Heating of aluminium alloy at

Heating of aluminium alloy at high temperature results in diffusing Si out of the lattice [25]. Since Si–Ag is an eutectic system which is practically immiscible in solid state, the diffusion of Si across the silver interlayer is restricted, resulting in higher concentration of Si at interface of aluminium NF-κB Compound Library metal and silver (Fig. 7). This observation is supported by quantitative analysis of Si across the weld interface for joints with silver interlayer (Fig. 8). Higher concentration of Si retards the formation of Fe2Al5 at weld interface by acting as a barrier to Fe diffusion [6,18,26,27]. Silicon segregation at the weld interface is not observed in a joint without silver interlayer, which allows more Fe to diffuse towards Al, resulting in Fe2Al5 formation and poor tensile property. Higher value of microhardness in a joint without silver interlayer can be attributed to the formation of Fe2Al5 and FeAl3[28]. Slight reduction in hardness with silver interlayer can be observed due to the presence of silver/aluminium intermetallic compounds which are soft in nature. Low tensile strength and elongation of joints without silver interlayer can be attributed to the formation of Fe2Al5 and FeAl3 intermetallics which are brittle in nature. Introduction of silver interlayer results in improvement in tensile strength and elongation since, and Fe–Al based intermetallic compound are partially replaced by Al–Ag based compounds which are ductile in nature.
Conclusions A friction welding technique has been developed for joining aluminium (AA6061) to low alloy steel (AISI 4340) using an interlayer of silver. Silver as an interlayer partially reduces the formation of Fe–Al based intermetallic and replaces it with Al–Ag based intermetallic, such as Ag3Fe2, Ag2Al and Ag3Al, resulting in better tensile strength and ductility of welds. Presence of silver as an interlayer reduces Mg concentration at the weld interface by intermittently replacing it with Si on AA6061 side, which restricts the interaction of Fe with aluminium. The higher strength and ductility of aluminium to low alloy steel dissimilar metal welds with silver as an interlayer was attributed to the formation of ductile phases like Ag3Fe2, Ag2Al and Ag3Al.
Acknowledgement The authors express their gratitude to Defence Research and Development Organization for the financial support to carry out this program and are thankful to Dr. Amol A. Gokhale, Distinguished scientist, Director, Defence Metallurgical Research Laboratory, India for his continued encouragement and support.
Introduction The surfaces of engineering materials are given the specific treatments that are different from those of the core. These treatments can alter the composition of the case by incorporating the specific species on the surface of the substrate material or it can be subjected to heat treatment which do not alter the composition of the substrates or the deposited layer can have a different material than the substrate. The surface treatments can be physical, physico–chemical, fusion, as well as non-fusion based. Solid state process that does not involve melting and solidification is versatile as it gives rise to the deposits which are free from solidification related defects it is an amenable process for many incompatible dissimilar metals owing to the short interaction time available for the extensive formation of deleterious intermetallics. Friction surfacing is one such solid state process currently being pursued extensively for various surfacing applications requiring wear and corrosion resistance properties. A schematic diagram of the surfacing process is shown in Fig. 1. The friction surfacing process involves a rotating coating rod called mechtrode that is brought in contact with the substrate under axial load. Intense friction heat is produced on the rubbing surface between the substrate and the coating rod. Generated heat is sufficient to plastically deform the end of the mechtrode. A layer of mechtrode material is deposited by moving the substrate across the face of the rotating rod. Metal coatings are made possible by the generation of high contact stress and intimate contact between the coating material and substrate which initiates solid-state adhesion between the coating and the substrate [1]. Being a solid state process, the friction surfacing offers several advantages over conventional fusion welding processes. Friction surfaced coatings exhibit zero dilution and wrought microstructures with very fine grain size. Since melting and solidification are not involved, the problems, such as solidification cracking, brittle intermetallic formation and porosity, do not arise. The critical areas of application include the deposition of hard facing materials on cutting edges of knives of various categories, punch, die, tools and blades required for food processing, chemical, agriculture and medical industries. It opens up a new area of repair and reclamation of worn and damaged components [2,3].