THE INFLUENCE OF SOLUTION TREATMENT ON THE HIGH-TEMPERATURE STRENGTH OF AL-SI FOUNDRY ALLOYS WITH NI Florian Stadler 1 , Helmut Antrekowitsch 1 , Werner Fragner 2 , Helmut Kaufmann 3 , Peter J. Uggowitzer 4 1 2 AMAG Casting GmbH, Postfach 32, 5282 Ranshofen, Austria Institute of Nonferrous Metallurgy, Montanuniversitaet Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria 3 AMAG Austria Metall AG, Postfach 32, 5282 Ranshofen, Austria 4 Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Straße 10, 8093 Zurich, Switzerland Keywords: Al-Si foundry alloy, Solution treatment, High-temperature strength, Hardening effect, Nickel Abstract 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. Introduction Multicomponent Al-Si based foundry alloys provide several advantageous characteristics such as good castability, high corrosion and wear resistance as well as high thermal conductivity and adequate strength at elevated temperatures. Therefore they are widely used in automotive industry, especially for cylinder heads, pistons and gearbox housings [1-8]. The accelerated need for weight reduction, however, leads to higher mechanical and thermal loading of these aluminum castings in future vehicles, requiring improved Al-Si foundry alloys concerning strength at elevated temperatures [1-3, 7, 9]. For the development of new Al-Si recycling foundry alloys with superior elevated-temperature strength a detailed physical understanding of the role of each particular alloying element concerning its ability to improve the high-temperature properties is required. In previous work Ni was identified to significantly enhance the high-temperature performance of Al-Si foundry alloys, though just to a certain level, depending on the fraction of eutectic phase in the alloy. Ni stabilizes the contiguity of the eutectic network by increasing the volume fraction of rigid phases (Si + Al 3 Ni) in the eutectic [8, 10-12]. Heat treatment (HT) is a commonly used technique to enhance the mechanical properties of the respective alloys, such as strength (HT: T6) or ductility (HT: T4, T7). However, during a solution treatment the eutectic Si spheroidizes and its aspect ratio decreases, which results in a loss of the contiguity of the eutectic platelets [8, 13]. Previous studies showed that in Ni-free alloys the Si contiguity is completely lost after a solution treatment at 540°C for 24 h, and (for example) an AlSi12 alloy changes from a ‘fibre-reinforced’ to a ‘particle-reinforced’ type of material, with the same strength as an AlSi1 alloy [8, 13]. Since heat treatment is energy-intensive and time-consuming it is generally desired to abstain from this expensive process, provided that the materials’ performance concerning their mechanical properties is not severely affected. Consequently the aim of the present work is to analyze the effect of a solution treatment on the high-temperature strength of Ni-containing Al-Si foundry alloys and to clarify its effect on the active strengthening mechanisms. Experimental Methods Eight near-eutectic alloys based on the system AlSi12 with varying Ni concentration were fabricated by the AMAG Austria Metall AG testing laboratory. The composition of the samples is given in Table I. The alloys can be divided into two groups: Mg- containing (1 – 4) and Mg-free (5 – 8). All materials were melted in a 100 kg induction furnace and cast into a steel mould with a wall thickness of the test section of 20 mm to form tensile test bars. The mould was preheated to a temperature of 320 ± 5°C and coated with boron nitride before casting. The melt temperature was held constant at 750 ± 5°C during the whole casting process. The applied heat treatment involved a solution treatment at 495°C for 8 h and quenching into water at room temperature, and was carried out for the half of the samples which are marked by the abbreviation ‘ST’ in the following. In order to simulate the ‘thermal load’ in service all samples were over-aged at 250°C for 100 h. Tensile test bars were designed according to the standard DIN60125:2004-01 (gauge length: 48 mm; diameter: 8 mm) and tested on a ZWICK universal tensile testing machine at 250°C using a strain rate of 0.006 sec -1 . The mechanical properties illustrated in the results section represent the average values of at least three separately cast rods. Metallographic specimens were taken within the gauge length region of the tensile bars to analyze the microstructure by means of light optical microscope (LOM) and transmission electron microscope (TEM) techniques. To identify the phase components occurring in the alloy, energy dispersive X-ray (EDX) analysis was performed. Light Metals 2012 Edited by: Carlos E. Suarez TMS (The Minerals, Metals & Materials Society), 2012 431