D. Sediako

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.

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.

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.

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.

Quantification of residual strain associated with reduction of hot tears by grain refinement in B206 aluminium alloy

Abstract Hot tearing continues to limit the industrial use of many aluminium casting alloys. The hot tearing susceptibility of an alloy is influenced by microstructure, solidification rate and the stress/strain conditions it experiences during solidification. In this study, a novel technique was used to quantify the residual strain associated with hot tearing in B206 aluminium alloy. Neutron diffraction strain mapping was carried out on three B206 castings with varying levels of titanium (i.e. unrefined, 0·02 and 0·05 wt-%). Titanium additions were effective in causing grain refinement of the alloy and eliminating hot tears. Neutron diffraction strain mapping was successfully used to demonstrate more uniform distribution of strain in the casting, resulting from grain refinement with consequent higher resistance to hot tearing. La fissuration à chaud continue de limiter l’usage industriel de plusieurs alliages moulés d’aluminium. La susceptibilité à la fissuration à chaud d’un alliage est influencée par la microstructure, par la vitesse de solidification et par les conditions de contrainte et déformation dont celui-ci fait l’expérience lors de la solidification. Dans cette étude, on a utilisé une nouvelle technique pour quantifier la déformation résiduelle associée à la fissuration à chaud de l’alliage d’aluminium B206. On a effectué la cartographie de la déformation par la diffraction des neutrons de trois moulages de B206 ayant des niveaux variés de titane (i.e. non affiné, 0·02% et 0·05% en poids). Les additions de titane avaient un effet positif sur l’affinement de grain de l’alliage et sur l’élimination des criques de solidification. On a utilisé avec succès la cartographie de la déformation par la diffraction des neutrons pour démontrer la distribution plus uniforme de la déformation dans le moulage, résultant de l’affinement de grain avec pour conséquence une résistance plus élevée à la fissuration à chaud.

Solidification Analysis of a Magnesium-Zinc Alloy Using In-Situ Neutron Diffraction

This research outlines some of the experimental procedures and data analysis techniques used to examine the solidification of a magnesium-5 wt.% zinc alloy using in-situ neutron diffraction. The magnesium-5 wt.% zinc alloy was first melted in a graphite crucible and cooled from 660 to 200°C while being irradiated by neutrons as it solidified. The resulting castings were characterized using optical microscopy, scanning electron microscopy and X-ray diffraction. The alloy had a coarse irregular grain structure containing a Mg matrix with Mg-Zn intermetallics. The in-situ neutron diffraction results showed that fraction solid curves can be generated for lower intensity planes such as (1120) by using larger sample sizes and oscillation. Improvement in fraction solid analysis was obtained by examining the neutron diffraction intensity of the samples well below its solidus temperature.

Neutron Diffraction Analysis of Light Alloys: A Review

The development and application of low density alloys, such as Al and Mg alloys, has rapidly increased in the automotive sector in recent years. This necessitates advanced characterization techniques to assess the evolution of microstructure and phases during casting and processing. Further, understanding the mechanism of evolution of the defects is important in ensuring their minimization. Neutron diffraction has provided a method to determine the factors that trigger hot tearing in Al and Mg alloys as well as determining factors compromising integrity of powertrain components. In addition, neutron diffraction has been applied to examine the phase evolution during solidification of Al and Mg alloys enabling a better understanding of the effect of inoculants and solute additions on the solidification characteristics, resulting in improved castability. This paper highlights the frontiers of neutron diffraction analysis undertaken by the Centre for Near-Net-Shape Processing of Materials, Ryerson University and the CNL-Canadian Neutron Beam Centre.

Advanced Aluminum Alloy Development and In Situ Fitness-for-Service Testing for Automotive Lightweighting

Lightweighting has led to an increased use of aluminum alloys in many automotive systems, including the powertrain, body-in-white, and suspension. Fitness-for-service certification of new alloys for these applications frequently requires development of new testing methods that would subject the test components to realistic conditions of temperature and load, while studying the long-term materials’ response. As the typical lifetime of a vehicle exceeds 3000 h, the new testing methods must provide clear indication on the material’s suitability for a target application over a more realistic timeframe. An in situ study of the creep behavior using neutron diffraction quickly reveals the response of individual crystallographic planes to the applied load under the in-service operating conditions, yielding the critical information on the expected lifetime of the targeted component. This knowledge helps to identify alloy chemistry and processing conditions that result in manufacturing components capable of sustaining the thermal mechanical loads over the expected life cycle of a vehicle. Two advanced aluminum alloys, based on Al–Si and Al–Cu systems, have been the focus of this research.

High Temperature Creep Evolution in Al-Si Alloys Developed for Automotive Powertrain Applications: A Neutron In-Situ Study on hkl-Plane Creep Response

Recent trend in the automotive industry towards lightweighting and downsizing the powertrain components, without compromising the power output, have led to increased engine power density. Increased power density frequently requires these lighter components to operate in conditions of increased temperature and pressure, which is challenging for many aluminum alloys in use today in the powertrain manufacturing. Meeting the challenge requires not only improving high-temperature performance of known alloys or developing new ones, but also developing new advanced techniques to understand the long-term behaviour of the alloys.