Effect of laser shock peening without protective coating on the surface mechanical properties of NiTi alloy
Introduction
The nickel-titanium alloy (NiTi) has been widely used for biomedical parts and microelectromechanical component due to its super-elastic properties and shape memory effect. When NiTi alloys are used as orthopedic instruments and dentures, severe wear would occur, and wear debris will induce the implanted material to fail eventually. The friction environment in the human body is very harsh, hence it is important to enhance the surface mechanical property of the component to decrease the wearing damage and extend the service time. In order to increase the surface hardness and abrasion resistance of NiTi parts, different surface treatment techniques have been studied and applied such as surface mechanical attrition treatment, ion implantation, and laser nitriding. The surface mechanical attrition treatment refine the grain size to nanometer-scale caused by the severe plastic deformation and meanwhile the treatment does not change the chemical composition in the top layer of NiTi parts [1], [2]. The enhanced surface layer with high hardness and small grain size can increase the wear resistance of NiTi alloy. Oxygen or niobium ion implantation into NiTi alloy can decrease its friction coefficient and increase its wear property through the formation of new layer with other chemical composition [3]. Laser nitriding is also an effective technique to form TiN compound layer on the surface of NiTi alloy to reduce friction coefficient and enhance surface hardness, which would induce a better surface mechanical property [4].
Different from the above surface treatment techniques, laser shock peening (LSP) is one kind of cold working and non-contact treatment method, which has been used to improve the mechanical properties of different metallic materials, such as magnesium alloys, aluminum alloys, steels, and titanium alloys. Zhang and Siddaiah et al. study the effect of LSP treatment on mechanical properties of AZ31 magnesium alloy and find that the LSP treatment is an effective surface treatment technique to increase hardness and wear resistance [5], [6]. When titanium alloys, such as TC11 alloy and Ti6Al4V, are processed by LSP, it is found that the enhanced microhardness and induced compressive residual stress would produce special layer with a better abrasion resistance [7], [8]. LSP is also used to modify NiTi alloy for improving its fatigue behavior and corrosion resistance, in which the higher microhardness is caused by the high-pressure shock wave [9], [10]. In most of the researches about LSP treatment, the protective coating, such as black tape or Al foil, is used to prevent the surface from laser ablation. However, few researches have done to investigate the feasibility and influence of LSP without protective coating on wear resistance of NiTi alloy. The aim of the present research is to study the effect of LSP treatment without protective coating on the wear resistance and microstructure of NiTi alloy. The surface topography, hardness, residual stress, and wear resistance of NiTi sample with and without LSP treatment are measured and studied.
Section snippets
Materials and methods
The NiTi alloy with 50.80%Ni-49.20%Ti (heated 800 ℃ for 30 min) from Ingpuls GmbH with thickness of 1 mm is treated by laser shock peening with a nanosecond switched Nd:YAG laser at room temperature. The water layer with a thickness less than 1 mm is used as a confinement layer to generate higher pressure of shock wave. The protective coating is not used in the study, so the laser beam penetrates the water layer and irradiates directly on the surface of NiTi alloy (as shown in Fig. 1). The
Surface and cross-section morphology
Fig. 2 shows the surface SEM images and EDX results on the surface NiTi alloy after laser shock peening without protective coating with different laser pulse energies. From the SEM images, the melting layer and small holes can be found. They can be assigned to direct laser melting of the NiTi surface because the laser peening was done without protective layer. Normally, the protective layer, such as black tape, Al film, and black paint, is used to prevent laser damage on the surface of the
Conclusions
In this study, the NiTi alloy is processed by laser shock peening without any protective coating. The ablative layer is produced due to the laser ablation, which increases the surface roughness of the NiTi specimen. Micro-cracks can be observed in the top region of the LSPed specimen with 5 J (20 ns). The Vickers hardness on the surface of NiTi alloy increases and the thickness of the affected layer is about 220 µm, but the difference of hardness induced by 3 J (10 ns) and 5 J (20 ns) is not
CRediT authorship contribution statement
Hao Wang: Experimental design and data analysis. Hao Wang: Writing − original draft preparation. Hao Wang, Sören Keller: Equipment and experiment preparation. Sören Keller: Writing − review & editing. Yuling Chang: Writing − review & editing. Nikolai Kashaev: Writing − review & editing. Kai Yan: Writing − review & editing. Evgeny L. Gurevich: Writing − review & editing. Andreas Ostendorf: Project administration.
Funding
This research was funded by the China Scholarship Council (CSC, No. 201706340042).
Declaration of Competing Interest
The authors declare no conflict of interest.
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