Helmut Kaufmann

Fundamentals of the New Rheocasting process for magnesium alloys

Casting of high strength, ductile and pressure tight components at low cost is the prerequisite for the introduction of magnesium alloys into hydraulic and structural applications. This paper introduces the New Rheocasting process (NRC) as a novel approach for semi-solid casting of light metals, in which the slurry is prepared from normal casting alloys directly at the foundry machine. The specialties of the process and the alloy requirements are explained. Ways for increasing ductility and process stability with slight alloy modifications and proper heat treatment are shown. The resulting mechanical properties are compared with data received from classical high pressure die casting parts. New Rheocasting of the alloy AZ71proves to be superior in strength and ductility, and shows excellent KJC values. 100µm AZ71

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.

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.

Structure and properties of AIMgSi1 alloy tailored for semi-solid forming

Thixoforming requires semi-solid slurries, which contain a high volume fraction of non-dendritic solid phase with special grain morphology. Volume fraction, geometry and connectivity of the solid α-phase has to be kept within narrow limits. The paper illustrates the development of a new aluminium based wrought alloy AIMgSi1 alloy, adapted with barium, characterised by a microstructure which exhibits a low sensitivity towards fluctuations in process parameters resulting in improved mechanical properties of the alloy. The rheological behaviour of the new alloy by means of backward extrusion experiments is described, and an overview of the static and dynamic strength properties is given that can be achieved when the alloy is processed by means of the New Rheocasting process.

Influence of compositional variations on microstructural evolution, mechanical properties and fluidity of secondary foundry alloy AlSi9Cu3

Abstract The combined effect of the main alloying elements on the mechanical properties and fluidity of the secondary foundry alloy AlSi9Cu3 has been investigated. Systematic compositional variations within the alloys tolerance limit illustrate the broad spectrum of attainable properties. The yield strength in the as cast condition can be adjusted from 100 to 200 MPa, while the elongation to fracture can be simultaneously varied between 0·35% and almost 4%. Additionally, variation in fluidity by more than 100% can be achieved. The microstructure–property relationship is interpreted in the light of thermodynamic calculations that reveal a significant mutual interaction of the alloying elements.

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.

Recent Achievements and Future Challenges for R&D on Aluminium Alloys and Processing

The focus of this paper is set on technical achievements and challenges - however, these are most often closely linked to economical or ecological targets set by customers or society. Ideally, an alloy or process optimization leads to improved properties, reduced cost, and reduced emissions. With a continuously growing understanding of the underlying materials science, supported by novel computer simulation, improved alloys and processing routes have been developed. Many of the recent improvements were related to the thermal-mechanical treatment of high strength alloys for enhanced light weight design. Currently and in the future, the focus will be on sustainable development along the entire process chain, with special attention to the recycling of used products and high recycled content in new products. The optimized utilization of resources (e.g. materials, energy, etc.) will require the close cooperation of materials suppliers, product designers and manufacturers as well as R&D facilities to reconsider given material specifications and processing routes.

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.