Effect of laser shock peening with and without protective coating on the microstructure and mechanical properties of Ti-alloy

https://doi.org/10.1016/j.optlaseng.2020.106052Get rights and content

Highlights

  • A high potential of LSP to improve surface and mechanical characteristics.

  • Laser interaction increases the surface parameters and average roughness increased.

  • Surface topology better results with the use of transparent and protective layer.

  • Interaction of laser beam increases the microhardness in all case.

Abstract

Owing to their good mechanical properties, titanium based alloys are used for machine parts and devices that operate in demanding regimes and environmental conditions. Laser shock peening (LSP) is an innovative technique for improvement of the surface and mechanical characteristics of the material. In this paper, LSP is employed for additional improvement of already good characteristics of Ti- alloy. Laser shock peening is performed on alloy surface, with and without protective and transparent layer and obtained microstructure, surface characteristics and mechanical properties are compared. LSP improved the surface topology and microhardness of material, producing overall better results when the Ti-alloy surface was coated by transparent and protective layer.

Introduction

Titanium and its alloys are very attractive metallic materials for aircraft, automobile, marine, medicine, chemical and energy industries, due to their low density, high resistance to corrosion, good biocompatibility and high strength-to-weight ratio, as presented by Vaithilingama et al. [1], Liu and Liu [2] and Sonntag [3]. They have been used as medical implants and biomaterials for long-term applications, owing to their excellent capability to prevent corrosion within the body and biocompatibility, as reported by Branemark [4].

Therefore, many research papers are devoted to the investigation of different surface treatments with the aim to improve surface characteristics and mechanical properties of these alloys. Tan et al. [5] have shown that smooth finish of the deposited layer surface can be improved by increasing the particle speed, and proved that deposited layer height decreases with the increase of particle speed. Ma et al. (2017) [6] have performed laser polishing of Ti alloy surface which had a result in enhancing of microhardness and wear resistance. In their research, Saleh et al. [7] proved that microhardness had to be directly related to the volume fraction and the size of the TiC phase, in the case of surface carburizing of Ti-6Al-4 V alloy. Many different surface modifications to improve wear and erosion surfaces characteristics have been employed, such as surface hardening [8], surface cladding [9], thermal spraying [10], plasma spraying [11], electron beam [12] and laser technologies [13], [14]. In the last few decades, laser shock peening appeared as an encouraging and powerful tool for improving stated characteristics, which allows introducing residual stress and strain hardening into surface layers of processed materials. Creation of compressive residual stress on the surface of Ti alloy helped to increase the fatigue strength over 10 percent, as presented by Thomas and Jackson [15] and Li et al. [16]. Compared to the traditional shot peening, LSP introduces the shock waves and higher values of the residual stresses, propagating two times deeper into the treated material than during the shot peening treatment, and additionally, the average surface roughness and peak to valley ratio values are notably lower than the ones arisen after shot peening. Pant at al. [17] and Fabbro et al. [18] compared LSP with SP and concluded that LSP reduced FCG rate more than SP when titanium alloy is treated [17], [18]. The main concept of laser shock peening is to introduce high level of surface compressive stresses into the depth of workpiece by using shock waves in order to modify and improve the microstructural and surface characteristics, as shown on titanium alloy by Shepard et al. [19], and on nickel-based superalloys arisen by LSP by Petronic et al. [20], [21].

Biocompatibility is mainly dependant on the surface chemistry of Ti alloys, i.e. presence of elements on the surface, and their distribution may form their corresponding oxides, and reduce the stability of the surface oxide layer, as is presented by Sotereanos et al. [22] and Iwakur et al. [23]. Surface characteristics like energy, roughness, and morphology of Ti alloy biomaterials that could affect the interaction of biomaterial with the human body environment is analysed by Garellik et al. [24], Hsu et al. [25] and Man et al. [26]. It is reported by El‐Labban, et al. [27], Ochonogor et. al. [28] and Lin and Lin [29] that Ti alloys containing 6 percent of Al and 4 percent of V are defined as the low wear and fatigue resistance difficult-to-machine material, and with low thermal conductivity, elastic modulus and hardness. Due to very good combination of their properties, i.e. low elasticity modulus, high biocompatibility, low level of toxicity, high specific strength, Ti and its alloys are nowadays the best option as metallic materials for orthopaedic and dental implants, even if their tribological properties are inferior in comparison with other metallic biomaterials. Titanium alloys are superior to pure titanium in terms of their significantly better fatigue properties, however, Colic et al. [33] reported that fatigue failure of the Ti alloys is still a problem, Čolić et al. [30] so the fatigue crack growth behaviour is of great interest and importance in the common application of these alloys and investigation such as demonstrated by Zhou et al. [31] and Abhay et al. [32]

For the purpose of the experimental investigation and analysis of the mechanical and microstructure characteristics of Ti-alloy after various laser treatments, e.g. direct laser ablation, laser shock peening using transparent layer only, and peening using both, protective and transparent layer are applied, and the results are presented and discussed in this paper.

Section snippets

Experimental setup, materials, and methods

Experimental investigations on Ti-alloy samples are performed in order to analyse possible surface modifications and improvements due to various laser shock peening treatments. The samples are prepared as CT specimens for standard fracture mechanics tests, as reported by Colic et al. [33], in order to initiate cracks in the material, and then analyse the mechanical and surface characteristics of Ti-alloy around the crack propagation zone. The chemical composition of the investigated Ti - alloy

Results and discussion

The SEM microphotograph of Ti-alloy is presented in Fig. 2 and it is evident that the microstructure of the base material is homogeneous.

Results of the energy dispersive spectrometry, corresponding to the microstructure presented in Fig. 2, are listed in Table 3.

Microphotographs in Fig. 3a) - c) present the Ti-alloy microstructure arisen after direct ablation of a picosecond laser beam, microstructure arisen after laser shock peening without the protective layer and microstructure after laser

Conclusion

In this paper, the effect of laser ablation, laser shock peening with and without protective layer on the microstructure and mechanical properties of Ti-alloy. Based on obtained results it could be concluded as follows:

  • After direct interaction of the laser beam, the grain boundary started to segregate and the cracks started to initiate. VC carbides started to form in unfavourable size and shape initiating the crack forming.

  • After LSP treatment without a protective layer, the segregation on grain

Acknowledgements

This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia under projects TR 35040, TR 37021 and OI 172045.

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