Werner Fragner

Effect of main alloying elements on strength of Al–Si foundry alloys at elevated temperatures

Abstract In this study, we describe the effect of the main alloying elements Ni, Cu and Mg on the mechanical properties of near eutectic and hypoeutectic Al–Si foundry alloys at 250°C after a long term exposure to test temperature. Systematic compositional variations illustrate the significant hardening effect of secondary precipitates such as Al2Cu and Mg2Si. It is also shown that the strength is increased by the addition of Ni, albeit only to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys are considered as coarse two-phase systems, where a hardening effect is caused by load transfer to the harder phase, which requires a certain contiguity of the latter. This paper discusses the individual influences of Ni, Cu and Mg on the high temperature strength and describes a potentially adverse effect of the combinations Cu/Ni and Cu/Mg.

The effect of Ni on the high-temperature strength of Al-Si cast alloys

In order to investigate the effect of Ni on the high-temperature strength of Al-Si cast alloys, tensile properties of hypoeutectic and eutectic alloys were determined at 250 °C after long-term annealing at test temperature. LOM- and SEM-analysis revealed the existence of Al3Ni-phases in close contact to eutectic Si. It was shown that the strength can be increased by the addition of Ni, though just to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys were considered as a coarse two-phase system where a hardening effect is caused by load transfer to the harder phase, which requires a certain connectivity/contiguity of the latter. The paper describes the extent of contiguity of the eutectic as well as the hard silicon and Al3Ni-phases within the eutectic, and discusses their contribution to an enhanced strength of Al-Si alloys at elevated temperatures.

Interface reactions of Al and binary Al-alloys on mild steel substrates in controlled atmosphere

To meet the requirements of weight-saving and low-cost production of components for future transport vehicles, the concept of multi-material mix is of increasing importance. In this context aluminum-iron compounds produced by means of compound casting are considered to be of particular importance. An essential and critical aspect of such compound castings is the formation of intermetallic phases (IMP) at the Al-Fe interface. Both the nature and the kinetics of potential IMPs are not well understood and require a systematic investigation. In this paper we document the interface formation of pure Al and binary Al-alloys on a mild steel substrate by means of isothermal wetting experiments. Tests were carried out employing the sessile droplet method in a controlled atmosphere. Using pure Al and Al7Si, Al7Cu, and Al7Zn alloys the interface reactions were investigated by quantitative metallography (LOM, SEM/EDX). Special attention was paid to the influence of the alloying elements on the type and sequence of IMPs at the interface.

Influence of variations in alloy composition on castability and process stability. Part 1: Gravity and pressure casting processes

Abstract The influence of variations in alloy composition on the fluidity of three different light metal alloys was investigated. The aluminium alloys AlSi9Cu3 (A380) and AlSi7Mg (A356) as well as the magnesium alloy AZ91 were cast into spiral fluidity test dies at their upper and lower compositional tolerance limits. The test results show significantly varied behaviour, which is explained by microstructural features and heat content. In alloy AlSi7Mg at the upper compositional limit the formation of intermetallic phases in the interdendritic liquid physically blocks the flow and leads to short flow length compared with the same alloy with low iron and manganese content. Alloy AlSi9Cu3 at the upper compositional limit is very close to eutectic composition and flows significantly better than the alloy at the lower tolerance limit. The intermetallic phases are formed simultaneously with the matrix during eutectic solidification and do not block the flow. In the magnesium alloy the intermetallic phase is incorporated in the primary magnesium phase and does not hinder the flow. A higher content of alloying elements increases the flow length owing to a higher heat content of the melt.

Using Scrap in Recycling Alloys for Structural Applications in The Automotive Industry

After studying CO2 emissions caused by vehicles itself, automotive manufacturers now also consider CO2 emission during the production of vehicles and attempt to reduce them according to a holistic approach. This is where AUDI AG as first OEM together with AMAG Casting applies this approach at the production of structural components, which have been produced by primary alloys up to now. This component segment, which is significantly growing by now, requests mainly high ductility values, in order to absorb as much energy as possible in the case of a crash. In addition to the Fe-content, recycling alloys also have further tramp elements, which occur inevitably at scrap treatment. Besides Cu and Zn there are also elements like Bi, Cr, Ni, Sb, Sn etc. which have to be considered, since they influence the requested alloy properties in a negative way. The results of the existing work show that skilled scrap input at the alloy production can reduce this negative effect close to zero.

THE INFLUENCE OF SOLUTION TREATMENT ON THE HIGH-TEMPERATURE STRENGTH OF AL-SI FOUNDRY ALLOYS WITH NI

Al-Si-Ni alloys can be considered as a coarse two-phase system where a hardening effect is caused by load transfer to an interconnected rigid network of eutectic Si and aluminides. In the course of a solution treatment the contiguity of the eutectic phase is reduced, which leads to a decrease of strength. However, solution treatment is necessary to obtain a high supersaturation of elements in the Al-solid solution, which contribute to high-temperature strength due to precipitation hardening. Despite Ostwald ripening, the distribution of secondary precipitates is still dense enough to act as dislocation obstacles, as was confirmed by TEM-analysis. This work discusses the influence of heat treatments on the elevated-temperature strength of Al-Si foundry alloys with Ni and analyzes the active strengthening mechanisms. In order to investigate the effect of a solution treatment on the high-temperature strength of Ni-containing Al-Si foundry alloys, the tensile properties of various eutectic alloys were determined at 250°C after long-time exposure to test temperature.

The Effect of Nickel on the Thermal Conductivity of Al-Si Cast Alloys

In this study the effect of Ni on the thermal conductivity of hypoeutectic Al-Si cast alloys is described. The Ni-containing alloys are considered as a two-phase system, which consists of a ‘matrix’, composed of α-solid solution, eutectic Si, primary Al12(Fe,Mn)3Si2 phases and secondary Mg2Si-precipitates, and Ni-containing intermetallic phases incorporated in this ‘matrix’. With increasing Ni-content the volume fraction of second phases increases, whereby the characteristics of the material change from a ‘homogeneous matrix’ to a ‘particle-reinforced’ two-phase composite. The thermal conductivity data are discussed on a systematic basis of thermodynamic calculations and compared to data for the electrical conductivity as well as theoretical models for the thermal conductivity of heterogeneous solids.