Wear Behavior of TiNi and TiNi-TiC Clads Deposited by TIG Surfacing

Transcription

1 Southern Methodist University From the SelectedWorks of Masoud Harooni 2012 Wear Behavior of TiNi and TiNi-TiC Clads Deposited by TIG Surfacing Masoud Harooni, Southern Methodist University Morteza Shamanian, Isfahan University of Technology Alireza Fadaei Tehrani, Isfahan University of Technology Available at:

2 Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition IMECE2012 November 9-15, 2012, Houston, Texas, USA IMECE WEAR BEHAVIOR OF TINI AND TINI-TIC CLADS DEPOSITED BY TIG SURFACING PROCESS Masoud Harooni Mechanical Department, IUT Isfahan, IRAN Morteza Shamanian Materials Department, IUT Isfahan, IRAN Alireza Fadaei Tehrani Mechanical Department, IUT Isfahan, IRAN ABSTRACT This paper deals with the investigation of wear behavior of TiNi and TiNi-TiC clads applied to plain carbon steel by tungsten inert gas (TIG) surfacing process. In this regard, ball milled Ti-Ni and Ti-Ni-C powder mixtures were used as feedstock materials. The clad layers were investigated using X-ray diffractometery (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), hardness measurements and reciprocating wear tests. The results of microhardness measurements indicated that hardness values of TiNi- TiC composites were higher than those of TiNi by approximately 100HV. It was also shown that the wear behavior of TiNi-TiC clads was better than that of TiNi. Further investigations indicated that the dominant wear mechanisms in TiNi and TiNi-TiC clad layers were delamination and surface fatigue, respectively. KEYWORDS Titanium-nickel; titanium carbide; surface cladding; wear; hardness; microstructure INTRODUCTION Wear is one of the most frequently encountered failure modes for moving metallic components. The surface modification of metallic parts by application of a high-performance wear-resistant coating is a beneficial approach to enhance the wear performance [1]. These techniques include thermal spraying, hardfacing, cladding, electroplating, hot dipping, chemical and physical vapor deposition processes and etc [2]. The high-energy density sources such as electron beam welding; electric arc and laser have been widely applied to enhance the wear and corrosion resistance of industrial parts [3]. The weld deposition is widely used for surface modification. The application of a hard, wear resistant surface cladding of various metals or alloys on a metallic substrate by weld deposition is a versatile and inexpensive technique, especially in the case of expensive and large components. The thickness of clads ranges from 750 µm to a few millimeters [4]. This process employs a welding method such as submerged arc welding (SAW) or gas tungsten arc welding GTAW. GTAW process would be beneficial due to its easy operation, high mobility and large-scale availability. In addition, GTAW has been proven to be useful for surface modification by using different filler metals [5]. Materials used for surface cladding are different and depended on substrate and required hardness. Many cladding materials are now available in the market such as Fe-based alloys, Ni-based alloys, Co-based alloys, Carbides, etc. [6]. TiNi alloy is a well-known wear-resistant material [7-11] which exhibits an excellent performance during cavitation and erosion [12-15]. This high erosion resistance of TiNi alloy arises from its high work-hardening rate and its pseudoelasticity. The production of components made of TiNi is expensive; consequently, interest in TiNi coating technologies such as cladding is on the rise [13-17]. In this regard, explosive welded TiNi alloys have shown good cavitation resistance [18,19]. The wear resistance of TiNi is dependent on chemical composition and microstructure. For decreasing this dependence, TiNi can be applied as a matrix reinforced with hard particles [15]. It has been reported that hard particles can improve wear properties of TiNi [20]. 1 Copyright 2012 by ASME

3 TiC ceramic particles have higher hardness and thermal stability than chromium carbide and can be used to reinforce Fe-based alloys. Economou et al. [21] reported that the TiC-reinforced materials have bond strengths exceeding 58MPa and sliding resistance of 20% higher than APS WC Co coatings and up to 100% higher than Stellite 6 coatings, depending on the TiC-reinforced matrix. Coatings reinforced with ceramic particles, especially carbide particles, are widely used in many wear resistant applications. Titanium carbide (TiC) can offer the necessary toughness to the coating to operate under high loads and remain stable, up to 1100 ºC, for high temperature applications. Low friction coefficient and density are other advantages of TiC to make it an excellent reinforcement for the cermet coatings [22]. TiC, is an extremely hard compound used for fabrication of hard composites such as cermets [23]. The sliding wear behavior of TIG cladded TiNi and TiNi-TiC has not been reported in the literature. The aim of the present work is to investigate wear behavior of in situ synthesized TiNi and TiNi-TiC clads. For this purpose, two different mixture of powders were prepared by ballmill procedure and then pressed to a bar shape. Then GTAW welding process was performed to melt preplaced coatings on the substrate. Then, the clad layers were investigated using optical microscope, X-ray diffractometery (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), hardness measurements and reciprocating wear tests. EXPERIMENTAL PROCEDURES The base metals ( ) mm for TIG cladding were prepared from DIN steel plates. The substrate was analyzed and it included 0.951% manganese and 0.218% silicon. Coating materials were in powder shape. The powder mixture was prepared by using different powders of titanium, nickel and graphite to produce TiNi for A sample and TiNi-TiC as B sample. Ti, Ni and graphite powders with a mesh size of 250, were mixed separately by equal stoichiometry ratio in argon gas atmosphere using a ceramic ball mill apparatus for 2 hours. Then, the mixed powders were separately pressed in approximately 2 mm thickness for A and B samples and were placed on the surface of substrate as shown in Figure 1. Welding process for DIN was GTAW with parameters shown in Table 1. Dilution rate was 20-25%. Calculated heat input was 1.78 kj/m (V=14, I=165, speed= 1.3 mm/s). Before cladding, the surface of substrate was cleaned with acetone for removal of any defects. During welding a rectifier was used as a power source. The electrode was connected to the positive pole. Welding was done in horizontal-flat position. Figure 1 Pre-placed bar shape samples on simple carbon steel After cladding, samples were cut from the alloyed specimens for microstructural evolution and microhardness measurement. For material characterization of coatings material, the samples were prepared for scanning electron microscopy (SEM), energy dispersive spectrograph (EDS), and X-ray diffraction. Table 1 The welding parameters used in this study Welding parameters Values Welding current (A) 165 Voltage (V) 14 Welding speed (m/s) Argon flow rate (L/min) 4 Electric current type Direct and constant For microstructure evolution, samples were prepared by standard metallographic procedure. Samples etched with 2% nital and the microstructure was investigated by scanning electron microscope (SEM) equipped to EDS (XL SERIES, Philips, Netherlands). This was exploited to survey microstructure, phases, wear surfaces and wear debris. Phases which were formed at the interface of substrate and the associated coatings were characterized by x-ray diffractometer (XRD) Phlips- X'Pert. Vickers microhardness was determined by Wolpert apparatus at a load of 100 gr and loading time of 30 s. The dry sliding wear tests were performed at room temperature by a pin on-disk reciprocating wear machine (Center for Tribology, IUT, Isfahan, Iran) in air. The counterpart was AISI steel ball with a hardness of 63 HRC and a diameter of 6 mm. The wear tests were carried out under a load of 9N and 1000 m distance. The sliding speed was 0.14m/s with amplitude of 8 mm. The dimensions of samples were 110x10x10 mm. Wear resistance in A and B samples were compared to observe TiC particles effect. The weight loss of samples was measured by a scale with a ±10-3 g tolerance, after a specific time interval. Before weight loss measurements, the samples were washed in acetone and then dried in warm air. Powders and wear debris were collected 2 Copyright 2012 by ASME

4 and studied by SEM to determine the wear mechanism. RESULTS AND DISCUSSIONS Hardness, microstructure and composition Figure 2 shows microhardness profile of two clad layers as a function of distance. As it can be seen in this figure, maximum hardness of A sample is about 650 HV and the average hardness is about 500 HV. By comparing this average hardness to the hardness of 200 HV for the DIN simple carbon steel, hardness is increased by more than three times. Also, the average hardness was compared to TiNi bulk material [20, 24-26]. The comparison result shows that this mean hardness is in agreement with finding reported in the literature. In Figure 2, microhardness is increased to above 700HV for B samples as compared to the microhardness of A samples. In other words, the purpose of improving hardness of TiNi matrix with TiC particles is achieved. Interface zone is completely clear and recognizable and after interface zone the hardness is decreased to 200 HV that is hardness of the substrate. It can be noted that, based on phase analysis, the peak points of the microhardness curves are related to the carbide while the low points represent the austenite in the coatings [27]. Figure 2 Hardness profile of A and B specimens Figure 3a shows SEM image of A sample coating on surface. In this figure, fish bone shape of the composition in the matrix is clear. A and B points were tested by EDS and as shown in Figures 3a and 3b, TiNi Intermetallic is formed as a matrix and black-gray regions is included Ti2Ni. In order to make sure that the matrix is a TiNi alloy rather than a mixture of Ti and Ni powder elements, the sample was examined by X-ray diffraction. Figure 3d illustrates the X-ray pattern, which indicates that alloying is achieved. As it is shown in this figure, TiNi and Ti2Ni were formed and this is due to nonequilibrium condition in GTAW process. Figure 3 Phase and microstructure analysis of A specimen: (a) SEM image of layer (b) EDS spectra of TiNi (c) EDS spectra of Ti2Ni (d) XRD pattern of clad layer 3 Copyright 2012 by ASME

5 An SEM surface image of B sample is shown in Figure 4a. In the central region, nets along grain boundaries are formed. The EDS analysis of point A shown in Figure 4b ensures that TiC particles are formed but for more confidence XRD pattern is used. Figure 4c illustrates the X-ray pattern of the B sample. As it is shown in this figure, TiNi Intermetallic and TiC particles were formed in the coating layer. Figure 4 Phase and microstructure analysis of B specimen: (a) SEM image of B specimen clad layer (b) EDS spectra of TiC (c) The XRD pattern of clad layer Wear behavior Wear tests were performed at 9N normal load for the comparison of the wear behaviors of the samples. The weight loss amount is shown in Figure 5. It is clear that the initial wear rate of the specimens is significantly high. At the beginning of the wear process, the contact area between the abrasive pin and the asperities on the surface of specimens is relatively low. This may have caused a higher amount of effective stress on the asperities of the specimen, which led to more plastic deformation of the asperities and more material removal in this stage, which is run- in wear. After a specific sliding distance, a stable wear rate has been achieved. The weight loss in substrate is high as expected and it is 70 mg for the first 100 meter distance. As it can be seen in this figure, the weight loss is considerably lower in the A sample than the substrate for the same load at a fixed sliding distance. Therefore, the sample A, in comparison to substrate lost a negligible amount of mass during 1000 m sliding distance. Hardness of the materials plays an important role in determining the wear characteristics. For metals and single phase materials, wear is generally inversely proportional to the hardness. However for the multiphase alloy, the microstructure also contributes a significant effect on the wear properties [28, 29]. Hardness of substrate is about 200 HV, and that of A sample is about 500 HV. By comparison of A and B samples, B sample lost about one forth amount of mass at the same load during 1000 m sliding distance. This difference in the weight loss may be primarily due to the hardness difference. As discussed earlier, hardness of A sample is about 500 HV, and the hardness of the B sample is about 700 HV. The pin weight loss was negligible in comparison to samples weight losses. Figure 5 Weight losses of substrate, A and B specimens This hardness difference can be attributed to this fact that in B sample with TiC, the clad layer was reinforced with hard carbides, caused hardness to be high and so wear resistance increased subsequently. According to results from other researchers [22-33], composite structure formed from intermetallic and TiC has a better wear resistance. Therefore, this improved wear resistance is attributed to the formation of hard carbides in the microstructure. 4 Copyright 2012 by ASME

6 mechanism by having chopped particles separated from the surface. Figure 6 SEM image of A specimen worn clad layer (a) worn surface morphology and (b) wear debris Examination of the surface wear tracks and debris formed after testing was conducted using SEM in order to identify the predominant wear mechanisms. The micrographs of wear surface and wear debris of the specimens, that is, samples A, B are shown in Figures 6 and 7, respectively. In the present investigation, the wear occurred due to the contact between the two moving surfaces. One was the studied samples A and B, and the other was a small steel pin. The wear mechanism may vary with the change of material, but for all specimens due to the plastic deformation that which is already discussed, adhesion may occur between the two components, followed by tear. In Figure 6a worn surface of A sample is shown. The figure shows wear lines which is indication of lamination and micro crack initiation. Also, this figure demonstrates that the high hardness and impact energy absorption of the pseudoelasticity property of TiNi could effectively counteract and resist the ploughing action [34]. Therefore, predominant wear mechanism was delamination. Wear debris of A sample were given at Figure 6b; also this figure shows that platelets were separated from the surface and confirms delamination wear mechanism. According to lamination theory, shear plastic deformation, crack initiation and propagation near the surfaces leads to debris lamination. Figure 7a shows the micrographs of worn surface of B sample. Carbides are also favorable for crack prevention. This figure exhibits plastic deformation around TiC particles and their separation from surface. This is due to the higher hardness of TiC particles in comparison with the disk material. Therefore, B sample predominant wear mechanism was surface fatigue. Figure 7b also confirms surface fatigue wear Figure 7 SEM image of B specimen worn clad layer (a) worn surface morphology and (b) wear debris CONCLUSIONS In the light of experimental work, the following conclusions can be made: TiNi and TiNi-TiC coatings are feasible to form by GTAW process from powder elements. In TiNi clad layer, overlay hardness is about 650 HV and in TiNi+TiC layer was about HV. Microhardness change profile from the coating surface to substrate is continuous and steady with low inclination. The coatings overlayed by this method resulted to better wear resistance especially in TiNi-TiC composite. The dominant wear mechanism in TiNi and TiNi-TiC clad layers were delamination and surface fatigue, respectively. ACKNOWLEDGEMENTS The authors would like to thank Mrs. M. Karbasi (SEM laboratory), Mr. Sadeghi (Welding laboratory, IUT), and Also Mr. Arabian (Metallography laboratory) for their valuable help in performing the laboratory tests. REFERENCES [1] Y.F. Liu, Z.Y. Xia, J.M. Han, G.L. Zhang, S.Z. Yang (2006) Surf. Coat. Technol. 201: [2] S. Grainger, J. Blunt (1989) Engineering Coating Design and application, Abington Publishing, Cambridge. 5 Copyright 2012 by ASME

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