C. Ravindran

Prediction of Hot Tear Formation in a Magnesium Alloy Permanent Mold Casting

A viscoplastic deformation model considering material damage is used to predict hot tear evolution in AZ91D magnesium alloy castings. The simulation model calculates the solid deformation and ductile damage. The viscoplastic constitutive theory accounts for temperature dependent properties and includes creep and isotropic hardening. The mechanical properties are estimated from data found in the literature. Ductile damage theory is used to find mechanically induced voiding, and hot tears are expected in regions of extensive damage. Simulations are performed for a test casting consisting of a 260 mm (10.2 in) long horizontal bar connected to a vertical sprue on one side and an anchoring flange on the other. The contraction of the horizontal bar is restrained during solidification and hot tears nucleate at the junction between the horizontal bar and the vertical sprue. The hot tearing severity is manipulated by adjusting the initial mold temperature from 140°C (284°F) to 380°C (716°F). For the simulation, a trial-and-error method is used to determine the mold-metal interfacial heat transfer coefficient from experimental thermocouple results. The simulation results suggest that the predicted damage is in agreement with the hot tears observed in the experimental castings, both in terms of location and severity. The simulation results also confirm the observed decrease in hot tear susceptibility with increasing mold temperature. These results suggest that the proposed viscoplastic model with damage shows promise for predicting hot tearing.

Onset of Hot Tearing in AE42 Magnesium Alloy

Abstract Magnesium alloy AE42 has long been recognized as a superior high temperature magnesium alloy for aerospace and automotive components. The elevated temperature strength of this alloy is attributed to the Mg-Alx-REy intermetallics on the grain boundaries preventing grain boundary sliding. However, these intermetallics also hinder interdendritic liquid feeding during casting solidification and contribute to the alloys high susceptibility to hot tearing. In this research, the conditions associated with the onset of hot tearing in the AE42 alloy were identified. Thermal analysis suggested that a casting with a hot tear experienced long vulnerable interval, when interdendritic feeding was minimal and the alloy was susceptible to hot tearing. Microscopic analysis revealed the presence of interdendritic shrinkage pores with Al-RE intermetallics at hot-tear nucleation sites. Further, the elastic residual strain measured by neutron diffraction indicates that tensile strain resulting from contraction of the casting during solidification was responsible for opening and propagation of hot tears in the AE42 alloy.

Influence of Mold and Pouring Temperatures on Hot Tearing Susceptibility of AZ91D Magnesium Alloy

This research focused primarily on studying the effect of mold temperature on the hot tearing susceptibility of permanent mold cast (PMC) AZ91D magnesium alloy. The results suggest that increasing the mold temperature from 140 °C (282 F) to 380 °C (716 F) had a significant impact on the severity of hot tearing in the alloy. Mold temperatures above 340 °C (644 F) were seen to virtually eliminate hot tears through a morphological change of the alloy’s β-phase regions, which enabled liquid metal feeding of the casting at later stages of solidification and high fractions of solid. Variation of the mold temperature also affected the alloy yield strength and casting porosity. The effect of increasing the pouring temperature in the 680–720 °C (1256–1328 F) range to reduce hot tearing was less pronounced than that of varying the mold temperature.

Interplay Between Residual Stresses, Microstructure, Process Variables and Engine Block Casting Integrity

The replacement of nodular cast iron with 319 type aluminum (Al) alloys in gasoline engine blocks is an example of the shift towards the use of lighter alloys in the automotive industry. However, excessive residual stress along the cylinder bore may lead to bore distortion, significantly reducing engine operating efficiency. In the current study, microstructure, mechanical properties and residual stress were characterized along the cylinder bridge of engine blocks following thermal sand reclamation (TSR), T7 heat treatment, and service testing of the casting. Neutron diffraction was effectively used to quantify the residual stress along both the Al cylinder bridge and the adjacent gray cast iron cylinder liners in the hoop, radial, and axial orientations with respect to the cylinder axis. The results suggest that an increase in cooling rate along the cylinder caused a significant refinement in microstructure at the bottom of the cylinder. In turn, this suggested an increase in alloy strength at the bottom of the cylinder relative to the top. This increased strength at the bottom of the cylinder likely reduced the susceptibility of the cylinder to rapid relief of residual stress at elevated temperature. In contrast, the coarse microstructure at the top of the cylinder likely triggered stress relief at an elevated temperature.

Solidification analysis of Al–5 wt-%Cu alloy using in situ neutron diffraction

Abstract In this study, in situ neutron diffraction was used to characterise the solidification of an Al–5 wt-%Cu alloy. Neutron diffraction patterns were collected in a stepwise mode during solidification between 660 and 440°C. The nucleation and evolution of the primary Al phase and Al2Cu phase was successfully detected and quantified. The solid fractions of these phases were determined from the intensity of neutron diffraction peaks over the solidification interval. Furthermore, the results from neutron diffraction showed good agreement with FactSage simulations and optical and scanning electron microscopy. This study aims to better understand the solidification mechanism of Al–Cu alloys, with a view to eliminating the formation of defects during solidification and thereby, enhancing the use of Al–Cu alloys in industrial applications.

Effect of melt superheat on maximum nuclei density in A356 alloy

Abstract The macro-micro modeling, in its present form, does not include the effect of melt super-heat. In this work, identical castings of sand, permanent mold, and lost foam processes were produced by pouring aluminum alloy A356 at 700, 750, 800, 850, and 900 °C. Grain size increased with pouring temperatures to different levels in the three processes. Cooling rates decreased with increasing pouring temperatures in sand and lost foam castings. Simulated cooling rates agreed closely with experimental values. The grain size-cooling rate relationship agreed well with published data. Maximum nuclei density, n 0 , varied with the casting process and pouring temperatures.

Investigation of solidification behaviour of Mg–6Al and Mg–9Al alloys using in situ neutron diffraction

Abstract In situ neutron diffraction was used to examine the solidification behaviour of Mg–6 wt-%Al and Mg–9 wt-%Al alloys. Samples of each Mg–Al alloy were heated above their liquidus temperatures and stepwise cooled to 420°C while simultaneously collecting neutron scattering intensities. The solidified alloys were examined using scanning electron microscopy. Mainly blocky Mg17Al12 was found in Mg–6 wt-%Al alloy while branched Mg17Al12 adjacent to a large network of fine lamellar Mg17Al12 was found in the Mg–9 wt-%Al alloy. The neutron diffraction data accurately described the fraction solid growth as represented by the () crystallographic plane over the entire solidification regime. The fraction solid of the Mg–6 wt-%Al alloy rose quickly at temperatures just below the liquidus point and rapidly approached 100% until solidification was complete while the Mg–9 wt-%Al alloy showed a more linear transition from liquid to solid. Neutron diffraction was also capable of detecting the formation of eutectic Mg17Al12 in the Mg–9 wt-%Al alloy. This research demonstrates unique possibilities in using neutron diffraction for further understanding of nucleation, eutectic formation and solid phase evolution of Mg alloys.

Comparison of Measurement Methods for Secondary Dendrite Arm Spacing

Secondary dendrite arm spacing (SDAS) is most commonly measured by what is referred to as the linear intercept method. However, substantial variation in the technique exists between researchers, and its influence on the measurements has not yet been elucidated. Given the strong correlations between SDAS and material properties, a consistent and accurate technique is essential for interstudy comparability and effective alloy design. In this study, the SDAS of an aluminum alloy cast at two different solidification rates was quantified using five methods typical in the literature. Each method enabled observation of the refinement in dendritic structure associated with faster solidification. Also, each method produced a very similar average SDAS value for castings at high solidification rates, since the error is reduced in fine and uniform microstructures. Yet, significant variation, inaccuracy, and inconsistency in SDAS values were found to be possible between the methods for castings at low solidification rates. The most accurate and precise of the five methods for both coarse and fine microstructures was identified, and its use is recommended to improve SDAS measurement practices in academia and the industry.

In situ neutron diffraction analysis of stress-free d-spacing during solution heat treatment of modified 319 Al alloy engine blocks

Abstract Aluminium alloy engine blocks have successfully replaced ferrous materials in order to maximise weight savings and improve vehicle fuel efficiency. However, the development of an optimal heat treatment process is required to improve engine block casting integrity and prevent potential problems such as in-service cylinder distortion. Optimisation of heat treatment parameters requires an in-depth study to determine how residual stresses are relieved with time during solution heat treatment. In order to perform this analysis, however, in situ neutron diffraction must first be carried out on stress-free samples of the same composition and processing history as the engine blocks to account for factors such as thermal expansion and changes in lattice parameter due to dissolution of secondary phases. The results from this study suggest that thermal expansion caused the largest change in d0 spacing, while prolonged exposure at the solutionising temperature resulted in relatively small changes in {311} and {331} d0 spacing due to phase dissolution.

Determination of the Rate of Latent Heat Liberation in Binary Alloys

For the simulation of casting solidification, it is necessary to model the liberation of latent heat from the solidifying metal or alloy. The existing models use simplifying assumptions that result in loss of accuracy. In this paper, a numerical method based on the thermodynamics of solidification is presented to calculate the modified specific heat that can incorporate the liberation of latent heat into any numerical solution procedure and yield more accurate results. The cases of (1) equilibrium solidification and (2) nonequilibrium solidification based on Scheils equation are used for calculating the fraction solid formed. The present method takes into account two important aspects namely: (1) the change in the fraction solid with decrease in temperature during solidification, and (2) the change in the composition of the solid formed during solidification. The calculated values of modified specific heat are compared with the results reported in literature.