Effect of grinding parameters on microstructure evolution of TC21 titanium alloy with bimodal starting microstructure
Introduction
Titanium alloys commonly worked in high-temperature and high-pressure conditions due to its thermal stability and high damage tolerance. Thermo mechanical process (TMP) was usually used to improve to the mechanical properties of titanium alloys [[1], [2], [3]]. Grind hardening (GH) has been an effective method to improve the hardness and strength as well as applied in many alloys processing [4,5]. The high temperature generated in the grinding zone leads to phase transformation on specimen surface. Besides, the grinding force between abrasive grains and specimen surface is larger than common grinding, which is helped to stress-induced martensitic transformation.
Compared with other high damage tolerant titanium alloys, Ti–6Al–3Mo–2Zr–2Sn–2Nb–1Cr (TC21) [6] has a better performance in mechanical properties, welding properties and comprehensive process performance, which has been broadly utilized in aerospace and aviation industries. In previous studies, the ideal microstructure with high strength and ductility was usually obtained by thermo mechanical process (TMP). It was known that, the critical factors involved temperature, stain rate and stain [7], which were extremely important for the final microstructure and performance. Until now, numerous researches have been given to discuss the hot deformation characteristics of titanium alloy. Li et al. [8] presented an investigation into the effect of deformation speed on martensitic transformation in metastable β titanium alloy. The results show that the yield strength and ductility is sensitive to strain rate and the excellent integrated mechanical properties appeared at strain rate of 10−2/s. Du et al. [9] carried out high-low duplex aging heat treatments to improve mechanical properties of near beta titanium alloy. Gao et al. [10] investigated the relationship between processing parameters and tri-modal microstructure development during isothermal local deformation experiment. The tri-modal microstructure was dominated affected by heating temperature, initial state and cooling mode. As reported by Syed et al. [11], the compression twins were generated by asymmetry stress of TMP process. However, except for the asymmetry stress, martensite failure under higher deformation condition also plays an important role in the stability of final microstructure, as reported by Naydenkin et al. [12]. Meanwhile, the thickness of TiN layer [13] formed on specimen surface has a directly effect on micro-hardness and friction coefficient, thus how to optimize grinding parameters is an urgent problem.
Nevertheless, the microstructure and hardening layer properties of TC21 alloy under GH process were few discussed in detail. In this work, a series of microstructural analysis tests were conducted to study the evolution mechanism of α phase and mechanical properties of hardening layer.
Section snippets
Materials and procedures
Grind hardening (GH) tests were carried out on a BLOHM surface grinder, and then aging treatment at 590 °C for 4 h. The experimental steps and initial microstructure of specimen is shown in Fig. 1. A forged TC21 titanium alloy was developed by Northwest Institute For Non-ferrous Metal Research, which was cut into 30 × 12 × 15 mm cuboid after grinding. The detailed composition of as-received alloy is presented in Table 1. As a two-phase titanium alloy, the β transus temperature (Tβ) was measured
True stress and strain
Fig. 2 shows the stress-strain curves of TC21 titanium alloy obtained under different TMP conditions (temperatures range 850–900 °C, strain rates rang 10-3s-1–10s-1). It can be noted that the flow stress drops as deformation temperature rise and strain rate reduce. A rapidly increase in flow stress until to a peak stress and then decrease continuously to a steady state can be observed from all the curves. The flow stress shows no obvious variations with increase of strain at relatively low
Conclusions
The effect of grinding parameters on microstructural evolution, mechanical property and splitting mechanism of α phase was discussed. The main conclusions could be summarized as follows:
- 1
According to processing maps, the ideal processing region is suggested to be in temperature of 880 °C, strain rate range 10−3–0.03s−1.
- 2
The volume fraction of primary α phase decreases and β grain boundary gradually faded away with increase of grinding depth. The αs phase has two types of changing trend, one is
Declaration of competing interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.
Acknowledgement
This project is supported by National Natural Science Foundation of China (Grant No. 51775101) and Natural Science Foundation of Liaoning Province (Grant No. 2019-KF-01-11).
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