Impacts of laser cladding residual stress and material properties of functionally graded layers on titanium alloy sheet

Abstract

Laser cladding induces high tensile residual stress (RS), which can compromise the quality of a specimen. Therefore, it is critical to accurately predict the RS distribution in cladding and understand its formation mechanism. In this study, functionally graded material (FGM) layers were successfully deposited on the surface of a titanium alloy Ti6Al4V sheet by laser cladding technology. A corresponding thermo-mechanical coupling simulation model of the laser cladding process was developed to investigate the formation mechanism of RS in the laser cladding FGM layers. The results show that high tensile RS forms in cladding components. Subsequent cladding can effectively alleviate the RS in cladding components although the position of maximum RS remains unchanged. The measurement results of the longitudinal RS on the top and bottom surfaces of cladding components by the X-ray diffraction (XRD) method agreed with the simulation results, thereby proving the accuracy of the simulation. In addition, the formation mechanism of RS in the laser cladding FGM layers was revealed by discussing the individual impact of each material property on RS. It was indicated that the RS distribution in the laser cladding FGM layers was significantly affected by material properties (in particular, coefficient of thermal expansion and Young’s modulus), except for the temperature gradient induced by the laser cladding process.

Introduction

Over the past decades, additive manufacturing (AM) has attracted widespread attention because complicated parts can be built by accumulating material layer by layer. Unlike traditional subtractive manufacturing, AM has considerable advantages in terms of (1) rapid fabrication of intricate parts and (2) rapid repairing of parts [[1], [2], [3]]. Among various AM techniques, laser-based additive manufacturing (LAM) is the most representative one because the laser beam can instantaneously concentrate significant amounts of heat in a micron region to melt and solidify metals in air due to the high effective energy of laser spot [4,5]. Ma et al. [6] successfully manufactured large-scale 316 stainless steel parts with high shape accuracy through direct laser fabrication. Baumers et al. [7] found that the use of direct metal laser sintering can save 36–46% in manufacturing cost compared with traditional processing methods. The direct laser writing method has been proposed to quickly print angular standards with micro/nanometer precision for the calibration of optical morphometry instruments, as reported by Ströer et al. [8]. All of the above indicates that LAM allows high-throughput, low-cost and high-precision manufacturing of most metal materials. In fact, LAM is potential in the rapid fabrication of functionally graded materials (FGMs) due to the above advantages.

FGMs refer to special composite materials that exhibit gradient-varying properties by designing gradient-varying compositions or structures. The concept was originally proposed by the Japanese space shuttle project in the 1980s [9]. Through designing the smooth transition between metal and ceramic, a high-quality thermal barrier system capable of withstanding 1000 °C temperature differences within a thickness of 10 mm was obtained. Since then, FGMs have been of particular concern from aerospace [10,11] to nuclear [12], automobile [13], bioengineering [14], and optoelectronics [15]. The FGMs can be manufactured in several ways, and LAM is the most promising today. It can directly produce FGMs by depositing powder materials with a gradient components layer by layer conveniently [16]. Balla et al. [17] used laser engineered net shaping to form a fully dense metal/ceramic functionally gradient coating for extremely high-temperature environments, and this technique controlled the composition and microscale with high accuracy. Mahamood et al. [18] reported the fabrication of Ti6Al4V/TiC FGMs with excellent wear resistance by laser metal deposition. Through selective laser melting, Han et al. [19] additively manufactured a FGM bone implant with the smooth transition from pure titanium (Ti) to 100% hydroxyapatite (HA), relieving the stress at the implant and bone interface. Besides, LAM can be used to prepare metal/metal [20,21], metal/polymer [22], polymer/ceramic [23], and glass/ceramic [24] FGMs.

The investigation illustrates that LAM has obvious superiority in the preparation of FGMs. However, the residual stress (RS) induced by the rapid melting and solidification process deteriorates the FGMs property significantly, though the composition gradient of FGMs can alleviate thermal stress partially. Thermal stress-induced cracks were observed and discussed in the Ti/HA FGM implant prepared by Han et al. [19]. Liang et al. [25] investigated the mechanical properties of Ti/Ti6Al2Zr1MoV FGMs fabricated by LAM, showing that cracks significantly affect the deformation ability of FGMs. Hence, it is essential to fully analyze the distribution characteristics of RS in LAM FGMs and reveal its formation mechanism.

Presently, research on LAM FGMs mainly focuses on experimental preparation and performance characterization. The RS distribution and its formation mechanism are rarely reported. The research on the RS in homogeneous materials is helpful to the analysis of FGMs. Gu et al. [26] illustrated that numerical simulation was suitable for RS research. The selective laser melting process induced complex thermal cycles and temperature distribution in the deposited layers, which would induce large tensile RS. Brückner et al. [27] indicated that compressive plastic strain and tensile RS were easily generated during the cooling of laser cladding, but they only discussed one cladding layer. Bailey et al. [28] simulated the laser direct deposition process of multilayers and indicated that RS was difficult to control in multi-thermal cycles. The above studies provide guidance for revealing the formation mechanism of RS in LAM FGMs. First of all, numerical simulation is suitable to analyze the RS. Secondly, the temperature and RS field should be analyzed in detail during the repeated heating and cooling in the LAM process. In addition, for FGMs, the influences of gradient variations in material properties should be appended.

In this study, the titanium alloy Ti6Al4V (TC4 for convenience), which is popular in extremely high-temperature environments, was chosen. SiC microparticles (SiCp) reinforced TC4-based FGM layers were coated on a TC4 sheet by the laser cladding technology, and the distribution of SiCp in FGM layers was confirmed. This structure not only improves the wear resistance [29] but also adjusts the coefficient of thermal expansion (CTE) to match ceramics [30]. Furthermore, a thermo-mechanical coupling simulation model of the laser cladding process was developed to analyze the transient temperature field and RS distribution in detail. The X-ray diffraction (XRD) method was adopted to verify the accuracy of the simulated RS. Importantly, the formation mechanism of RS in laser cladding FGM layers was revealed by discussing the effect of each material property on RS.