Abstract
There is a need to extend the application of semi-solid processing (SSP) to higher performance alloys such as 319s (Al-Si-Cu-Mg) and 201 (Al-Cu-Ag). The melting of these two alloys was investigated using differential scanning calorimetry (DSC) and thermodynamic prediction. The alloys had been processed by magneto-hydrodynamic (MHD) stirring before receipt to produce a microstructure suitable for SSP. The DSC results for the as-received MHD material were compared with those for material which has been taken through a complete DSC cycle and then reheated for a second DSC run. The effects of microsegregation were then analyzed. A higher liquid fraction for a particular temperature is found in the second DSC run than the first. Microstructural observations suggest this is because the intermetallics which form during the first cooling cycle tend to co-located. Quaternary and ternary reactions then occur during the second DSC heat and the co-location leads to enhanced peaks. The calculated liquid fraction is lower with 10 K/min DSC heating rate comparing with 3 K/min at a given temperature. The DSC scan rate must therefore be carefully considered if it is to be used to identify temperature parameters or the suitability of alloys for SSP. In addition, the starting material for DSC must represent the starting material for the SSP. With thermodynamic prediction, the equilibrium condition will provide better guidance for the thixoforming of MHD stirred starting material than the Scheil condition. The Scheil mode approximates more closely with a strongly microsegregated state.
Highlights
SEMI-SOLID metal (SSM) processing is a powerful technology for forming alloys in the semi-solid state to near net-shaped products (e.g., Reference 1)
They have identified that it is the coincidence between this eutectic knee and ~50 pct liquid which means that A356 alloy (Al ~7 pctSi) is eminently suitable for semi-solid processing (SSP); if the knee occurred at a higher amount of liquid, the billet would have a much greater tendency to collapse before the liquid which should be present throughout the structure has fully developed
This paper summarizes the influence of microsegregation on the differential scanning calorimetry (DSC) curves and the comparison with those from thermodynamic prediction
Summary
SEMI-SOLID metal (SSM) processing is a powerful technology (with several variants including thixoforming and rheocasting) for forming alloys in the semi-solid state to near net-shaped products (e.g., Reference 1). There are more challenges with high strength aluminum alloys (such as the silver-containing 201, or the Al-Si hypoeutectic Cu-containing alloy 319) often due to hot cracking during the final stages of forming. These alloys usually contain copper, a key alloying element which contributes to the strength and age hardening response, but can increase hot tearing.[2] The excellent castability and good mechanical properties of 319 Al alloy (Al-6Si-3.5Cu-0.1 Mg wt pct) and 201 alloy (Al-5Cu-0.35Mg-0.35Mn-0.7Ag) give them widespread application in the automotive and aerospace industry, respectively.[3,4] Alloy 319s is a primary version of the low-cost foundry alloy 319 to enhance the suitability for VOLUME 48A, OCTOBER 2017—4701. Note that these are for alloy 319 with composition Al-5.7Si3.4Cu-0.62Fe-0.1Mg-0.92Zn-0.14Ti-0.36Mn, wt pct) rather than 319s (see Table III)
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