Gas Tungsten Arc Welding of Titanium Nickel Overlay on Carbon Steel and Stainless Steel

J.M. Chen, J.L. He, K.C. Chen, J.T. Chang

Department of Materials Science and Engineering, Feng Chia University,

No.100 Wenhwa Rd., Seatwen, Taichung 40724, Taiwan

Abstract: Gas tungsten arc welding with the TiNi intermetallic filler material was used to weld overlays onto the SUS 304 stainless steel and AISI 1045 medium carbon steel. The elements from substrate materials diluted the overlay and caused the formation of dendrite structures in overlays. The crystalline phases in overlays on SUS 304 are TiNi-B2, Ti-Ni-B19’, TiNi3 and Ti3Ni4, while those in overlay on AISI 1045 are TiNi-B2 and TiNi3. Keywords: gas tungsten arc welding; titanium nickel.

1. Introduction

Gas tungsten arc (GTA) is the most widely employed type of the plasma torch. The arc plasma is generated between a tungsten electrode and the workpiece material. Its high heating efficiency, low equipment cost and highly controllable characteristics enable it to be widely utilized for material processing such as melting, cutting and welding (GTAW), thermal spraying or decomposition, waste treatments, etc.

TiNi intermetallics, with pseudoelasticity, high work-hardening rate and corrosion resistance, have been demonstrated to reduce cavitation erosion, fluid-jet ero-sion and wear rate in various ways [1-5]. However, the high price and poor machining ability of intermetallic materials limit their use. Coating is a method to reduce the process cost and to extent the applications [6-12]. Compared with other Ti-Ni coating processes such as thermal sprayings and vacuum vapor depositions, gas tungsten arc welding has relative lower equipment cost, easier operation, and higher field applicability which enables it to be a suitable process to overlay or to repair the Ti-Ni cladding onto fixed or huge facilities.

The idea in this study is to weld a titanium nickel (Ti-Ni) overlay onto commonly used steels via using GTAW and to evaluate the enhancement of surface strength. The microstructure of the overlay is also cha-racterized.

2. Experimental procedure

The filler material wire with a composition of 50Ti-50Ni atomic percent and a diameter of 3.5 mm was provided by Hsu-Yang Technologies Company. The size of the SUS 304 and AISI 1045 steel substrate specimens was 20×20×3 mm. The steel specimens were ground and degreased before cladding. The welding was carried out at a current of 85 A and an argon flow rate of 15 l/min. The cladding parameters are listed in Table 1. During the cladding, the tungsten cathode was kept at a distance of 5 mm above the touch point of TiNi filler wire and steel specimen anode, and the torch was ignited to melt the filler and the steel surface at the same time. By

moving the torch and filler wire together back and forth, 5 mm wide welding beads were formed repeatedly to stack Ti-Ni material until the specimen was completely covered by the overlay and the total thickness of welded specimen reached 5.5 mm. Micro Vickers hardness test of the Ti-Ni filler wire, overlays and steel substrate was carried out with an indentation load of 500 g. The crystal structure of the filler wire and overlays was characterized by X-ray diffraction (XRD) with Cu Kα radiation source. The composition of each phase in overlay and the composi-tional depth profile was analyzed using a wavelength dis-persive spectrometer (WDS).

3. Results and discussion

During the trials for obtaining proper arc current, it was found that there are cracks and delamination between the weld overlay and SUS 304 substrate when arc current was set for over 130 A. On the contrary, no delamination occurred between the AISI 1045 steel substrate and the overlay welded at such a high arc current. This may be as a result of less segregation of brittle phase or the lower tensile residual stress due to less difference of hardness between the overlay and AISI 1045 substrate. This im-plies the structure and hardness of weld overlays could be totally unlike the TiNi filler material.

Although there was no delamination on the weld over-lay on AISI 1045 when using high arc current, cracking was found on the overlays on both kinds of substrates during later surface grinding. A relatively lower arc current, 85 A, was therefore chosen in later research to minimize the heat input and the cracking induced by heating and cooling cycles.

Table 1 The cladding parameters used in this study Arc current 85 A Argon flow 15 l/min Substrate material AISI 1045, SUS 304Filler material 50Ti-50Ni at% Filler wire diameter 3.5 mm

By comparing the metallographs of TiNi filler wire, overlays on SUS 304 and on AISI 1045 as shown in Fig. 1, 2 and 3, respectively, it is obvious that the structure of overlay is completely changed as above presumption. In Fig. 2 and 3, the overlays present a dendritic structure with various dendrite sizes. It is a common phenomenon on welded alloys, which is induced by the rapid solidifi-cation of the melt. The size and appearance of the den-drite of the two overlays on SUS 304 and AISI 1045 are different, which may be due to different thermal conduc-tivity and composition of the melt.

From the XRD pattern of TiNi filler wire and overlays on two steel substrate as shown in Fig. 4, it was found that the pattern of TiNi filler wire exhibit broad peaks of martensitc B19’ only, which agreed with its composition [13] and it had been cold-worked during drawing forming. Distinctively, Ti-Ni overlays on both SUS 304 and AISI 1045 steel exhibit a main structure of austenitic B2 phase, but other minor phases are different. The structure of the overlay on SUS 304 comprises minor B19’, TiNi3 and Ti3N4 phases while only TiNi3 phases is detected in the overlay on AISI 1045 steel. The formation of these phases with lower Ti ratio may be due to the dilution of the overlay by various elements from substrate materials or the consumption of titanium which oxidized during cladding. The presence of phases with lower Ti ratio indicates that significant amount of iron elements has been alloyed with Ti-Ni filler material.

The WDS is used to quantitate the composition of overlay and specific dendrite structures. These results shown in Fig. 5 confirm the above inference about dilu-tion of the overlay by the iron or chromium elements from steel substrate. The iron atom tends to substitute nickel in Ti-Ni intermetallics [14], this may promote the forma-tion of B2 and AB3 structure phase and contribute to the diffraction intensity of nickel-rich phases in XRD patterns. Differently, the chromium atom tends to substitute tita-nium which may be the reason for the existence of B19’ phase in the overlay on SUS 304 steel. Similar results had also been reported in the work by Cheng et al. [11]. By comparing the composition of different phases, the brighter dendrites in the metallograph which contain

Fig. 1 The metallograph of the TiNi filler wire.

relatively higher Ti ratio than the darker area might be the B2 phase detected in XRD. The darker region with tiny round precipitations may be a composite of other minor phases. The compositional change and the complication of overlay structure may result in completely different mechanical properties.

(a) (b)

Fig. 2 Metallographs of (a) the upper overlay and (b) the fusion boundary of the overlaid SUS 304 steel by TiNi fillers at the current of 85 A.

(a) (b)

Fig. 3 Metallographs of (a) the upper overlay and (b) the fusion boundary of the overlaid AISI 1045 steel by TiNi fillers at the current of 85 A.

20 μm

20 μm 20 μm

20 μm 20 μm

40 50 60 70 80Diffraction angle 2

Inte

nsit

y

TiNi filler wire

Overlay on SUS 304

Overlay on AISI 1045

B2 B19'TiNi3 Ti3Ni4

Fig. 4 XRD patterns of the TiNi filler wire and the weld overlays on AISI 1045 and SUS 304 steel substrate.

Fig. 5 The metallograph and elemental composition of overlays on (a) SUS 304 (b) AISI 1045 steel substrate.

Table 2 The hardness of different phases in overlays welded at arc current of 85 A on different steels and the TiNi filler wire.

Material Hardness (MPa)

Substrate Dark area

Bright area

Overlay on SUS 304 1993 8359 7183 Overlay on AISI 1045 2718 7263 5989 TiNi filler wire 2232

The EPMA depth profile of specimens shows that the

fusion zone between the overlay and the SUS 304 sub-strate is about 0.1 to 0.2 mm thick, and that of AISI 1045 is about 0.4 to 0.6 mm. The thinner fusion zone might not provide an adequate structural gradient to mitigate the residual stress which causes delamination between the overlay and SUS 304 substrate at high arc current.

The hardness of different phases in overlays, TiNi filler wire, SUS 304 and AISI 1045 steel is listed in Table 2. The hardness of overlay is much higher than that of TiNi filler wire. The reasons could be high residual stress formed during solidification, solid solution hardening, and precipitation hardening by minor phases such as TiNi3 [15], oxides, or carbides. The large difference in the hardness and sharp structure gradient between the the overlay and SUS 304 might be the reason of the delami-nation of overlays at high arc current. The overlays on both substrate materials exhibit a brittle behavior and no expected pseudoelasticity.

4. Conclusion

Gas tungsten arc welding with TiNi intermetallic filler material was used to overlay a Ti-Ni cladding onto the SUS 304 and AISI 1045 steel substrates. By examining the microstructure and composition of overlays, it was concluded that: 1. Due to the compositional dilution and the rapid solidi-

fication during cladding, the overlays present a feature of dendrites surrounded by various precipitates.

2. The fraction of Ti and Ni in overlays is diluted by elements from substrate materials. This composi-tional change induced the formation of nickel-rich phases including major B2 phase and the precipitation of minor TiNi3 and Ti3Ni4 precipitates in overlays.

3. The hardness of overlay is higher than that of TiNi filler wire about 2-fold. This can be attributed to the high residual stress formed during solidification, solid solution hardening, and precipitation hardening by minor phases such as TiNi3, oxides, or carbides.

4. The thinner fusion zone and higher difference in hardness between the overlay and the SUS 304 sub-strate compared to that of AISI 1045 may cause the cracking and the delamination of the overlay on SUS 304.

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