liquid erosion behavior of gas tungsten arc welded TiNi overlay

Wear 255 (2003) 219–224

Solid/liquid erosion behavior of gas tungsten arc welded TiNi overlay J.R. Weng, J.T. Chang, K.C. Chen∗ , J.L. He Department of Materials Science, Feng Chia University, P.O. Box 25-221, Taichung, Taiwan, ROC

Abstract The gas tungsten arc welding (GTAW) process has been widely used in the surface damage reparation of industrial and hydraulic handling components to yield a hardfacing overlay typically composed of nickel based alloys. The pseudoelasticity of TiNi intermetallic alloy provides excellent fatigue resistance and cavitation erosion resistance. GTAW was chosen to yield a TiNi alloy overlay onto AISI 1048 medium carbon steel substrates. The microstructure of the weld overlay was characterized. Solid/liquid mixture impact tests were carried out to explore the erosion behavior of the overlay. The X-ray diffraction pattern shows that the TiNi overlay preserves the B2 phase structure as TiNi rod source material, hence the TiNi overlay exhibits pseudoelasticity and high fracture toughness. The hardness value of the overlay is increased to 8.6 GPa, far higher than the TiNi rod source material, mostly due to the trace titanium oxide formed during welding. The erosion tests show that the TiNi overlay exhibits ductile feature erosion loss with the maximum erosion loss occurring at a 30◦ impact angle. The work-hardening effect induced by the impact was also found on both the TiNi overlay and AISI 1048 substrate. This feature was intensified in for the TiNi overlay. The GTAW TiNi overlay can thus reduce the erosion rate of AISI 1048 substrate significantly over a wide range of solid/liquid erosion impact angles because of its higher work-hardening effect and high hardness for the as-welded structure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Gas tungsten arc welding; TiNi; Intermetallic; Erosion

1. Introduction TiNi alloy is a well-known shape memory and pseudoelastic alloy. It has also shown great promise as a wear-resistant material [1–5]. Its excellent performance during cavitation erosion and water-jet erosion has also been demonstrated [6–9]. The high erosion resistance of TiNi alloy arises from its high work-hardening rate and pseudoelasticity (reaching to 7–20%, two orders of magnitude larger than ordinary elasticity), which enable its effective impact energy absorption properties with little damage from thermal–elastic phase transformation deformation. Accordingly, TiNi alloy seems to be a good choice for hydraulic system applications. In most cases, hydraulic machines are bulky and the TiNi alloy is expansive. Coating is a way to reduce the cost of TiNi alloy applications. As a result, explosively welded TiNi and thermal spray TiNi coatings have been studied [7,8,10,11]. It was found that TiNi coatings could improve cavitation erosion resistance. Thin TiNi film coatings produced using cathodic arc plasma ion plating have been proven to enhance the cavitation resistance of steel substrates [12,13]. These

∗ Corresponding author. E-mail address: [email protected] (K.C. Chen).

coating processes were proposed to meet a particular requirement in practice, for example, explosive welding are beneficial for large area construction with the cost of intolerable noise, thermal sprayings are used by considering the easy supplying of raw powder materials with, however, the necessity of post-treatment such as surface remelting. Ion plating is capable of producing uniform film on complex-shape work piece but suffering the relatively low film deposition rate. In many cases, the erosion damaged parts require a field repair surface welding by using a portable repair facility. Gas tungsten arc welding (GTAW) process is advantageous to this. Although, GTAW has been proven to be useful for surface repair by using different filler metal, it has never been demonstrated to deposit nickel titanium as an overlay for erosion protection. A GTAW welded TiNi process would be beneficial due to its easy operation, high mobility and large-scale availability, which are basic requirements for the repair of hydraulic systems. In this study, GTAW was chosen to yield a TiNi alloy overlay onto AISI 1048 medium carbon steel substrates. The microstructure of the weld overlay was characterized and solid/liquid mixture impact tests were conducted that modeled water/sand mixture impacts on hydraulic machinery components to explore the erosion behavior of the overlay.

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00120-0

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2. Experimental procedure

Table 2 Erosion test parameters used in this study

The weld overlay source material was prepared by vacuum arc remelting (VAR) using a Ti48.7 Ni49.3 V1 Co1 composition ratio. A minor amount of vanadium was used in this alloy system to increase the stress-induced martensitic transformation (pseudoelasticity) start stress, and lower the martensitic transformation temperature [14,15]. The cobalt addition also reduced the grain size to increase the hardness and lower the martensitic transformation temperature [16]. The vanadium and cobalt addition stabilized the source material into pseudoelastic B2 phase material at room temperature. The pseudoelastic B2 phase structure was determined using X-ray diffraction. The AISI 1048 substrate specimen size was 25 mm × 20 mm × 2.6 mm. The specimens were polished and degreased before welding. The GTAW welding parameters are shown in Table 1. During welding, the TiNi source rod and the tungsten arc was moved on substrate together, and the TiNi source material was melted to form a 5 mm wide welding bead. By moving the tungsten arc and TiNi source rod back and forth, complete specimen surface overlay coverage was obtained. After welding, the specimen surface was machined and polished using abrasive paper. Vikers micro-hardness test of the weld overlay and bare substrate was measured using an indentation load of 500 g before and after the erosion test. The erosion test was carried out according to ASTM G76-83. Fig. 1 shows the erosion tester. Specimens were tested under solid/liquid conditions by mixing silica and water injected at different impingement angles. The erosion test parameters are shown in Table 2.

Test parameters

Value

Erodent material Particle velocity (m/s) Carrier media (liquid) Impingement angle (◦ ) Erodent particle flux (g/min) Erodent particle size (␮m) Test duration (min)

Silica 80 Water 15, 30, 45, 60, 90 2 ± 0.5 50–70 30, 60, 90, 120

Table 1 The welding parameters used in this study Welding parameters

Value

Working current Argon flow Substrate material Rod source composition Rod diameter

150 A 14 l/min AISI 1048 Ti48.7 Ni49.3 V1 Co1 3.5 mm

3. Results and discussion 3.1. Microstructure characterization Fig. 2 shows the hardness of the TiNi source rod material, substrate, weld overlay, and heat-affected zone. The hardness of the weld overlay was much higher than that of the source material. A possible reason is the titanium oxide phase formed during welding. This hard oxide phase appears in the form of a precipitation island grown in the TiNi matrix as darkest spots in Fig. 3(a) and evidenced in Fig. 3(b) by titanium elemental mapping. These islands increase the hardness of the overlay. Micro-segregation in the TiNi matrix is also observed from Fig. 3(a) where the brightest areas are nickel-rich B2 phase and evidenced by nickel elemental mapping in Fig. 3(c). From the XRD patterns for the source rod material and weld overlay, as shown in Fig. 4, the major phase of weld overlay is B2 phase with a minor B19 martensite phase. The as-received source rod material presents a B2 phase, which is the target phase for pseudoelasticity. The width of the diffraction peak corresponding to B19 is broader for the weld overlay than regular peak width. This can be explained by the lattice distortion caused by the residual stress induced by the rapid cooling during welding process. This could also be one of the reasons for the high hardness of the overlay. The diffraction peaks at 2θ = 37◦ and 44◦ are contributed by the aluminum holder used to fix the small sized rod material during X-ray diffraction. It is also interesting to find that no peak corresponding to titanium oxide was present in the XRD pattern of the weld overlay probably due to the relatively too few amount detected. The rapid solidification of the weld overlay gives the coexistence of B2 and B19 phases. Such a nonequilibrium structure is difficult to identify weather it preserve pseudoelasticity from raw material or not. 3.2. Erosion behavior

Fig. 1. Illustration of erosion tester.

The erosion mass loss as a function of time at 30◦ and 60◦ impingement angles, shown in Fig. 5, indicates that the TiNi overlay can enhance the erosion resistance of AISI 1048 steel substrate. The cumulative mass loss exhibits a linear

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Fig. 2. The hardness of source rod, substrate, overlay and heat-affected zone.

response to the test time revealing a constant erosion rate at each impingement angle test. Fig. 6 shows the variation in erosion rate with impact angle for the weld overlay and substrate. Both reached a maximum value at the 30◦ medium impact angle and a minimum value at the 90◦ high impact angle. This shows that both the substrate and weld overlay exhibited ductile behavior during erosion loss, although the erosion loss rate of the weld at high impingement angles was comparable to the base material and presents the feature of brittle material. This can be explained by the multiphase structure of the overlay, as identified in Figs. 3 and 4. The oxides increased the surface hardness by sacrificing fatigue resistance and increasing the consequent brittleness. The micro-cracking and thermal stress induced during the welding process also raised the brittle fracture in erosion mass loss. Further identification of whether ductile or brittle erosion loss in the weld overlay could be obtained by observing the work hardening on the eroded surface. Fig. 7 shows the surface hardness of the weld overlay and substrate after erosion tests at different impact angles. The increasing rate of hardness increased with the impact angle. At low impact angles, the erosion loss was due mainly to cutting, gouging and ploughing. The material was removed fast and the surface hardness increment, shown in Fig. 7, was lower. At normal incident angles the surface experienced a

Fig. 3. (a) SEM micrograph (SEI) and EDS elemental mapping of (b) titanium and (c) nickel of the weld overlay, where the titanium depleted spot indicates the precipitation of titanium oxide. The vertical arrow shown in schematic figure indicates the SEM sampling area.

Fig. 4. The XRD pattern of (a) weld overlay and (b) source rod.

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Fig. 7. The hardness (Hv) as a function of impingement angle of eroded substrate and overlay. The sampling area is at eroded crater center.

Fig. 5. The erosion mass loss as function of time for AISI 1048 substrate and TiNi weld overlay at impingement angle 30◦ and 60◦ .

fatigue process via plastic deformation due to the repeated silica erodent impacts. The specimen thus exhibited higher hardness and lower mass loss. The hardness increment of the TiNi overlay at all impact angles was higher than that for AISI 1048, i.e. a higher work-hardening rate than the

Fig. 6. Erosion rate as a function of impingement angle for AISI 1048 and TiNi weld overlay.

substrate material as expected [6,16], could be ascribed to the mechanical deformation [1,4]. When the impact stress was over the pseudoelastic deformation limit, the ordinary plastic deformation with a dislocation-slip pinning mechanism would occur. Since the work-hardening coefficient of the TiNi alloy was relatively high (2–11 GPa) [16], and the oxide or nickel-rich phase precipitates in TiNi weld overlay also reduced the dislocation mobility. These reasons cause the higher work hardening of TiNi weld overlay than the substrate material. It is therefore unlikely to the contribution of the pseudoelasticity to the decrease of erosion loss as the work-hardening ability does. The pseudoelasticity of the weld overlay in this regard can be out of concern. Fig. 8 macroscopically shows the erosion damage in specimens at different impact angles. The damage craters changed from an elliptical shape at low impact angles into a circular shape at normal impact angles. The comet-tail blurred area in low angle impacted specimens was caused by the ploughing of scattered silica in the erosion test. No visually observable differences could be found between the weld overlay and bare substrate at any particular angle.

Fig. 8. The surface morphologies of specimen after erosion test at different impingement angles: (a) AISI 1048 substrate; (b) welded specimen.

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Fig. 10. The scanning electron micrograph of TiNi overlay surface after (a) 30◦ and (b) 90◦ erosion test.

Fig. 9. The surface morphology of (a) AISI 1048 and (b) TiNi weld overlay after 2 h erosion test at different angles. The arrows indicate impact direction.

To observe the eroded surface morphology microscopically, as shown in Fig. 9, the ploughing scratches on the AISI 1048 specimen were longer with lip-like deformation features more obvious than that for the TiNi weld overlay along the impact direction at low angles. This demonstrates that the high hardness and impact energy absorption from the pseudoelasticity of TiNi could effectively counteract and resist the ploughing action. At high impact angles, most damage appears in the platelet features formed by a backward extrusion process and repeated impacts. This confirms the work-hardening effect and surface fatigue as a function of the impact angle. Higher SEM magnification observations of the eroded overlay surface are shown in Fig. 10, where the ploughing and extrusion induced by particle impingement produces a wavy surface at low impact angles. Some chips shaved by particles can also be seen. In the case of the specimen eroded at normal angles, shown in Fig. 10(b), some stacked thin platelets were formed by the repeated impingement. No brittle fracture events were observed. In view of these results the GTAW process is feasible for welding a TiNi overlay for erosion resistance applications. Even though the precipitated oxide hardens the overlay, it

exhibits ductile behavior and relatively lower loss during erosion progress. By contract, the GTAW weld overlay, produced by remelting of filler material, is less suspicious to the spallation of overlay than any other developed process and could be an alternative to the erosion damaged surface repair. Further studies on the optimization of the welding parameters are necessary. 4. Conclusion In this study, GTAW was chosen to yield TiNi alloy overlay on AISI 1048 medium carbon steel substrates, and the solid/liquid mixture erosion behavior was explored. It can be concluded that the GTAW TiNi overlay maintains the B2 phase structure as source rod material, and the hardness of overlay is increased to threefold of source material by the trace oxide formation. The GTAW TiNi overlay can thus reduce the erosion rate of AISI 1048 substrate significantly over a wide range of impact angle solid/liquid erosion because its higher work-hardening effect and high hardness of the as-welded structure. Acknowledgements The authors wish to thank the National Science Council of Taiwan for its financial support under project “NSC 90-2216-E-035-015”. References [1] J. Singh, T.T. Alpas, Wear 181–183 (1995) 302. [2] D.Y. Li, Scripta Mater. 34 (1996) 195.

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