Influence of annealing on the microstructure and mechanical properties of Ti/steel clad plates fabricated via cold spray additive manufacturing and hot-rolling

Highlights

• Ti/steel clad plates were fabricated by cold spraying and hot-rolling.

• Effect of annealing on the structure and performance of the samples was studied.

• Annealing improves mechanical properties through recovery and recrystallization.

• Annealing reduces the dislocation density, promoting elongation of the sample.

• High temperature promotes the growth of TiC, harming the sample's performance.

Abstract

A novel method was designed to fabricated Ti/steel clad plates via cold spray additive manufacturing (CSAM) and hot-rolling. In order to study the influence of annealing on the microstructure and mechanical properties of the Ti/steel clad plates, the as-rolled clad plates were subjected to different annealing temperatures in the order: 450, 550 and 650 °C. It was revealed that CSAM and hot-rolling promote metallurgical bonding between Ti/Ti as well as Ti/steel interfaces, thus resulting in Ti/steel clad plates with excellent mechanical properties. The annealing treatment promotes inter-diffusion of Ti and Fe across the Ti/steel interface and improves the microstructures of the as-rolled Ti/steel clad plates. The results show that when the annealing temperature is low (i.e. 450 °C), the elongation of the specimen increases while the ultimate tensile strength and shear strength both decrease through recovery and recrystallization mechanisms. However, when the annealing temperature is too high (i.e. 650 °C), the elongation and strength of the specimen decrease sharply due to the nucleation and growth of brittle TiC and FeTi compounds. The annealing, at 550 °C for 3 h, was pin-pointed as the optimum post-rolling treatment to achieve superior ultimate tensile strength (564 MPa), shear strength (280 MPa) and elongation (18%) in the clad plate.

Introduction

In the recent years, Ti and its alloys have been widely used in aerospace and nuclear industries because of their excellent corrosion resistance and high specific strength [1,2]. However, the high cost and limited resources of Ti hamper their widespread applications for engineering structures [3,4]. In order to address this issue, Ti/steel clad plates have been developed and are being widely used in ocean engineering, petrochemical and other industries [5,6]. Ti/steel clad plates synergize the advantages of Ti alloys and carbon steel thereby exhibiting superb corrosion resistance and excellent mechanical properties [5,7]. Currently, roll bonding [8,9], explosive bonding [10,11], and diffusion bonding [12,13] are the main industrial techniques to manufacture clad plates. However, all these methods have their own shortcomings. For example, in roll bonding process, it is difficult to avoid formation of cracks and pores due to interface oxidation. Consequently, the mechanical properties of the clad plates are degraded [14]. Explosive bonding has a problem of uneven interfacial shear strength which originates from inhomogeneous deformation induced by an uncontrollable explosion wave [15]. Likewise, diffusion bonding is not a suitable choice for the mass scale industrial production due to prolonged process time and size limitation [3]. Therefore, in order to cope with the above-mentioned challenges, it is necessary to develop a new and viable technique to fabricate clad plates.

In the recent years, cold spraying additive manufacturing (CSAM) has emerged as a potential material processing technology, which can directly process powders of pure metals and their alloys (such as Ti, Al, Cu, steel, Ti6Al4V, 6061Al, etc.) into structural components [[16], [17], [18], [19], [20]]. In CSAM, the raw feedstock powder is accelerated by pre-heated high-pressure gas while passing through a converging-diverging nozzle. Wherein, the thermal energy of the accelerating gas is converted into kinetic energy to form a supersonic solid/gas stream which is layer by layer deposited on a substrate to achieve a thick metallic deposit [[21], [22], [23]]. Due to the unique feature of CSAM, i.e. low process temperature, there is a limited chance for thermal residual stresses or oxidation in the deposited layer. This makes CSAM an attractive industrial prospect [24,25].

However, weak mechanically interlocked (coating/substrate) interface, formed in CSAM, does not meet the required strength criteria for the clad plates [26]. Therefore, post-spray treatment is essential for CSAM samples. For instance, hot-rolling facilitates metallurgical bonding at the coating/substrate interface [27]. Similarly, annealing has been proved to be a simple and effective way to modify the microstructure as well as to improve the mechanical properties of clad plates [[28], [29], [30]]. Literature suggests that selection of annealing temperature plays a pivotal role in modifying the microstructure and resulting mechanical properties, since, the atomic diffusion coefficient is a function of temperature [31]. The atomic diffusion at the interface leads to metallurgical bonding and greatly improves the shear strength of the clad plates [32]. Appropriate annealing temperature activates recovery and recrystallization mechanisms, which improve the plasticity of the clad plates. However, if the annealing temperature is on the higher side, it may lead to the growth of brittle intermetallic compounds at the interface which adversely affect the shear strength of the clad plates [33,34].

Therefore, we recently developed an innovative approach, based on CSAM and hot-rolling, to fabricate Ti/steel clad plates with excellent properties. In our previous work [35], Ti powder was initially deposited on steel sheets via CSAM and subsequently hot-rolling was performed to achieve metallurgical bonding between Ti/Ti particles as well as Ti/steel interface. In this work, the Ti/steel clad plates were subjected to a series of annealing treatments with the aim to further improve their mechanical properties. Evolution of microstructure and formation of interfacial compounds in the annealed samples were systematically investigated by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) techniques. The mechanical properties were evaluated by performing tensile test, tensile shear test and microhardness test. The influence of annealing temperature on the microstructure and mechanical properties of Ti/steel clad plates was discussed.