Investigation on compatibility of laser additive based remanufacturing for AISI 4140 alloy steel substrate: Bonding behavior and failure mechanism

A medium carbon and low alloy steel such as AISI 4140 has been widely used for many industrial applications such as gears, shafts, rotors, and so on due to its good balance of strength, toughness and wear resistance. Under the harsh working conditions such as high temperature, high speed sliding contact, and frictional heat generation, however, the surfaces of critical components made of this alloy steel undergo severe damage such as micropitting, wear, and corrosion, which is detrimental to the service life of the high-value components and leads to the complete failure. Thus, it is necessary to extend their service life and refurbish the worn or damaged parts in order to minimize economic losses, waste of expensive materials, and downtime, thereby increasing the industrial competitiveness.
Recently, laser additive manufacturing technology like laser engineered net shaping (LENS) has been used to fabricate or repair high-value components. Not only can it fabricate a complex, functional, and structural part, but also can be used for surface treatment such as coating, hardfacing, as well as repairing worn and damaged parts. Additionally, it has shown excellent metallurgical bonding to the substrate with a minimum heat affected zone (HAZ) compared to other surface coating processes. High-performance materials such as superalloys have demonstrated great potentials to achieve significantly enhanced mechanical properties at high temperature due to their excellent wear, corrosion, and oxidation resistance. Thus, the superalloys will be one of the most promising materials for surface coating and repair applications when the LENS process is utilized, which could provide potential solution to meet the industrial requirement. LENS of superalloys bas been studied by researchers to investigate its microstructure and mechanical properties. However, to date there has been a paucity of research on LENS-deposited superalloys powders on AISI 4140 alloy steel substrate in the literature. Particularly, there was a lack of knowledge on the interface bond and fracture behavior of a hybrid part. Most coating and hardfacing research focused on wear and hardness due to thin coated thickness and layer. In case where coating or repair area is deeper and larger, thick and multi-layer coating is highly expected. Interface bonding between the laser repaired coating and the original substrate plays a vital role in determining overall performance of the whole component since low bond strength leads to failure resulted from the coating peeling off, cracking or corrosion along the interface, which leads to the premature and catastrophic failure. Therefore, joining two materials with good interfacial bonding is of critical importance to ensure structural integrity and reliability of the part. The coated or repaired material will be functional only when the interface between the coating and the substrate is durable and strong. Unfortunately, knowledge on the compatibility of the superalloys with the substrate processed by laser additive manufacturing still remains unknown. Thus, the fundamental research is needed to prove the unknown mechanisms. The goal of this fundamental research is to provide new knowledge that fills the current knowledge gaps on compatibility of LENS-processed superalloys with the substrate by proving the hypotheses, and extend the LENS process capability and industrial application such as surface repair and coating. In order to achieve the goal, the research objectives are established: (1) to investigate the compatibility of LENS-deposited nickel-based and cobalt-based superalloys with AISI 4140 alloy steel substrate with a focus on interface bond performance and fracture behavior of the hybrid fabricated parts, and (2) to reveal bonding fracture failure mechanisms of the hybrid fabricated parts and provide a theoretical basis for compatibility and LENS process. By proving the hypotheses through theoretical and experimental investigation, this fundamental research fills the gaps and will have the following contributions: 1) creating a new knowledge on the effects of powder materials on compatibility with the substrate and bonding failure mechanism during the LENS process and 2) revealing fundamental mechanisms on phase formation, microstructure evolution, and bonding properties during laser additive manufacturing. The research will give theoretical basis to guide feasible material selection for AISI 4140 substrate in laser additive remanufacturing application. It will help engineers to create more reliable adhesive bonds and make more efficient use of high performance repairing materials. Other areas of application such as aerospace and wind turbine will be benefited.