A. Sambasiva Rao

INFLUENCE OF WELDING PROCESSES ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF DISSIMILAR AUSTENITIC-FERRITIC STAINLESS STEEL WELDS

Abstract Dissimilar metal welding of austenitic (AISI 304)-ferritic (AISI 430) stainless steel has been taken up to understand the influence of the welding process on microstructure and mechanical properties. Fusion welding processes, namely, gas tungsten arc welding (GTAW), electron beam welding (EBW), and friction welding, have been employed. The GTAW and EBW processes were selected to understand the heat input effects, while friction welding was included to compare fusion and solid-state welding processes. The material used for fusion welding studies is 20-mm-thick, hot-rolled, and annealed plate. Rods of 18 mm diameter machined from the same plate material were used for friction welding studies. In GTAW, ER 430 filler material was employed for dissimilar metal combination, while other welds are autogenous. Gas tungsten arc welds consisted of coarse columnar grains. In electron beam welds, the microstructure consisted of predominantely equiaxed grains on the austenitic stainless steel side, while columnar grains were observed on the ferritic stainless steel side. Epitaxial solidification was noted on the ferritic stainless steel side, while no such features were evident on the austenitic stainless steel side. Electron probe micro analysis (EPMA) revealed that inter diffusion of elements was significant in GTAW, intermediate in EBW, and insignificant in friction welds. Notch tensile and impact properties of ferritic stainless steel and dissimilar metal combination of austenitic-ferritic stainless steel friction welds are superior to gas tungsten arc welds and electron beam welds. Electron beam welds of austenitic stainless steel exhibited superior notch tensile and impact toughness compared to friction welds. Gas tungsten arc welds exihibited the lowest pitting corrosion resistance, while friction welds possessed the highest pitting corrosion resistance. In general, pitting was confined to Cr-depleted regions adjacent to the carbide precipitates.

Influence of scandium on weldability of 7010 aluminium alloy

Abstract The commercial 7000 series aluminium alloys are based on medium strength Al–Zn–Mg and high strength Al–Zn–Mg–Cu systems. The medium strength alloys are weldable, whereas the high strength alloys are non-weldable. This is because the amount of copper present in these alloys gives rise to hot cracking during solidification of welds. As a result, the high strength Al–Zn–Mg– Cu base alloys are not used for applications where joining of components by welding is an essential step. In the present study, using a combination of qualitative Houldcroft test and quantitative Varestraint test, it is shown that a small addition of scandium to the commercial 7010 alloy reduces the hot cracking susceptibility during solidification of welds produced by the gas tungsten arc welding process. The improvement in weldability is found to be the result of the considerable grain refinement in the weld structure following the scandium addition. The results of microhardness and tensile tests are further described within the context of the present work to demonstrate that the 7010+Sc welds also exhibit a combination of improved strength and ductility.

Microstructure and tensile properties of dissimilar metal gas tungsten arc welding of aluminium to titanium alloy

Abstract Commercially pure aluminium and titanium alloy Ti–6Al–4V were welded by gas tungsten arc welding (GTAW) process using AA 4047 Al–Si filler material. The chemical composition of reaction layer at the interface of titanium alloy and weld zone was determined by electron probe microanalyser (EPMA) showing that continuous layer of intermetallic compound of TiAl base phase was formed and aluminium is partly replaced by silicon. Transmission electron microscopic examination confirmed formation of (AlSi)3Ti intermetallic compound formed at the interface of Ti alloy and weld. In the weld zone, intermetallic phases containing Fe and Si were identified by EPMA analysis. These intermetallics were confirmed by X-ray diffraction technique. No intermetallic compound was found at the interface of weld and aluminium. Optical microscopy, scanning electron microscopy, microhardness, and tensile testing of the joints were used to characterise the resulting joints. The transverse strength of the weld joint is higher than that of the aluminium.

Friction Stir Surfacing Route: Effective Strategy for the Enhancement of Wear Resistance of Titanium Alloy

Titanium alloys are widely used in aerospace applications due to their properties like high strength to weight ratio, good corrosion and creep resistance. Poor wear resistance of these alloys limits their use in tribological applications. Friction surfacing technique is now recognized as an effective solution to surface engineer the light weight high strength alloys to make them suitable for general engineering applications involving wear and corrosion. The present work pertains to a study on wear resistance of surface coating of boron carbide on Ti–6Al–4V alloy using friction surfacing technique. Coating was formed by placing the boron carbide powder into the holes of predetermined depth on the surface and was characterized by metallography, electron probe micro analysis and dry sliding wear testing. The present study revealed that titanium alloy could be friction surfaced with boron carbide powder. The coating exhibited excellent wear resistance, which is attributed to the formation of strong metallurgical bond with the substrate. In the present work an attempt has also been made to compare the wear behaviour of surface composite layer on titanium alloy with that of conventionally used engineering materials such as mild steel and austenitic stainless steel. Wear data clearly revealed that wear resistance of friction stir surfaced composite layer is better than that of mild steel and stainless steel. This study demonstrated that friction stir surfacing is an effective strategy for the enhancement of wear resistance of titanium alloys.

PRODUCTION OF Cu–Cr ALLOYS BY IN SITU REDUCTION OF CHROMIUM OXIDE DURING ELECTRO SLAG CRUCIBLE MELTING (ESCM)

An electro slag crucible melting process for production of copper–chromium alloys is described. The process uses fine copper scrap as a raw material. After the copper scrap is melted, chromium is alloyed with copper by direct reduction of chromium oxide added to the slag. Carbon and aluminum can be used as reductants and the reduction is carried out in situ in the molten slag. Copper chromium ingots containing up to ∼1 wt % chromium were produced by this process. The process serves the dual purpose of recycling copper scrap and alloying remelted copper by chromium. This is the first time that direct reduction has been employed during an electro slag melting process. The in situ reduction technique described has the potential of being a production route for a variety of alloys. It is particularly suitable for production of difficult-to-melt alloys such as copper–chromium.

Electroslag cladding of low alloy steel with stainless steel

Abstract Cladding of a low alloy steel slab with stainless steel was carried out using a modified electroslag remelting technique. It is shown that the thickness of the cladding that can be achieved via electroslag remelting is dependent on the fill ratio used. The effect of power input on the joint profile obtained is reported. A combination of low fill ratio and relatively low power input is essential to minimise penetration of the base slab by the liquid metal. A satisfactory joint profile and defect free joint can be obtained via the optimisation of these process parameters. The clad product was successfully forged and rolled, which indicates satisfactory strength of the clad joint.

Effect of Ti, W, Mn, Mo and Si on microstructure and mechanical properties of high carbon Fe–10·5 wt-%Al alloy

Abstract Effect of quaternary alloying elements Ti, W, Mn, Mo and Si on the microstructure and mechanical properties of Fe–10·5Al–0·7C (wt-%) alloy has been investigated. Six different alloys were prepared by a combination of air induction melting with flux cover and electroslag remelting (ESR). The composition of the quaternary alloying element was ∼2 wt-% and was substituted for iron. The ESR ingots were hot forged and hot rolled at 1375 K. The hot rolled alloys were characterised with respect to microstructure and mechanical properties. Alloys containing W, Mn and Si exhibited two phase microstructure of Fe3AlC0·5 precipitates in α Fe–Al matrix. Whereas alloys containing Mo and Ti exhibited three phase microstructures, the additional phase being the respective carbides. Both α Fe–Al matrix and Fe3AlC0·5 precipitates have considerable amount of solubility for W, Mn and Mo whereas Si has very high solubility in α Fe–Al matrix as compared with Fe3AlC0·5 precipitates and titanium has very low solubility in both α Fe–Al matrix and Fe3AlC0·5 precipitates. Greater improvement in room temperature tensile properties was observed by the addition of Mn as compared with the addition of W. Significant improvement in tensile and creep properties was observed by the addition of Mo. Though the addition of silicon has resulted in poor room temperature ductility, there has been remarkable improvement in high temperature strength and creep properties.