Gas tungsten arc welding assisted droplet deposition manufacturing of steel/lead bimetallic structures

Abstract

Droplet deposition of lead alloy to C45 steel with direct metallurgical bonding was conducted by adopting a gas tungsten arc welding (GTAW) pre-melting heat source on the surface of C45 steel substrate. It was found that a nonlinear relationship existed between the heat input of the welding arc and the feature sizes (deposition height and width) of the lead alloy deposited layers. The chemical composition, phase and microscopic analyses were carried out. The results showed that no evidence of cracks and micro voids was found at the bonding interface even at higher magnification. The diff ;usion reaction of elemental Fe, Sn, and Sb still exists, which ensures the metallurgical connection between lead alloy deposited layers and C45 steel substrate. In the lead alloy deposited layer, microhardness keeps unchanged, but in the C45 steel beneath the bonding interface, the microhardness value increases suddenly to about 650 HV. This can be attributed to the formation of martensite phase and the annealing effect during GTAW assisted droplet deposition manufacturing.

Introduction

Compared to single-material structures, multi-material structures can provide unique solutions to many challenging engineering problems by varying properties at different locations of the same structure. Among structures made of multi-materials, bimetallic structures comprise of two different metals joined together to achieve the distinct properties of the base materials, or to selectively improve the overall performance of one of the components (Onuike, 2019). Bimetallic structures have been used in high temperature aerospace applications through reducing fabrication lead times and minimizing weight for liquid rocket engine components (Gradl et al., 2019). Other areas include energy (Bai and Liu, 2020), optics and photonics (Asano et al., 2018) industries for coatings to improve high-temperature strength and lead-bismuth eutectic (LBE) corrosion resistance, absorptivity of copper to blue diode lasers, respectively. But, manufacturing of bimetallic structures has always been a challenge because of the significant difference in their mechanical, thermo-physical and metallurgical properties. Conventionally, bimetallic structures are fabricated through fusion processes such as laser welding (Joshi and Badheka, 2019) and electron beam welding (Tomashchuk et al., 2013), including friction stir welding (Jimenez-Mena et al., 2018). Other techniques include, brazing (Dong et al., 2020), diffusion bonding (Cooke and Atieh, 2020) and transient-liquid-phase (TLP) (AlHazaa et al., 2019) method. However, butt joints and lap joints are the most frequently used configuration of dissimilar metal joints. Thus, the multi-material structures with complex geometries are difficult to be fabricated by the above-mentioned joining processes.

Additive Manufacturing (AM) techniques have been proved that they can be applied to fabricate multi-material structures, as reported in the work by Bandyopadhyay and Heer (2018). However, there are only few AM techniques which are capable of processing dissimilar metals, namely Laser Engineered Net Shaping (LENS) (Zhang et al., 2020), Wire and Arc Additive Manufacturing (WAAM) (Abe and Sasahara, 2016), Selective Laser Melting (SLM) (Chen et al., 2019), and Ultrasonic Additive Manufacturing (UAM) (Sridharan et al., 2017). Improvements in properties such as wear resistance, thermal conductivity and mechanical properties could be obtained within one single additively manufactured multi-material structure. For example, Zhang et al. (2020) used laser cladding to synthesize high-entropy alloy coatings with TiAlNiSiV on Ti-6Al-4 V alloy, thus, the wear-resistance properties and hardness of the TiAlNiSiV high-entropy alloy coatings are significantly improved. A comparative study on the conductivity of pure Inconel 718 alloy and Inconel 718-copper alloy bimetallic structure fabricated by LENS was performed by Onuike et al. (2018), the bimetallic structure comprised of Inconel 718 substrate at 0.77 mm thick and GRCop-84 deposit at 1.22 mm thick. Thermal diff ;usivity was measured perpendicular to the bimetallic interface by using a Netzsch LFA 447 NanoFlash® thermal diff ;usivity system. The combined density and thickness of the bimetallic samples were used for the system’s testing parameters. Abe and Sasahara (2016) conducted dissimilar metal deposition using nicked-based alloy and stainless steel, stainless steel YS308 L weld bead was firstly accumulated on a stainless steel SUS304 substrate by the wire arc additive manufacturing process. Then, Ni6082 weld bead was accumulated on the stainless steel weld bead. A tensile test was conducted to investigate the joint strength and tensile strength of the fabricated sample. They found that the bond strength of the YS308 L/ Ni6082 bimetallic structure was comparable to the tensile strength of the YS308 L weld metal and Ni6082 weld metal. Sridharan et al. (2017) showed that it is possible to achieve high bond strengths during UAM of steel and Al. Butt et al. (2016) developed a composite metal foil manufacturing (LOM) process, which combinates brazing and LOM, to fabricate some layered composites of pure copper foils and aluminum 1050, mechanical testing revealed that the load value of fracture test for deposited aluminum alloy layers was greater about 11 % than its corresponding value for aluminum alloy. However, the preparation of brazing paste and the joining with heat and pressure add time and material costs.

C45 steel has high strength and good machinability. Lead and its alloy exhibit excellent ray absorption and radiation shielding ability due to its high density and atomic mass, as had been previously reported by Loewen and Tokuhiro (2003). Steel/lead bimetallic structures, which have many advantages such as compact size, light weight and significant shielding effect, have promising application prospects in the nuclear industry and electrochemistry as stated by Liu et al. (2013) and Li et al. (2018). Non-direct bonding of lead alloy with diff ;erent types of steels have also been attempted through adhesive bonding by Li et al. (2018) and transient liquid phase diffusion bonding by Fang et al. (2013). The conventional processes (e.g., fusion welding, mechanical fastening, solid-state welding, etc.) used for joining dissimilar metals in butt, lap and T-joint configurations are not capable of fabricating the steel/lead bimetallic structures with complex geometric shape. Currently, adhesively bonded steel/lead bimetallic structures often form many different types of defects, such as porosity, voids, cracks and dis-bonds during storage and use. These defects were analyzed and summarized in the review work by Calvez et al. (2012), they can severely damage the integrity of the adhesively bonded structures, and further lead to the decrease of load capacity, even serious radiation leakage. On the other hand, the surface preparation prior to adhesive bonding greatly increases time cost as reported by Jairaja and Narayana (2019). Up to now, there is only a very limited literature by Hsu and Lee (2016) to describe the metallurgical behaviors and microstructure of steel/lead composites. Different from the published literature on additively manufactured bimetallic structures, the thermal and physical properties of C45 steel and lead alloy in the present study are quite different, such as the melting point of lead alloy is 283℃, the melting point of C45 steel is 1540℃. Additive manufacturing of the steel/lead bimetallic structures has not yet been reported. Therefore, development of a novel and reliable forming method to ensure the establishment of a true metallurgical bond and the complete continuity of grain structure between lead alloy and C45 steel is urgently needed.

Metal droplet deposition manufacturing is another important method to realize the additive manufacturing of metal components, in which the ejected liquid metal droplets are often employed as input material instead of powder or wire. Thus, material and process costs can be significantly reduced as stated by Murr and Johnson (2017). However, according to the investigations of Chao et al. (2013), the deposition defects in the components made by droplet-based additive manufacturing, such as micro-void and cold lap, are always very difficult to avoid due to the insufficient re-melting and coalescing of adjacent liquid metal droplets. So far, it has not been reported that the bimetallic structures were fabricated by metal droplet deposition manufacturing.

In this study, a novel GTAW assisted droplet deposition manufacturing process is proposed to fabricate steel/lead bimetallic structures, the basic principle of the novel process is described in detail in Section 2. The effect of the heat input of GTA welding arc on the feature sizes of single-track lead alloy deposited layers is discussed.