLM specimens distinct and multiple crack initiation sites could be observed when specimen failed under high cycle and low cycle regime respectively. However, for all the cases, fatigue life is found to be less compared to its wrought counterpart. Present study could be used as a guideline to select proper surface modification methods for SLM 15–5 PH stainless steel for a desired fatigue life.

Introduction

With recent advancement in manufacturing technologies, Additive Manufacturing (AM) seems to be a viable option for many industries including aerospace, engineering and medical. The process allows fabrication of a geometrically complex part layer by layer from its digital footprint. Among various AM processes, Selective Laser Melting commonly known as SLM process is a Laser Powder Bed Fusion (LPBF) process in which a fine layer of metal powder is spread over a build plate and a focused laser beam is scanned following a particular scan path and strategy along the powder bed under controlled/inert gas environment. Due to high heat flux associated with laser, material gets melted in a controlled amount wherever laser beam is focused followed by rapid solidification after the laser beam moves away. Patterns of these solidified tracks are governed by the laser scan strategy, and the combination of individual tracks within a plane forms a thin metal layer. These steps are repeated until the part is completely built. SLM process is known for better powder catchment efficiency and precision in part features. However, since the final part is submerged in the powder bed, powder particles stick to the surface along the outer periphery of the part and degrade the surface quality. This calls for post-processing such as machining/electro-polishing to improve the surface quality before its functional usage.

In spite of having several advantages, wide spread adoption of SLM parts in industries seems to be still challenging due to uncertainty in its mechanical properties, mainly in dynamic loading conditions e.g. fatigue life [1]. Among various factors which influences fatigue life of SLM parts, surface condition is one of the critical parameters [2]. Smoother the surface i.e. lesser the stress concentration points on the surface, more the fatigue life is. One of the highly researched materials in SLM process is TiAl6V4 because of its numerous applications in aerospace and medical industries. Kasperovich and Hausmann [3] compared high cycle fatigue life of SLM TiAl6V4 alloy under two different surface conditions, as-built and machined. It was reported that the surface undulations of as-built SLM TiAl6V4 specimens act as stress concentration points and result in lower fatigue life when compared to machined SLM specimens. Since there may be some faces of geometrically complex SLM components for which machining is not feasible, it was recommended to consider the lower fatigue strength of rough as-built surface while calculating overall life of the SLM component. Greitemeier et al. [4] studied the effect of surface roughness on fatigue life of both as-built SLM and EBM Ti–6Al–4V specimens. Relatively lower surface roughness was reported for SLM specimens while compared to specimens built using EBM. Both as-built SLM and EBM Ti–6Al–4V specimens reported to have poor fatigue life compared to the machined specimens. Mower and Long [5] reported relatively lower fatigue life of as-built SLM AlSi10Mg and Ti6Al4V compared to that of the conventionally manufactured material. Lower fatigue strength was attributed to the multiple crack initiation sites originated from surface defects, internal voids and micro-cracks. Fatigue strengths of horizontally built SLM 316L and 17–4 PH stainless steel were found to be comparable to wrought materials; while the specimens built in vertical direction showed relatively lower fatigue life. Post-processing such as Hot Isostatic Pressing (HIP) was recommended to improve fatigue life of SLM specimens. Uhlmann et al. [6] compared fatigue life of SLM SS 316L under as-built and machined condition. Another batch of as-built specimens was subjected to vibratory finishing process and its fatigue life was also evaluated. It was found that SLM specimens undergone machining operations has highest, while as-built SLM specimens has lowest fatigue life. Specimens undergone vibratory finishing process did not show significant improvement in fatigue life.

In a recent time, efforts have been made to investigate the effect of laser surface re-melting (LRM) to improve surface roughness and reduce sub-surface pores. Mohammadi and Asgari [7] studied the effect of different combination of skin-core parameters on the surface roughness of SLM AlSi10Mg. It was found that the selected skin-core parameters yield better result in terms of both reduced porosity and surface roughness compared to that of built using standard process parameters suggested by the powder manufacturer. Effect of laser re-melting on deposition quality, residual stress, microstructure, and mechanical property of SLM Ti–5Al-2.5Sn alloy has been investigated by Wei et al. [8]. In this study, LRM of top surface of the SLM specimen in cube form was carried out and reduced surface roughness was reported. However, improvement of surface roughness of side-surfaces of the cube was not reported. A strong texture along (30° 45° 0°) and (90° 90° 20°) in the Euler space as well as tensile residual stress was reported for LRM SLM specimen.

Precipitation hardening (PH) stainless steels are getting popular day-by-day due to their excellent mechanical and corrosion properties at both room and high temperatures. Among various PH stainless steel, 15–5 PH stainless steel which is martensitic steel is very popular in aerospace, medical, chemical and many other engineering applications [[9], [10], [11]]. Rafi et al. [12] studied fatigue life and fracture behavior of SLM Ti–6Al–4V and 15–5 PH stainless steel and reported two different types of fatigue fracture behavior. For 15–5 PH stainless steel crack initiated from the surface, whereas crack initiation occurred deep in the subsurface for Ti–6Al–4V. Specimens of both materials were built vertically and age hardened, and the estimated fatigue life was comparable to that of the wrought material. Spierings et al. [13] reported fatigue life of the polished SLM 316L specimens to be 25% lower than the wrought material. However, fatigue life of low cycle regime was found to be similar. Endurance limit of 15–5 PH stainless steel was found to be 20% lower than the wrought material. Some of the SLM specimens of both materials were machined from round cylindrical bars and polished using a buffing wheel to a surface roughness of 0.1 μm (R_a). Polishing could improve high cycle fatigue life while it has little effect on low cycle fatigue life of SLM 316L specimens. Fatigue life at higher stress cycles were reported to be relatively low for both as-built and polished 15–5 PH stainless steel specimens. In a recent study, Nezhadfar et al. [2] investigated synergic effects of heat treatment and surface roughness on fatigue life of SLM 17–4 PH stainless steel. They reported that the fatigue life of machined and polished heat treated SLM specimens is comparable to its wrought counterpart. From fractography analysis, it was concluded that surface micro-notches and internal pores are the primary crack initiating factors in as-built and machined specimens, respectively. Working in the same direction, effect of different surface conditions e.g. as-built and machining on HCF life of SLM 17–4 PH stainless steel was reported by Stoffregen et al. [14]. They also reported higher fatigue life of machined SLM specimen than its as-built counterpart. Yasa et al. [15] reported influence of laser re‐melting on density, surface quality and microstructure of SLM 316L parts. 90% improvement in surface roughness for LRM specimens was reported. Reduction in sub-surface porosity and 100% density in the outer shell of the part at the cost of higher production time were reported. However, flatness of the top surface after LRM was compromised due to the ‘edge effect’ [16]. While the above effort was made in-situ i.e. LRM was carried out in SLM machine itself, limited build volume of the machine is a bottleneck in carrying out of LRM of side surface of SLM parts with relatively higher dimensions.

Reported studies mostly compare fatigue life of SLM parts either in as-built and machined or as-built and polished conditions. However, it may not be always feasible to carryout machining of parts having geometrically complex features. Electro-polishing can be used to reduce surface asperities of as-built SLM specimens. It is an electrochemical process in which atoms of a workpiece (anode) submerged in an electrolyte convert into ions and are removed from the surface as a result of a passage of an electric current [17]. The process can reduce micro-asperities imparting better fatigue life of the part. However, large surface defects or roughness cannot be removed and also electro-polishing multiphase alloys may cause roughening due to selective dissolution of different phases.

Another alternative could be ex-situ laser surface re-melting which can be used to reduce surface asperities and thus improve fatigue life of as-built SLM specimen. With suitable geometrical optics and integrating optical fiber with a robotic arm, the laser beam can be manipulated and scanned over the surface of a geometrically complex SLM parts with ease.

It may be mentioned that it is a common practice to carryout heat treatment (mainly solution annealing and ageing) of SLM 15–5 PH stainless steel to remove thermal residual stress and induce precipitation strengthening of the matrix respectively. However, heat treatment causes changes in the bulk material properties which may not be always desirable. In one of the authors’ works [10], it has been shown that ageing heat treatment makes the material brittle and thus increases its wear rate. In another recent work [18] by the authors, it has been shown that the material becomes more sensitive to defects in high cycle fatigue regime and thus gives lower fatigue life for aged specimen when compared to as-built condition. A detailed investigation on the effect of different heat treatments on both quasi-static data and fatigue life of SLM 15–5 PH stainless steel has been carried out and benchmarked against wrought counterpart under similar test condition in this study. Also, for SA heat treatment, when the material is air cooled, there is a formation of oxide scale which needs to be removed and calls for additional post-processing involving both time and cost.

Considering the above facts and inferring from the study of Rafi et al., 2013 [12] that the surface and sub-surface defects are the major factors affecting the fatigue characteristics of SLM 15-5PH stainless steel, the present study is aimed to investigate possible alternative methods, namely machining, laser surface re-melting and electro-polishing to reduce surface and sub-surface defects with minimum post-processing and achieve desirable/improve fatigue life of SLM 15–5 PH stainless steel parts with no heat treatment subjected. To best of authors’ knowledge, effects of ex-situ laser surface re-melting on fatigue life of SLM 15–5 PH stainless steel is being reported for the first time. Fatigue test has been conducted under rotating bending fatigue testing condition. Fatigue behavior has been analyzed and correlated with the data obtained from various micro-structural characterizations, and fatigue life of SLM parts has been benchmarked against the wrought counterpart.