Two patterns of functionally graded materials (FGMs) were successfully fabricated whose compositions gradually varied from 100% stainless steel 316L to 100% Inconel718 superalloy using laser engineered net shaping process. The microstructure characterization, composition analysis, and microhardness along the graded direction were investigated. The comparison revealed the distinctions in solidification behavior, microstructure evolution of two patterns. In the end, the abrasive wear resistance of the material was investigated.
Since the mid-1980s, the processing of Functionally Graded Materials (FGMs) and structures has become an academic interest. They were developed mainly to satisfy the demand of ultra-high-temperature environment and to eliminate the stressed singularities [
Laser Engineered Net Shaping (LENS) process, as one type of rapid manufacturing technology [
In this paper, attempts have been made to fabricate two patterns of Stainless Steel 316L(SS316L)/Inconel718 superalloy FGMs by LENS process, which will expand the application of the technology. Also, the microstructure characterization, composition analysis, microhardness, and abrasive wear-resistance along the graded direction are investigated.
In the present study, two patterns of FGM samples with compositions gradually changing from 100% (volume percent, v/o) SS316L to 100% Inconel718 are successfully fabricated (30 mm × 1 mm × 36 mm, 35 mm × 30 mm × 2 mm, resp.). Figure
Schematic of SS316L/Inconel718 FGMs.
FGM1-tranditional.
FGM2-novel.
The LENS system (Figure
Schematic of of LENS system.
Low-carbon steel (100 mm × 100 mm × 10 mm) is used as the substrate for the experiment. The compositions of SS316L and Inconel718 powders are listed in Table
Chemical composition and size of the powders.
Chemical composition and size of the powders | ||
Powders | ||
SS316L | ≤0.03 C, ≤1.0 Si, ≤2.0 Mn, ≤0.035 P, | 45~90 |
≤0.03 S, 2~3 Mo, 12~15 Ni, 16~18 Cr, Fe (Bal.) | ||
Inconel 718 | <0.10 C, <0.75 Si, <0.50 Mn, <0.015 S, <0.015 P, 17~21 Cr, 50~55 Ni, 2.8~3.3 Mo, 0.2~1.6 Al, 0.6~1.3 Ti, 4.7~5.5 Nb+Ta, <0.006 B, Fe (Bal.) | 45~90 |
The Laser Processing Parameters.
Input Variables | Variation Range |
---|---|
Laser power (W) | 400 |
Scanning velocity (mm/min) | 500 |
Spot diameter (mm) | 1 |
Powder feed rate (g/min) | 4.3~4.4 |
Samples were polished and etched after being cut by wire electron discharge machining. Optical microscope was used to evaluate the microstructure. Quantitative analysis of elemental constituents was determined with electron microprobe (EPMA-1600, with secondary electron resolution as 6 nm). The hardness of the sample along the composition graded was measured by DMH-2LS knoops microhardness tester. The wear properties such as friction coefficient and wear scars were investigated by micro-tribometer (UMT-2).
Figure
Transverse cross-section: (a)–(d) SS316L; (b)–(e) 50%SS316L+50%Inconel718; (c)–(f) Inconel718, (a)(b)(c)- FGM1, (d)(e)(f)- FGM2.
It is indicated that columnar dendritic growth has prevailed in the entire FGM1 sample, except at the edge of the cross-section (Figure
Figure
Figure
The compositional distribution.
Figure
The microhardness along the graded direction.
Just as the former discussion the microstructure of FGM2 in transverse section is rich in directional growth columnar dendrites (Figure
It is considered to be due to the “ferritic-austenitic solidification mode” [
Figure
The wear properties of FGM2: (a) wear scars; (b) friction coefficient; (c) width of wear scars.
Micro cutting mechanism predominates in the dry sliding condition, accompanied by adhesive wear. It can be due to the lower friction coefficient and the deformation-induced phase change that results in the slight damage in 100% SS316L. Meanwhile, the elements of N, Mn can lower the stacking fault energy of austenite stainless steel; this will enhance the ability to defend the deformation. With the adding of Inconel718, brittleness such as carbide particles and intermetallic compounds formed, which will flake easily during the friction. It can be seen in the graded region that the flake brings about the abrasive wear such as the plough and tear, leading to a severe damage. It is considered to be due to the inherent high microhardness (328.6~360.3 HK) that makes 100% Inconel718 keeping the excellent wear-resistance, although the friction coefficient is much higher than other layers.
In the present study, linear composition graded materials of Stainless Steel 316L/Inconel718 superalloy in two patterns have been fabricated using LENS process. The study may expand the scope of FGMs prepared by LENS technology. The microstructure in FGM1 exhibits a columnar to equiaxed transition. It indicates a change of solidification mode in FGM1, and the microhardness shows a corresponding decrease between 0~50 v/o; while the microhardness exhibits a good linear increase in FGM2. In the end, the friction wear behavior of FGM2 is investigated, indicating that both 100% 316L and 100% Inconel718 layers are with excellent wear-resistance.
This research paper was supported by the National Key Basic Research Program of MOST (Grant no. 2009CB724307), Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education (Grant no. 20070141002).