Effects of SiC content on phase evolution and corrosion behavior of SiC-reinforced 316L stainless steel matrix composites by laser melting deposition

Highlights

• Laser melting deposition of SiC dispersed 316L stainless steel MMCs was attempted.

• Constituent phase of the MMCs evolved from single γ-Fe to γ-Fe + α-Fe + SiC phases.

• The formation mechanism of α-Fe in the MMCs were explored with the addition of SiC.

• The microhardness of MMCs increased from 362 HV to 974 HV with the SiC content.

• The corrosion resistance and corrosion mechanism of the MMCs were investigated.

Abstract

SiC dispersed (4, 8, 12 and 16 wt%) 316L stainless steel metal matrix composites (MMCs) have been prepared by laser melting deposition (LMD). The constituent phases, microstructure, microhardness and electrochemical properties of the MMCs were investigated as a function of SiC content. Experimental results showed that constituent phases of the MMCs evolved from single γ-(FeCrNi) phase with fcc structure for 4 wt% SiC dispersed MMC to γ-(FeCrNi) + α-(FeCrNi) + SiC phases for the 8, 12 and 16 wt% SiC dispersed MMCs. The presence of α-(FeCrNi) phase was due to the tensile stress resulting from the different coefficient of thermal expansion between SiC ceramic reinforcement and the γ-(FeCrNi) matrix in the MMCs. In addition, iron silicides (Fe₃Si and FeSi) appeared in 16 wt% SiC dispersed MMC. The microstructure was dense, uniform and the addition of SiC obviously refined the solidification microstructure of the MMCs. In the 16 wt% SiC dispersed MMCs, a micro-crack can be clearly observed. The microhardness of MMCs increased obviously from 362 HV to 974 HV with the addition of SiC. Accompanying the increase in hardness, the corrosion current density increased and the charge transfer resistance decreased, and the corrosion resistance of 4 and 8 wt% SiC dispersed MMCs was superior compared to 12 and 16 wt% SiC dispersed MMCs in 3.5 wt% NaCl solution.

Introduction

Laser melting deposition (LMD), as one of laser additive manufacturing techniques, is a promising candidate to produce metal prototypes using metal-based powders, which can directly build near net shaped and complexly formed component parts with a minimum waste of materials from computer aided design (CAD) data without using any molds or tools [1], [2]. LMD involves layer-by-layer materials melting and deposition process by introducing a powder stream into a high energy laser beam, which has a broad application prospects in aircrafts, automotive and chemical industry as long as a three-dimensional CAD project can be created [3], [4]. This technique has the advantage of a fast processing speed, relative cleanliness, high scope of automation and rapid solidification rate, which contributes to the formation of non-equilibrium phases, a dense bonded metallurgically between the layers and a fine microstructure with uniform size distribution of carbides and nonmetallic inclusions [5], [6], [7]. In addition, the alloying flexibility and geometric freedom afforded by LMD can be achieved to produce highly complex components with special compositions that are difficult to fabricate by conventional melting and casting route due to segregation-related hot-workability problems [8]. Recently, LMD has been successfully applied to develop various Fe- or Ni-based alloys with improved properties [9], [10], [11]. Compared with LMD Ni-based alloys, LMD Fe-based alloys have the advantage of low cost in commercial applications and a changeable mechanical properties by varying the volume fraction of various component phases (ferrite, austenite, martensite, and so on) [12]. Among various Fe-based alloys, 316L stainless steel is particularly suitable for preparation by LMD as it is relatively expensive to machine them. 316L stainless steel is widely used in marine, biomedical equipment, chemical and petrochemical plants due to its excellent corrosion resistance in severe corrosive environments, such as sea water, acidic and alkaline media [13], [14]. However, the low hardness and strength usually exert a limitation on it for a wider range of applications. Therefore, particulate-reinforced metal matrix composites (MMCs) offer a solution to the problem, which has the advantage of providing the combined favorable properties of the ductile matrix phase and the harder reinforcing phase that can be tailored as desired. So far, the studies on the Fe-based composites have been extensively reported by various investigators [15], [16], [17], [18], [19]. Li et al. [15] have thoroughly investigated the effect of different kinds of ceramic particles (SiC, Cr₃C₂, TiC and Ti(C,N)) on the strength, Young’s modulus and hardness properties of Fe-based composites. This particular study revealed that SiC reinforced particles exhibited the strongest effect on improving the strength of the composite, which can be attributed to its high fracture toughness and hardness as well as a limited decomposition. Ramesh et al. [16] have developed direct metal laser sintering of iron-SiC composites for enhancing the mechanical properties. They summarized that with the increase of SiC content, an increase in microhardness and a decrease in density of laser sintered iron-SiC composites were achieved. The wear rate for iron was 300% more when compared with iron-SiC composite where the SiC content was 3 wt%. Hu et al. [17] have concerned the effect of SiC content on the microstructure of SiC reinforced ferritic steel composites. They reported that the densification of the iron-SiC composites declined and phase transformation from α-Fe to γ-Fe was inhibited with the addition of SiC. Apparently, SiC was commonly employed as the reinforcing phase in Fe-based alloys owing to its high strength and chemical stability, especially in harsh environments [20]. In addition, previous studies mainly focused on the hardness and wear resistance of iron-SiC composites [21], [22], while studies on the effect of SiC content on the phase evolution and corrosion resistance of the austenitic 316L stainless steel are scarcely reported in the literature.

In this work, SiC dispersed (4, 8, 12 and 16 wt%) 316L stainless steel MMCs have been prepared by LMD under optimum processing conditions. The phase evolution, microstructure, hardness and corrosion resistance of SiC dispersed 316L stainless steel MMCs were investigated and discussed. The current study aims at (a) synthesizing the SiC dispersed 316L stainless steel MMCs by LMD; (b) exploring the effect of SiC content on the phase evolution of the 316L stainless steel and (c) optimizing the SiC content with the excellent properties and providing essentials for further composition design.