Elsevier

Materials Letters

Volume 62, Issue 15, 31 May 2008, Pages 2325-2328
Materials Letters

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

https://doi.org/10.1016/j.matlet.2007.11.082Get rights and content

Abstract

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti–50.0 at.%Ni and Ti–50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.

Introduction

NiTi shape memory alloys (SMA) exhibited good shape memory effect (SME) and pseudoelasticity (PE), and had been extensively used for industrial applications. Unfortunately a relatively low formability of NiTi alloys had been found during the joint of different NiTi alloys pieces in the device and components. Recently laser welding had gradually become one of the most efficient and important jointing techniques for NiTi alloys, and the welding of NiTi alloys with different compositions had been of great research interest because it allowed the production of smart components with very attractive functional properties [1], [2], [3], [4]. The aim of the present work was to investigate the effect of the Nd:YAG laser welding process on the microstructure and properties of the welded joints of binary NiTi wires with different compositions.

Section snippets

Experimental procedures

The binary TiNi alloy wires with different compositions (Ti–50.0 at.%Ni and Ti–50.9 at.%Ni) which had the same diameter of 1 mm were used throughout this study. The Ti–50.0 at.%Ni wires were fabricated by hot drawing, and a heat treatment was performed at 850 °C for 0.5 h with water quenching before the welding. The Ti–50.9 at.%Ni wires were fabricated by cold drawing; no heat treatments were performed before welding. The surface oxide layer was removed with a mixed acid solution (HF:HNO3:H2O = 

Microstructure of welded joints

Fig. 1A depicted the OM micrograph of the welded joints. The left side (a) was the base alloy with 50.9 at.%Ni. The right side (c) was the base alloy with 50.0 at.%Ni. As can be seen, the welded joint (b) had a little excessive penetration. Fig. 1B–D were the MM micrographs of the welded joints. As can be seen, the fusion zone of Route I and Route II exhibited columnar and layer microstructure and the heat affected zone consisted of coarse equiaxed grains obviously. The grain size in the fusion

Conclusions

In summary, the welding current and the impulse width had great influence on the quality of the welded NiTi alloy joints by changing the microstructure and mechanical property at the joint position. The highest fracture strength and elongation of the welded joints were obtained with 119 A of the welding current and 2.5 ms of the impulse width. The laser welding had a few effect on the reverse martensitic transformation behavior of the welded joints. All laser-welded joints exhibit the brittle

Acknowledgements

This work was supported by the Outstanding Youth Funding of Heilongjiang Province (JC200608), Department of Science and Technology of Heilongjiang Province, China.

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