Laser engineered net shaping of Co-based superalloys

XUE Chun-fang(薛春芳)¹, DAI Yao(戴 耀)¹, TIAN Xin-li(田欣利)²

1. Department of Mechanical Engineering, Academy of Armored Force Engineering, Beijing 100072, China;

2. National Key Laboratory for Remanufacturing Institute, Academy of Armored Forces Engineering,

Beijing 100072, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Laser engineered net shaping(LENS) process was investigated using Co-based superalloy powder with a high power continuous wave CO₂ laser. Thin wall part with smooth surface was obtained by LENS of layer-by-layer deposition of the powder materials. This thin wall sample was tested for metallographic examinations, micro-hardness, X-ray diffraction and mechanical property test. Microstructural results show that the layers possess rapid solidification microstructural feature, fine dendritic crystal and M7C3-type carbides (essentially chromium-rich carbide) dispersed in the γ(Co,Cr) phase matrix. Dendrite spacing as well as the solidification mode can be controlled through control process parameters. In addition, this microstructural feature of the as-formed Co-base sample leads to an evident hardening and a superior tensile strength and toughness.

Key words: laser engineered net shaping (LENS); Co-based superalloy; microstructure; mechanical properties

1 Introduction

Laser engineered net shaping (LENS) is a upcoming rapid manufacturing technology developed around the world. LENS is capable of near net shaping the components by layer-by-layer deposition of material directly from CAD model using laser. In addition to the processing advantages in net shape, solid freeform and dramatically shortened cycle times are developed for engineering systems. LENS can be used not only in the new production but also in the restoration of worn components. There have been various contributions which have improved the understanding of the process[1,2], and emphasized that it was important that an optimal setting of the processing parameters was required to achieve the deposition of dense, porosity-free coatings. Also, various studies[3-5] have been performed using powder materials such as Ni-based alloys, titanium alloys and iron-based alloys. Among these materials, cobalt-based alloys offer excellent high temperature corrosion resistance and attractive mechanical properties over a wide temperature range.

In this study, the manufacturing by LENS of Co-based alloy on a stainless steel was presented. A thin wall-like component fabricated by LENS was characterized by various mechanical and metallographic examinations. The purpose of this study was focused on the microstructural characterization and mechanical properties of as-formed thin wall sample, and the influence of processing conditions on the microstructure, microhardness, tensile strength and toughness were studied.

2 Experimental

2.1 Materials

The metal powder was used to form the coating consisted of Co-based alloy with a particle size of 40-100 μm, whose chemical compositions(mass fraction, %) are as follows: Ni 10.10, Cr 26.41, W 7.31, C 0.81,  Si 0.44 and balance Co. The melting temperature for this alloy is 1 400 ℃.

The substrate used was a 1Cr18Ni9Ti stainless steel with main compositions of 18Cr, 9Ni and 12C. Before treatment, specimen that was 200 mm in length, 150 mm in width, and 20 mm in thickness was sand-blasted to improve the surface absorptivity and the adhesion between the coating and substrate.

2.2 Test conditions and LENS system

The experiments are carried out with a continuous wave CO₂ laser as a heat source with a variable output power up to 5 kW producing a beam composed of a fundamental mode TEM00 and a wavelength of 10.6 μm. The tests were performed at a power (P) of 2-2.4 kW, and the distance between focal point and surface (Δf)  was 6-10 mm. Fig.1 shows the experimental set-up.

Fig.1 Schematic diagram of set-up for LENS process

Powder was continuously supplied by a powder injection system to substrate, in which the powder was injected through a lateral nozzle with argon gas acting as the carrier gas. The substrate plate was positioned on the x-y axis CNC gantry table. Cladding tracks were formed by moving the specimen relative to the beam in the x-direction. During deposition, the substrate displacement speed under the laser beam was 500 mm/min. In addition, layer was built-up with layer thickness of 0.25 mm. 

2.3 Microstructural and mechanical research

After the samples were cut, polished and etched, the microstructures of layers were examined metallographi-

cally by scanning electron microscopy(SEM). The crystallographic phases were also determined by standard X-ray diffraction(XRD) techniques.

Vickers microhardness (HV_0.5) measurements were made on the cross-section. The hardness tests were carried out with a microhardness testing system(HX-Ⅰ). The mechanical strength and elongation of the deposited component were tested by electronic tensile testing equipment(ZD10/90) at room temperature.

3 Results and discussion

Figs.2 and 3 show the photos of single layer and multi-layer sintering thin wall like sample of Co-based alloy by LENS with smooth surface, under the laser processing parameters of a laser power of 2 kW, the distance between focal point and surface of 10 mm, the powder feed rate of 20 g/min. Its total height is about 20 mm. It can be seen that the coatings are metallurgically bonded to the substrate, and have the thin wall part with smooth surface.

Fig.2 Morphology of laser clad single layer

Fig.3 Thin wall like sample of Co-based alloy deposited by LENS

The results of the mechanical properties test show that tensile strength (σ_b) of the deposited component is 946.5 MPa and the elongation(δ) is 27%. The results also prove that the sample by LENS possesses high strength and high plastic deformation ability.

3.1 Microstructural analysis

Fig.4 shows the SEM images of the multi-cladding layers after etching. It can be seen that the interface between each layer is very clear, no solidification induced defects are found in the deposit, and the deposit appears fully dense as shown in Fig.4(a). Extremely fine and homogeneous microstructure characteristic of rapid solidification structure (dendrite structure) is shown in Fig.4(b).

Since the microstructure determines the material properties, it is essential to understand, and control its formation. The solidification microstructure in the component can be controlled. This means that the mechanical properties of a component can also be controlled through region specific manipulation of the microstructure. Mechanical properties such as fatigue can be improved through dendrite arm spacing refinement. In addition, the significant grain refinement leads to an increase in hardness, strength and toughness. The parameters, such as laser power density, scanning speed, powder-feeding rate, are manipulated to alter the cooling rate and thermal gradient and thus the    solidification dendrite arm spacing (d) can be descrybed as a function of local solidification time (t_f) or the average cooling rate  during solidification as:

                          (1)

where a and b are constants, the exponent n is usually 0.3-0.5[6].

Fig.4 Microstructures in coating: (a) Multi-cladding layer; (b) Dendrite structure

The constants a and b are related because the local solidification time t_f is a function of the average cooling rate during solidification and the solidification temperature range ΔT, which is a physical property of the alloy. Thus:

                              (2)

The average cooling rate in turn is the production of the temperature gradient at the solid/liquid interface (G) and the velocity of the solidification front (R).

Thus:

                   (3)

It is evident that the temperature gradient and the solidification rate are easily manipulated through laser power density, scan speed, scan spacing and thermal boundary conditions at the melt-pool[7]. Therefore the dendrite spacing as well as the solidification mode can be controlled through these parameters.

The higher solidification rates induce a fine microstructure consisting of crystalline phases and an extended solid solubility of key alloying elements. Such microstructures may greatly enhance the high- temperature oxidation characteristics[8].

3.2 Microanalysis

Fig.5 shows the X-ray diffraction pattern of the layers. The result indicates that the major phases in the laser deposited layers are cobalt-rich γ-phase and a chromium-rich carbide M₇C­₃ with typical composition of Cr₇C₃. The presence of chromium-rich carbide Cr₇C₃ in the matrix indicates a high solid state cooling rate experienced in the sample. The austenite is a non-equilibrium phase with extended alloy contents. The existence of austenite may be attributed to the low martensite starting temperature, the high concentration of austenite stabilizing elements, and the rapid cooling rate[9]. The carbide particles are uniformly distributed in a dense Co-based matrix.

Fig.5 X-ray diffraction pattern of clad layers

Vickers microhardness(HV) of the thin-wall sample was measured on the cross-section and along the lines at 90? to the clad-substrate interface. The microhardness results are shown in Fig.6. The results show that there is little variation in hardness within the clad layer. The microhardness values of the coating are less than those of the heat-affected zone(HAZ). This is because that the laser heating is sufficient to austenitise the substrate material close to the surface and the rapid self-quenching provided by the bulk of the material results in the formation of martensite[10]. The hardness values also show that the laser cladding layers are homogeneous. The mean value of microhardness in this region is HV540. The hardness of the base material is about HV320. The reason for this increase in hardness and uniformity for the laser layers is due to high cooling rates, the significant reduction in grain size and strong convection currents in melt pool region. Also, the high hardness of the Co-based alloy layers is attributed to the presence of M₃C₃-type carbides (essentially chromium-rich carbides) dispersed in the γ-(Co) phase matrix. Thus, sample properties can be improved by modifying the microstructure of the layers. In general wear resistance increases with increasing hardness[10].

Fig.6 Variation of microhardness with distance from cladding substrate interface

4 Conclusions

1) Using the LENS processing, single layer and thin wall part of Co-based superalloy with smooth surface, superior tensile strength and toughness are obtained. Extremely fine and homogenous microstructures, characteristic of rapid solidification are obtained in the deposited Co-based superalloy. Furthermore, the solidification microstructure and the mechanical properties can be controlled by changing the laser parameters and the powder parameters.

2) The layers present a dendritic structure composed of a solid solution of γ(Co) and chromium-rich carbide Cr₇C₃ dispersed in the γ(Co) phase matrix, which leads to the high hardness of Co-based superalloy coating

3) LENS has the potentiality to achieve the deposition of dense, crack-free metal parts with superior mechanical properties.

References

[1] ZHANG X D, BRICE C, MAHAFFEY D W, ZHANG H, SCHWENDNER K, EVANS D J, FRASER H L. Characterization of laser-deposited TiAl alloys[J]. Scripta Mater, 2002, 44: 2419-2424.

[2] HIDOUCI A, PELLETIER H M, DUCOIN F, DEZERT D, GUERJOUMA R E. Microstructural and mechanical characteristics of laser coatings[J]. Surface and Coatings Technology, 2000, 123: 17-23.

[3] LI Peng, YANG Tai-ping, LI Sheng, LIU Dong-sheng, HU Qian-wu, XIONG Wei-hao, ZENG Xiao-yan. Direct laser fabrication of nickel alloy samples[J]. International Journal of Machine Tool & Manufacture, 2005, 45: 1288-1294.

[4] WU Xiao-lei. Rapidly solidified nonequilibrium microstructure and phase transformation of laser-synthesized iron-based alloy coating[J]. Surface and Coating Technology, 1999, 115: 153-162.

[5] SHEPELEVA L, MEDRES B, KAPLAN W D, BAMBERGER M, WEISHEIT A. Laser cladding of turbine blades[J]. Surface and Coatings Technology, 2000, 125: 45-48.

[6] SEXTON L, LAVIN S, BYRNE G, KENNEDY A. Laser cladding of aerospace materials[J]. Journal of Materials Processing Technology, 2002, 122: 63-68.

[7] FENG Li-ping, HUANG Wei-dong, LIN Xin, YANG Hai-ou. FGH95 superalloy laser metal forming directional solidification[J]. The Chinese Journal of Nonferrous Metals, 2003, 13(1): 181-187.

[8] DAS S, FUESTING T P, DANYO G, BROWN L E, BEAMAN J J, BOURELL D L. Direct laser fabrication of superalloy cermet abrasive turbine blade tips[J]. Materials and Design, 2000, 21: 63-73.

[9] KAPLAN A F H, GROBOTH G. Process analysis of laser beam cladding[J]. Journal of Manufacturing Science and Engineering, 2001, 123(11): 609-614.

[10] MERKLEIN M, HENNIGE T, GEIGER M. Laser forming of aluminum and aluminum alloys—microstructural investigation[J]. Journal of Materials Processing Technology, 2001, 115: 159-169.

(Edited by YANG You-ping)

Foundation item: Project(51461010101JB3501) supported by National Key Laboratory for High Energy Density Beam Processing Technology Foundation of China

Corresponding author: XUE Chun-fang; Tel: +86-10-66718148; E-mail: xchunfang@sina.com