Segregation band formation in Al-Si die castings

Abstract

Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer.

Defect Band Characteristics in Mg-Al and Al-Si High-Pressure Die Castings

“…We, therefore, conclude that ESCs are not necessary for defect band formation in HPDC. This conclusion is also consistent with both gravity flowthrough experiments, where defect bands can form when fully liquid alloy is used as feedstock material, [34] and rheology experiments, which do not involve ESCs. [35] The presence of ESCs may indirectly affect defect band formation, however, as discussed in Section V-E.…”

“…[1,10] The assembly of crowded crystals within this stagnant material experiences a developing shear stress as mobile material flows past, which could lead to dilatant shear bands in the stagnant material, similar to the mechanism proposed to occur in gravity flowthrough experiments. [34] The outer band in Figure 11(a) could have formed during die filling in this way. Localized deformation could also occur during the pressurization stage: As the solidifying alloy shrinks and thermally contracts, a pressure differential exists between the high pressure applied to the biscuit and the decreasing pressure in the cavity.…”

“…Laboratory casting experiments have shown that features similar to HPDC defect bands can be reproduced by shearing partially solid alloys: In ''gravity flow-through'' experiments, in which flow and solidification occur concurrently, bands form that contain positive macrosegregation in AlSi7Mg, [34] and concentrated porosity in AM60 [34] and AZ91, [35] similar to HPDC.…”

Solidification Behavior of Intensively Sheared Hypoeutectic Al-Si Alloy Liquid

“…The mechanism for band formation and effect of HPDC process parameters on defect band are explained in Ref. [4][5][6]20] By contrast, the sheared melt will have a uniform temperature, composition and well-dispersed nucleation sites, which results in the formation of considerably smaller solid fraction of ESC particles ( Figure 4) which are well distributed throughout the volume of the MC-HPDC, cast specimens ( Figure 2(c)). At low solid fractions, the strength increases at a relatively low rate.…”

Effect of Solution Heat Treatments on the Microstructure and Mechanical Properties of a Die-Cast AlSi7MgMn Alloy

“…Generally, in die-cast Al-Si alloys, these bands contain a segregated eutectic fraction, and their position and appearance are influenced by silicon content and solidification history. [25,28,29] In order to study how the local eutectic amount varies over the cross section of as-die-cast samples, the eutectic area fraction was calculated by quantitative optical microscopy. The distribution of eutectic area fraction is overlapped to the macrostructure of a sample ( Figure 1).…”