Sumanth Shankar

Mechanism and preventive measures for die soldering during Al casting in a ferrous mold

This work provides a comprehensive understanding of the reactions at the ferrous die/molten metal interface in a metal mold casting operation. The literature has shown that several important factors influence reactions at the ferrous die/molten aluminum interface, including temperature of the melt, temperature of the die, alloy chemistry of the melt and die, die surface engineering, topographical features, and coatings. This article discusses the effect of the more critical factors on soldering, based on the authors’ investigations. Inaddition, based on a mechanistic understanding of the interface reactions between ferrous die and molten aluminum, recommendations are given for specific processing issues to alleviate soldering during die casting of aluminum alloys.

X-ray nano-diffraction study of Sr intermetallic phase during solidification of Al-Si hypoeutectic alloy

The evolution of strontium (Sr) containing intermetallic phase in the eutectic reaction of Sr-modified Al-Si hypoeutectic alloy was studied with high energy synchrotron beam source for nano-diffraction experiments and x-ray fluorescence elemental mapping. Contrary to popular belief, Sr does not seem to interfere with the Twin Plane Re-entrant Edge (TPRE) growth mechanism of eutectic Si, but evolves as the Al2Si2Sr phase during the eutectic reaction at the boundary between the eutectic Si and Al grains.

Effect of mixing rate on the morphology of primary Al phase in the controlled diffusion solidification (CDS) process

Controlled diffusion solidification is a novel and promising process wherein near-net-shaped cast product of a desired Al wrought alloy is obtained by mixing two precursor alloys at specific individual composition, mass, and temperature each to obtain a non-dendritic morphology of the primary Al phase in the solidified microstructure. This study is devoted to quantify the effect of the rate of mixing of the two precursor alloys on the morphology of the primary Al phase in the cast component. The results show that the lower mixing rate with a higher mixing velocity is more favorable for the CDS process.

Controlled Diffusion Solidification of 2024, 6082 and 7075 Al Alloys via Tilt-Pour Casting Process

Controlled diffusion solidification (CDS) is an innovative casting route for Al alloys to obtain a cast part with a non-dendritic morphology of the primary Al phase during solidification, similar to those obtained in the semi-solid rheocasting process. The process involves mixing two alloy melts with specific individual compositions and temperatures to produce the desired final alloy by mixing and immediately casting in a mould. CDS enables sound shaped casting of Al based wrought and cast alloys with a non-dendritic microstructure. The two pre-cursor alloys will have to be chosen from a critical study of the various isopleths of the multi-component phase diagrams for the respective alloys along with laboratory experiments. Further, the favorable melt superheat temperatures of the two pre-cursor alloys were evaluated from laboratory experiments as well. A tilt-pour mould designed and validated to cast tensile test bars was used for the study. The aim of the study was to demonstrate the feasibility of manufacturing sound shaped castings via the CDS route for alloys which have been impossible to shape cast in a permanent mould process due to the detrimental hot-tearing problem during solidification. The present work defines process conditions to enable shaped casting of the Al wrought alloys 2024, 6082 and 7075 by the tilt-pour casting technique. Sound casting and a favorably non-dendritic microstructure were obtained for these alloys during solidification in a ceramic crucible as well as in a metallic mould via the tilt-pour casting process. Further work is underway to optimize various process conditions and develop new heat treatment cycles for parts cast by CDS to maximize mechanical properties for these alloys.

Valid mould and process design to cast tensile and fatigue test bars in tilt pour casting process

Abstract Tilt pour gravity casting technology is increasingly being used for shape casting various components with aluminium alloys. The ASTM B108/B108M-08 standard exists for a metal mould to evaluate the mechanical properties of castings made by gravity permanent mould process, yet there is no standard mould for the tilt pour process. We have designed, developed, tested and validated a standard mould to cast tensile and fatigue test bars in a tilt pour casting process. The new mould has demonstrated abilities to cast sound castings of A356·2 aluminium alloy, and the uniaxial tensile properties were superior to those obtained from conventional direct pour gravity casting process.

Partial pair correlation functions and viscosity of liquid Al–Si hypoeutectic alloys via high-energy X-ray diffraction experiments

The liquid structure of Al–Si hypoeutectic binary alloys was characterized by diffraction experiments using a high-energy X-ray (synchrotron) beam source. The diffraction experiments were carried out for liquid pure Al, Al–3 wt% Si, Al–7 wt% Si, Al–10 wt% Si and Al–12.5 wt% Si alloys at several temperatures. The salient structure information such as structure factor (SF), pair distribution function (PDF), radial distribution function (RDF), coordination number (CN) and atomic packing densities (PD) were quantified as a function of Si concentration and melt temperatures. Reverse Monte Carlo (RMC) analysis was carried out using the diffraction experimental data to quantify the partial pair correlation functions, such as partial structure factor, partial pair distribution function (PPDF) and partial radial distribution function. Furthermore, the partial pair distribution function and the liquid atomic structure information were used in a semi-empirical model to evaluate the viscosity of these liquid alloys at various melt temperatures. The results show that the viscosity determined by semi-empirical methods using the atomic structure information is in good agreement with the experimentally determined viscosity values.

Characterization of the Flow Behavior of Near Eutectic Composition Aluminum-Silicon Alloys

The cone and plate method is used to quantify the effect of shearing rate, shearing time, and molten alloy temperature on the rheological properties of strontium modified and unmodified near eutectic Al-Si alloys. It is found that in the temperature range 583°C (1,081°F) to 598°C (1,108°F) at relatively low shear rates, these alloys behave as Non-Newtonian fluids. In addition, they exhibit shear-thinning, and at all the shear rates tested, the viscosity of the Sr-modified alloys is higher than that of the unmodified alloys.

Control Diffusion Solidification (CDS): An Overview of Mechanism and Application

The Controlled Diffusion Solidification (CDS) is an innovative casting technology that is being successfully employed in the near net shaped casting of Al based wrought alloys; specifically the 2xxx and 7xxx series. The novelty of the CDS technology is that it could mitigate the notable casting defect in conventional solidification of the Al wrought alloys, namely, hot tearing and enables the alloy to be cast into near net shaped components. This publication presents an overview of the phenomenological mechanism that lead to the non-dendritic morphology of the primary Al phase in the as-cast microstructure that facilitates a viable opportunity to cast the wrought alloys into high integrity and structural near net shaped castings. The salient mechanical properties of the cast component with AA7050 alloy in various heat treatment tempers will be presented along with micro structural characterization. Additionally, potential application for the technology to be adapted to various Al shaped casting processes will be explored and presented.

Rheology of Liquid Al, Zn and Zn-7wt%Al Systems

The flow behavior and viscosity of pure aluminum, zinc and Zn-7wt%Al liquids were quantified with the effects of temperature and shear rate by rotational rheometry experiments. These systems exhibited a non-Newtonian, shear thinning and non-thixotropic flow behavior where in the liquid metal viscosity decreases with increasing shear rates. The temperature dependence of viscosity followed the Arrhenius equation. Moreover, at high shear rate regimes the flow resembles a nearly Newtonian behaviour.

Effect of Incubation Coupled with Artificial Aging in T6 Heat Treatment of A356.2 Aluminum Casting Alloy

A356.2 is one of the most popular Al casting alloys for structural applications in automotive and aerospace sectors. Typically, the cast components from these alloys undergo a T6 heat treatment to significantly improve the mechanical properties. Solution heat treatments, incubation at room temperature, followed by artificial aging at an elevated temperature are the typical steps in a T6 heat treatment process. The incubation process (IP) is typically carried out between 4 to 12 hours on commercial foundry products and typically longer on aerospace castings. There has been a few studies on the mechanisms of precipitation reactions occurring during the incubation and natural aging processes, however all these are for the Al-Si-Mg wrought alloys which are markedly different in composition from Al-Si-Mg casting alloys such as the A356.2. A hypothesis for the sequence of precipitation reaction in the primary Al phase during incubation and artificial aging processes, and the quantified effect of these aging treatments on the resultant tensile properties of the cast components with A356.2 Al alloy is presented. The results show that depending on incubation time, the formation of individual clusters of Si and Mg, breakdown of the individual-clusters of Mg and formation of the co-clusters of Si and Mg (Mg5Si6) would take place. The formation of Mg1.8Si and the final stable Mg2Si phases occurs during artificial aging. The sequence of the precipitation reactions were formulated from micro-hardness data on the primary Al phase matrix, tensile properties and information from the prior-art in this field of study. Further, the time of incubation has a significant effect on the resultant mechanical properties because the precipitation reaction during artificial aging is directly affected by the duration of incubation time.