Microstructure characterisation and process optimization of laser assisted rapid fabrication of 316L stainless steel

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1 Applied Surface Science 247 (2005) Microstructure characterisation and process optimization of laser assisted rapid fabrication of 316L stainless steel J. Dutta Majumdar c, *, A. Pinkerton a, Z. Liu b, I. Manna c,l.li a a Department of Mechanical, Aeronautical and Manufacturing Engineering, UMIST, P.O. BOX 88, Manchester M60 1QD, UK b Corrosion and Protection Centre, UMIST, P.O. BOX 88, Manchester M60 1QD, UK c Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur , India Available online 25 February 2005 Abstract In the present study, laser assisted fabrication of 316L stainless steel has been attempted using a high power (1.5 kw) continuous wave diode laser. The main process variables for the present study were applied power density, scan speed and powder feed rate. A detailed microstructural study of the surface and cross-section of the fabricated layer were carried out using optical and scanning electron microscopy to understand the influence of laser parameters on microstructure of the surface and interface between the successive layers. The microstructure of the top layer was equiaxed, the near substrate region was fine dendritic, however, at the interface between two successive layers, it was coarsened. The morphology and degree of fineness of the microstructure was found to vary with laser parameters. The range of grain size (maximum grain size minimum grain size) was taken as a measure of homogeneity. It was found that with increasing the scan speed, the range of grain size was minimized. Micro-porosities were present in the microstructure that reduced with increasing scan speed and found to be minimum at a medium powder feed rate. The optimum processing conditions have been established by correlating the characteristics of the fabricated layer with process parameters. # 2005 Elsevier B.V. All rights reserved. Keywords: Laser; Fabrication; Microstructure; Stainless steel 1. Introduction Laser assisted fabrication is a technique where, fabrication of solid component is achieved by laser melting of the materials in the form of particles/wire, deposition of molten materials on a substrate in a * Corresponding author. Tel.: ; fax: address: jyotsna@metal.iitkgp.ernet.in (J.D. Majumdar). layer-by-layer fashion and thereby, building-up of the full component from the computer aided design (CAD) [1]. This technique has several advantages over conventional fabrication techniques. These advantages include faster processing speed, no requirement of tooling, ability to fabricate complex shapes and retention of metastable microstructure/composition [2,3]. The process has been referred by different names, such as laser engineered net shaping (LENS) [4], direct laser fabrication (DLF) [5] and laser metal /$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 J.D. Majumdar et al. / Applied Surface Science 247 (2005) forming (LMF) [6]. Atwood et al. [7] developed LENS to fabricate components from steel, Ni based superalloys and titanium. Though the properties were satisfactory, the fabrication speed was slow due to a slow rate of powder accumulation. Arcella and Froes [8] reported on the laser forming of titanium. Srivastava et al. [9] reported on the direct laser fabrication of Ti48Al2Mn2Nb alloy and established the role of process parameters on the microstructure of the fabricated product. A homogeneous and refined microstructure was achieved, and thermal stability of the microstructure was established. 316L stainless steel is an important class of stainless steel having a wide range of applications in oil and gas industry, refineries, chemical and petrochemical plants and as biomaterials because of its excellent corrosion properties [10]. Attempts on laser assisted fabrication of AISI 316L stainless steel has already been made by Pinkerton and Li [11] who studied the influence of pulse frequency and duration on the microstructure, surface roughness and hardness of fabricated 316L stainless steel using pulsed wave CO 2 laser. As an extension of previous effort, an attempt has been made to fabricate solid-built made of 316L stainless steel by direct laser deposition technique using a continuous wave diode laser as a source of heat. A detailed study of the influence of laser parameters on the microstructure of the fabricated layers was carried out to optimise the process parameters. 2. Experimental procedure In the present study, gas atomized 316L stainless steel (C < 0.03%, Si 0.7%, Mn 1.7%, Mo 3%, Ni 11%, Cr 18%, balance Fe) powders of particle size mm was used as feedstock material. A laser-line diode laser of wavelengths 940 nm (maximum power of 1.5 kw) with a square beam of spot size of 3 mm was used for processing. Fabrication was done by melting the feedstock powder (delivered by an external powder feeder) using the laser and deposition of the melt on the substrate (mild steel) in a layer-bylayer fashion. The whole process was carried out using Ar as shrouding environment (at a flow rate of 6 l/min) to avoid oxidation during lasing. The substrate was mounted on a CNC controlled X Y sweeping stage and a relative speed between the laser beam and substrate was maintained to ensure the area coverage and a finite time of interaction between the laser beam and powder. The main process variables for the present study were applied power density, P ( kw/mm 2 ), scan speed, v ( mm/s) and powder feed rate, F P ( mg/s). Detailed process variables used in the present study are summarized in Table 1. Microstructural characterization of the top surface and cross-section of the fabricated component was carried out by optical and scanning electron microscopy. Grain or interdendritic spacing was measured by linear intercept technique [12]. Microdefects associated with rapid quenching and its distribution was measured by linear intercept technique [12]. Finally, the optimum processing zone for the fabrication of 316L stainless steel has been derived correlating the characteristics of the formed layer with that of the laser parameters. 3. Results and discussions 3.1. Microstructure Microstructure plays a very crucial role in determining the property of a component. In the present study, a detailed observation of the optical and scanning electron micrographs of the top surface and Table 1 Summary of the applied and optimized process parameters for laser assisted rapid prototyping of AISI 316L stainless steel Applied process parameters Optimum process parameters Power density (kw/mm 2 ) Range of scan speed (mm/s) Range of powder feed rate (mg/s) Power density (kw/mm 2 ) Range of scan speed (mm/s) Range of powder feed rate (mg/s) ,

3 322 J.D. Majumdar et al. / Applied Surface Science 247 (2005) cross-section of laser assisted fabricated layer were undertaken to study the morphology, grain size, defect density of the fabricated components and the effect of laser parameters on it. Fig. 1 shows the scanning electron micrograph of the cross-section of (a) built layer and higher magnification view of (b) zone 1, (c) zone 2, (d) zone 3 of Fig. 1a of the 316L stainless steel layer lased with a power density of kw/mm 2, Fig. 1. Scanning electron micrograph of the (a) full layer and higher magnification view of (b) zone 1, (c) zone 2, and (d) zone 3 of (a) for laser assisted fabricated 316L stainless steel lased with a power density of kw/mm 2, scan speed of 2.5 mm/s and powder feed rate of 203 mg/s. The arrowheads represent micro-porosities.

4 J.D. Majumdar et al. / Applied Surface Science 247 (2005) Fig. 1. (Continued). scan speed of 2.5 mm/s and powder feed rate of 203 mg/s. From Fig. 1(a) it is relevant that the built layer is continuous and homogeneous without presence of any macrocracks. However, the observation of the microstructure of the cross-section at a higher magnification shows that the morphology and degree of fineness depends on the thermal history and hence, varies with the position and laser parameters. The microstructure of the top surface is predominantly equiaxed, due to a uniform heat flow in all the directions (cf. Fig. 1(b)). On the other hand, coarsening of grains with a larger aspect ratio (ratio of diameter along horizontal to vertical directions) is observed at the interface between two successive layers. The grain coarsening is attributed to heating effect by the conducted heat of the solidified pool of the next layer (cf. Fig. 1(c)). In the subsequent layer, the interface was coarsened in the same fashion as in

5 324 J.D. Majumdar et al. / Applied Surface Science 247 (2005) Fig. 2. Area fraction of porosities (%) as a function of (a) scan speed, v (at a powder feed rate of 203 mg/s) and (b) powder feed rate, F p (at v = 5 mm/s) for applied power density of (1) kw/mm 2 and (2) kw/mm 2. Fig. 1(c). On the other hand, the microstructure near the substrate region is predominantly columnar and highly refined (as evident from Fig. 1(d)). The detailed variation of morphology and grain size of the microstructures with laser parameters is under investigation. Fig. 1(b) (d) reveals the presence of a large number of micro-porosities (as shown by arrowheads). Presence of micro-porosities is detrimental to mechanical and electrochemical properties and hence, should be minimized. Hence, a detailed study on the variation of area fraction of microporosities with laser parameters was undertaken and correlated with laser parameters. Fig. 2(a) and (b) shows the area fraction of porosities (%) present in the microstructure at different (a) scan speed (at a powder feed rate, F p = 203 mg/s) and (b) powder feed rate (at a scan speed, v = 5 mm/s) for an applied power density of (1) kw/mm 2 and (2) kw/mm 2, respectively. It shows that increasing scan speed reduces the area fraction of porosities in the microstructure (cf. Fig. 2(a)). On the other hand, at an intermediate value of powder feed rate the area Fig. 3. Range of grain size as a function of applied power density for laser assisted fabricated 316L stainless steed lased with (a) scan speed, v of 5 mm/s, powder feed rate, F p of 203 mg/s; (b) v = 5 mm/ s, F p = 68 mg/s; (c) v = 2.5 mm/s, F p = 203 mg/s, respectively. fraction of porosities is minimized (cf. Fig. 2(b)). Area fraction of porosity was however, independent of applied laser power density. It may be concluded that porosities was originated partially by entrapment of gas from atmosphere or shrouding environment. As a result of which increase in scan speed (reduced interaction time) reduces the area fraction of porosities. Hence, choice of increased scan speed is beneficial to reduce the porosity content in the microstructure.

6 J.D. Majumdar et al. / Applied Surface Science 247 (2005) A microstructural homogeneity plays an important role to produce a uniform mechanical and electrochemical properties. In laser assisted fabricated part, variation in grain size at different position of the fabricated layer is a commonly encountered problem, especially coarsening of grains at the interface between two successive layers (as shown in Fig. 1). However, the degree of coarsening and the homogeneity were found to vary with laser parameters. Hence, the range of grain size (maximum minimum) was chosen as a measure of the degree of homogeneity and the lower the grain size range, the more homogeneous the microstructure is. It was found that the range of grain size was dependent on the combination of applied power density and scan speed. Application of low scan speed and high power Fig. 4. Scanning electron micrographs of the top surface of laser fabricated AISI 316L stainless steel with a power density of (a) kw/mm 2, and (b) W/mm 2 and at a scan speed of 2.5 mm/s, powder feed rate of 136 mg/s.

7 326 J.D. Majumdar et al. / Applied Surface Science 247 (2005) (0.091 kw/mm 2 ) was found to cause a very large range of grain sizes and hence, detrimental to properties (cf. Fig. 3(a)). However, under the similar combination of scan speed and powder feed rate, the grain size range may be minimized by the application of a lower power density (0.031 kw/mm 2 ). On the other hand, it was found that by increasing scan speed (reducing interaction time), range of grain size can be minimized at any level of applied power density (cf. Fig. 3(c)), attributed to a faster rate of heat conduction and less accumulation of heat at the interface between two successive layers. The range of grain size was however, found to be independent of powder feed rate (cf. Fig. 3(c) vis-à-vis Fig. 3(b)). Hence, a proper combination of applied power density and scan speed is essential to minimize the range of grain size and so, homogenize the microstructure Parametric optimization From the above-mentioned discussions, it may be concluded that laser processing at an increased scan speed is beneficial to reduce area fraction of porosities. On the other hand, a proper combination of applied power density and scan speed should be chosen to minimize the range of grain size, i.e. develop a homogeneous microstructure. Taking into consideration of all the independent effects of applied laser power density, scan speed and powder flow rate on the micro-porosities and microstructural homogeneities, the optimum processing conditions have been established and are summarized in Table 1. The optimum processing zone mentioned in Table 1 ensures uniform microstructure and hence, properties throughout the component, however, it does not ensure improved properties of the fabricated layer. Fig. 4 shows the microstructures of the laser fabricated AISI 316L stainless steel with a power density of (a) kw/mm 2, and (b) kw/mm 2 and at a scan speed of 5 mm/s, and a powder feed rate of 136 mg/s (optimum processing conditions). A comparison of Fig. 4(a) with Fig. 4(b) reveals a significant grain coarsening with application of a higher power density, predominantly because of a slow rate of cooling at a higher applied power density [13]. The morphology was also found to vary with laser parameters. At a lower power density, the morphology is a mixture of cellular and dendritic. On the other hand, increasing power forms mainly cellular morphology with a larger grain size. In this regard, it may be concluded that the morphology of the microstructure is dependent on the ratio of the thermal gradient of the liquid at the solid liquid interface to solidification velocity which varies with applied energy density and position [14]. 4. Conclusions In the present study, the detailed microstructural characterization of laser assisted fabrication of 316 stainless steel was carried out with a power density of kw/mm 2, scan speed of mm/s and powder feed rate of mg/s. From the results, the following conclusions may be drawn: 1. The microstructure of the fabricated component was found to be equiaxed near the surface region, very course and columnar at the interface between two layers and fine columnar near the substrate region. 2. The grain size was coarsened with increase in applied power density and decrease in scan speed. The range of grain size (maximum minimum) was reduced with increasing scan speed. 3. A large area fraction of micro-porosities were present in the microstructure which was found to be reduced with increase in scan speed. It was also found that at an intermediate powder flow rate the porosity content was minimized. 4. The optimum processing conditions for laser assisted fabrication of AISI 316L stainless steel are as follows: for an applied power density of kw/mm 2, scan speed of mm/s and powder feed rate of mg/s; for an applied power density of kw/mm 2, scan speed of mm/s, powder feed rate of 136 mg/ s; for an applied power density of kw/mm 2, scan speed of 7.5 mm/s and powder feed rate of 203 mg/s. Acknowledgements The financial support from Department of Science and Technology, India (BOYSCAST SCHEME) and

8 J.D. Majumdar et al. / Applied Surface Science 247 (2005) Board of Research in Nuclear Science (BRNS), Bombay to JDM for the present study is gratefully acknowledged. References [1] W.M. Steen (Ed.), Laser Material Processing, Springer-Verlag, New York, [2] J. Laeng, J.G. Stewart, F.W. Liou, Int. J. Prod. Res. 38 (2000) [3] J. Dutta Majumdar, I. Manna, Sadhana (2003) [4] W.M. Steen, M.A. McLean, G.J. Shannon, in: M. Glenna, M. Vollertsen (Eds.), Laser Assisted Net Shape Engineering (LANE97), Meisenbach, Bamberg, 1997, p [5] G.K. Lewis, R.B. Nemec, J.O. Milewski, D.L. Thoma, M.R. Berbe, D.A. Cremers, Directed light fabrication, in: Proceedings of the ICALEO 94, Laser Institute of America, Orlando, FL, 1994, p. 17. [6] M. Gaumann, S. Henry, F. Cleton, J.-D. Wagniere, W. Kurz, Mater. Sci. Eng. A 271 (1999) 232. [7] C.L. Atwood, M.L. Griffith, L.D. Harwell, et al., Sandia Report, [8] F.G. Arcella, F.H. Froes, J. Met. 52 (5) (2000) 28. [9] D. Srivastava, I.T.H. Chang, M.H. Loretto, Intermetallics 9 (2001) [10] C.J. Novak, in: D. Peckner, I.M. Bernstein (Eds.), Handbook of Stainless Steels, McGraw-Hill, New York, 1977, p. 1. [11] A.J. Pinkerton, L. Li, Appl. Surf. Sci. 208 (2003) 405. [12] J. Hilliard, in: R.T. Dehoff, F.N. Rhines (Eds.), Quantitative Microscopy, McGraw-Hill Book Company, London, 1968, p. 45. [13] J. Dutta Majumdar, I. Manna, Lasers Eng. 12 (2002) 171. [14] M.C. Flemmings (Ed.), Solidification Processing, McGraw- Hill, New York, 1974, p. 60.