Murugaiyan Amirthalingam

Brazing of galvanised advanced high strength steels (AHSS) by conventional gas metal arc welding (GMAW)-based processes results in the deterioration of mechanical and corrosion resistance properties in the brazed region, leading to premature failure of the components. Brazing of galvanised AHSS sheets using low melting point fillers with minimal heat input is considered as a best alternative to avoid damage to the base metal microstructure and zinc evaporation. Cold metal transfer is a low heat input variant of GMAW in which droplet transfer generally occurs by the combination of pulsed free flight and controlled dip short circuiting transfers. The productivity of the cold metal transfer brazing process can significantly get affected if the droplet transfer behaviour is not optimised against the mechanical properties of the joints. The objective of this study is to achieve maximum droplet transfer rate with minimum heat input during brazing of a AHSS using copper-based fillers (Cu-3Si-1Mn and Cu-8Al, wt. %). In this study, increased droplet transfer rate is observed when the metal transfer is only by short circuiting (CMT) than short circuiting with pulsed free flight (CMT-Pulse) for the same mean current for both the fillers. Analysis of bead geometry indicated that for a given mean current, increased bead width is observed for Cu-Si filler than for Cu-Al filler due to the narrow solidification temperature range and high liquid metal viscosity of the latter. CMT-Pulse brazed joints exhibit good mechanical properties due to good wetting characteristics than those deposited by CMT process.

The original version of this article unfortunately contained a mistake. The symbol below eq. 2 on page 397 to denote deposition speed is displayed in an Asian character. It should be “v” instead of the Asian charater. The sentence should read ‘.and v is the deposition speed.

Welding of thin aluminum sheets with high heat input conventional welding processes often results in heat-affected zone softening, distortion, and burn-through problems. In this study, metal transfer characteristics are optimized in a gas metal arc-welding process to achieve low heat input and high productivity to weld aluminum sheets. The main objective of this study is to achieve maximum droplet transfer rate at low heat input in AA 1050 alloy using short circuiting (CMT) and short circuiting with pulse (CMT-P) gas metal arc-welding processes. Metal transfer characteristics are studied while depositing the AlSi5 and AlSi12 fillers on to an AA 1050 aluminum plate. Experiments are conducted to investigate the influence of mean current (90 to 110 A) and pulse frequency (5, 6, and 7) on heat input, droplet transfer rate, and its diameter. Kinetics of the metal transfer during welding is correlated with the corresponding current and voltage waveform and bead geometry. Results showed that for a given heat input, the droplet transfer rate is high in CMT-P at all mean current amplitudes compared to short-circuiting process for both the fillers. Analysis of the bead geometry indicated that for a given mean current, bead width of the AlSi12 filler is always higher than the AlSi5 filler in both the modes of metal transfer due to the near complete eutectic solidification in the former case. It is also found that the filler wire composition does not affect the droplet transfer rate for a given mean current in CMT and CMT-P processes used in this study.

Copper-based shape memory alloys (SMAs) are prone for grain growth during thermomechanical and betatising treatments. The grain growth of the alloy leads to intergranular cracking on quenching to form martensite in the alloy, which in turn leads to poor mechanical properties including corrosion resistance of the alloy. In the present work, grain refinement of a CuZnAl shape memory alloy was done by adding 0.2 to 0.4 wt.% of zirconium, titanium and boron as grain refiners. The effect of these additions on the microstructure and shape memory properties of the alloys were studied. The results show that the Zr and Ti additions reduce the grain size from 1.5 mm to 200 μm and 500 μm respectively. The Zr-added alloy shows good strain recovery and corrosion resistance compared with the alloy in the other conditions.

Application of electric current pulses while deforming a material, commonly referred to as electric-assisted forming (EAF), is known to have desirable effects over its formability. In the finite element simulation of this electric-assisted deformation, the time-temperature profile is obtained by providing various temperature dependent thermo-physical properties of the material. Out of all the required properties for such analysis, effective heat transfer coefficient and Joule heat fraction are sensitive to the microstructure of the material, geometry of the specimen and the ambient conditions. Generally, these coefficients are identified by iterative FE simulations. A clear methodology to estimate these parameters has not been established yet. In the present work, a procedure is developed using a genetically evolved meta-model of the time-temperature profile, which is experimentally obtained from the pulsed current assisted uniaxial tension and compression tests. For this purpose, various multi-objective optimization techniques such as BioGP, EvoNN and cRVEA have been utilized to estimate the temperature profile in each case. It is shown that the tri-objective optimization procedure predicts the experimental temperature profile with greater accuracy (within ± 5%) and is best suited to obtain the thermal modelling parameters of electric-assisted deformation, than other optimization techniques used in this work. Graphic Abstract: [Figure not available: see fulltext.].

Microstructural evolution during the strain-induced phase transformation of austenite in an Austempered ductile iron (ADI) under various thermomechanical processing conditions is studied in the present study. An alloyed ductile iron is taken as the base material, and thermomechanical treatment is carried out on a Gleeble 3800 thermomechanical simulator coupled with dilatometry. The effect of deformation on the austempering process has been studied by microstructure characterization using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. The variations in retained austenite volume fraction and its carbon content with respect to different austempering times are analyzed to study the effect of strain-induced transformation of austenite. It has been observed that the thermomechanical treatment significantly influences the phase transformation kinetics during the austempering process. The thermomechanical treatment produced a martensite free ausferritic microstructure for all austempering times with a high volume fraction of carbon enriched retained austenite as compared to the conventional heat treatment.

Despite of more than two decades of research in white etching areas (WEAs), this phenomenon still persists in wind turbine gearbox bearings leading to a reduction of L10 life by 90 percent. There are various drivers for WEAs formation among which hydrogen is considered one of the root causes which accelerates the microstructural degradation. Diffusible atomic hydrogen reduces the threshold for dislocation motion leading to localized strain accumulation. The tendency to form WEAs in bearing steel depends on the stability of carbide precipitates during plastic deformation and resistance to diffusible hydrogen. In this research work, the role of size and volume fraction of carbides on their relative stability against decomposition and hydrogen embrittlement is studied. The performance of bearing ball samples with different carbide sizes and distribution is tested in the presence of two different lubricants in the dynamic load pin-on-disc (PoD) test rig. The results from the study show that reducing the size of carbide precipitates improved their stability against plastic deformation leading to the stagnation of WEAs formation.

The flow stress reduction during plastic deformation superposed with electric current, commonly referred as 'electroplasticity' has been actively researched over the past few decades. While the existence of an electron-dislocation interaction, independent of Joule heating is established, the exact rate controlling mechanism of the observed behaviour lacks consensus. Understanding the governing mechanism is complex due to the combined effect of Joule heating and electron-dislocation interaction. The present work attempts to establish the electroplastic mechanism in AA 6063 alloy and its nanocomposites. The role of microstructure on the electron interaction is investigated by preparing four distinct microstructure from the base alloy. All the samples were subjected to constant amplitude direct current during plastic deformation. The Joule heating effect is decoupled using the experimentally measured temperature history. The potential electroplastic mechanism for the alloy is elucidated by analysing the trend of flow stress reduction with strain and strain rate. It is inferred that micro Joule heating and electron wind effect cannot completely explain the observed electroplastic behaviour in AA 6063. The SiC particles in nano-composites suppressed the electroplastic effect. The observed mechanical behaviour under electric current is in agreement with the trend predicted assuming magnetic depinning mechanism. The reduction of dislocation density quantified using X-ray diffraction is found to concur with the inferred mechanism.

Functionally graded materials (FGM), whose structural properties are varied along their volume to perform an intended function, have a full application in the marine industry. Under the corrosive marine environment, a corrosion-resistant alloy of duplex stainless steel, functionally graded with carbon-manganese steel along with the inner layer, is proposed here for the risers. The interface strength of FGM, fabricated using Cold Metal Transfer – gas metal arc welding, based on Wire Arc Additive Manufacturing (WAAM) in the lab-scale, is compared with an X-52 Carbon Manganese steel to highlight the superiority of the proposed FGM. The role of metal transfer characteristics on the microstructure formation at the layers adjacent to the interface is analyzed and correlated with the voltage-current waveform those obtained during the deposition process. Based on the experimental studies carried out, it is seen that the yield strength of the FGM interface is very close to that of X52 steel, with a marginal increase of about 6% in the ultimate strength. Based on the energy dispersive X-ray analysis, it is observed that there is no enrichment of chromium across the interface towards the carbon-manganese steel side. It is vital for the corrosion-resistance of duplex stainless steel. Proposed FGM shows advantages both in strength and durability and hence seen as a promising candidate for marine riser applications.

The effect of temperature (350 °C ¡ MS ¡ 450 °C) on the partitioning mechanisms and the final microstructure evolution in a CMnSiAl quenching and partitioning (Q&P) steel was investigated. The microstructure of both the Q&P specimens, comprised of distorted BCC or pseudo tetragonal martensite structure with two different characteristics namely (i) tempered or carbon depleted martensite that formed during initial quenching (Mf ¡ 240 °C ¡ MS) and partitioning step and (ii) carbon enriched fresh martensite that formed after partitioning step and final quenching (RT) together with blocky and inter-lath films of retained austenite. In addition, packets of M/A constituents were observed in Q&P-350-1min specimen and some traces of carbide and plate martensite were observed in Q&P-450-1min specimen. The increase in partitioning temperature led to nearly 2% increase in the amount of retained austenite (both blocky and inter-lath) with increased carbon content of 0.27 wt.%. Along with carbon partitioning, slight interface mobility/isothermal martensite formation was also observed in the case of specimen partitioned at 350 °C, whereas tempering effect was predominantly seen in the case of specimen partitioned at 450 °C. Irrespective of the partitioning temperature, the amount of carbon required to stabilize the retained austenite at RT was found to be about 1.15 wt.% and was confirmed through APT analysis.