TECHNICAL DEVELOPMENT REPORT CONTROLLED DIFFUSION SOLIDIFICATION OF
2024, 6082 AND 7075 Al ALLOYS VIA TILT-POUR CASTING PROCESS P. Ashtari, G. Birsan, A. Khalaf and S. Shankar
Light Metal Casting Research Centre (LMCRC), McMaster University, Hamilton, ON, Canada Copyright © 2011 American Foundry Society Abstract
Controlled diffusion solidification (CDS) is an innova- tive casting route for Al alloys to obtain a cast part with a non-dendritic morphology of the primary Al phase dur- ing 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. Fur- ther, the favorable melt superheat temperatures of the two pre-cursor alloys were evaluated from laboratory experi- ments as well. A tilt-pour mould designed and validated to cast tensile test bars was used for the study. The aim of the
Introduction
Controlled diffusion solidification (CDS) is a novel casting process to enable high integrity shaped casting of Al cast and wrought alloys alike with a cast microstructure similar to those observed in semi-solid metal (SSM) casting products. CDS is a cost effective route wherein conventional casting equipment could be used in both the High Pressure Die Cast- ing (HPDC) and permanent mould casting processes with the installation of an additional melting/holding furnace. The unique feature of the CDS process is that it enables the shaped casting of Al based wrought alloys with the aim of achieving superior properties and performances when com- pared with their casting alloys counterparts.1
The advanta-
geous feature of the CDS technology is the significant ease of obtaining an alloy mixture with a homogeneous tempera-
Nomenclature Alloy 1
Alloy 2 Alloy 3
TL1 TL2 T1
, T2 , TL3
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 cast- ing 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.
Keywords: CDS, controlled diffusion solidification, 2024, 6082, 7075, SSM, Al wrought alloys, rheocasting
ture (without any substantial additional capital equipment) within a few seconds (< 5s) of the intermediate holding time to equalize the temperature in the alloy mixture.
In the CDS process, two alloys of specifically chosen sol- ute compositions and melt superheat temperatures are mixed such that the alloy with the higher thermal mass (tempera- ture and mass) is poured into the alloy with the lower ther- mal mass to obtain a resultant mixture in which the tempera- ture and solute composition fields equalize in a controlled manner; and the resultant mixture is poured into a mould to yield a shaped casting with a non-dendritic morphology of the primary Al phase in the solidified microstructure.2 a previous publication,2
In we have elaborately discussed the
mechanisms involved during the mixing and solidification stages of the CDS process. Figure 1 shows a schematic of
Pre-cursor alloy with higher thermal mass (higher temperature and higher mass). Pre-cursor alloy with lower thermal mass. Resultant required Al wrought alloy.
Liquidus temperature of Alloy 1, Alloy 2 and Alloy 3 respectively. Melt Temperature of Alloy 1 and Alloy 2 respectively.
International Journal of Metalcasting/Spring 11 43
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