Optimisation of heat treatment of Al–Cu–(Mg–Ag) cast alloys

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Influence of sludge particles on the fatigue behavior of Al-Si-Cu secondary aluminium casting alloys

Al-Si-Cu alloys are the most widely used materials for high-pressure die casting processes. In such alloys, Fe content is generally high to avoid die soldering issues, but it is considered an impurity since it generates acicular intermetallics (\u3b2-Fe) which are detrimental to the mechanical behavior of the alloys. Mn and Cr may act as modifiers, leading to the formation of other Fe-bearing particles which are characterized by less harmful morphologies, and which tend to settle on the bottom of furnaces and crucibles (usually referred to as sludge). This work is aimed at evaluating the influence of sludge intermetallics on the fatigue behavior of A380 Al-Si-Cu alloy. Four alloys were produced by adding different Fe, Mn and Cr contents to A380 alloy; samples were remelted by directional solidification equipment to obtain a fixed secondary dendrite arm spacing (SDAS) value (~10 \ub5m), then subjected to hot isostatic pressing (HIP). Rotating bending fatigue tests showed that, at room temperature, sludge particles play a detrimental role on fatigue behavior of T6 alloys, diminishing fatigue strength. At elevated temperatures (200\u25e6C) and after overaging, the influence of sludge is less relevant, probably due to a softening of the \u3b1-Al matrix and a reduction of stress concentration related to Fe-bearing intermetallics

The complex interaction between microstructural features and crack evolution during cyclic testing in heat-treated Al–Si–Mg–Cu cast alloys

Abstract The study aimed to investigate crack initiation and propagation at the micro-scale in heat-treated Al–7Si–Mg cast alloys with different copper (Cu) contents. In-situ cyclic testing in a scanning electron microscope coupled with electron back-scattered diffraction and digital image correlation was used to evaluate the complex interaction between the crack path and the microstructural features. The three-nearest-neighbour distance of secondary particles was a new tool to describe the crack propagation in the alloys. The amount of Cu retained in the α-Al matrix after heat treatment increased with the Cu content in the alloy and enhanced the strength with a slight decrease in elongation. During cyclic testing, the two-dimensional (2D) crack path appeared with a mixed propagation, both trans- and inter-granular, regardless of the Cu content of the alloy. On fracture surfaces, multiple crack initiation points were detected along the thickness of the samples. The debonding of silicon (Si) particles took place during crack propagation in the Cu-free alloy, while cracking of Si particles and intermetallic phases occurred in the alloy with 3.2 wt% Cu. Three-dimensional tomography using focused ion beam revealed that the improved strength of the α-Al matrix changes the number of cracked particles ahead of the propagating crack with Cu concentration above 1.5 wt%

Tailoring Al-7Si-0.3Mg cast alloy properties to represent HPDC tensile and fatigue behaviour in component prototypes

To produce prototypes with mechanical properties expectable from EN AC 46000 HPDC components, prototyping related processes such as sand and plaster gravity casting as well as proper alloying and post solidification processes need to be understood and adjusted. Therefore, the influence of process, composition and heat treatment on tensile and fatigue behaviour has been investigated for an EN AC 42100 alloy. Sand cast test samples comprised the base alloy in as-cast condition and T5 treated as well as a 2 wt. % Cu addition in as-cast condition. Plaster cast test samples consisted of the base alloy in ascast condition and T6 treated as well as a 1.7 wt. % Cu addition in as-cast condition. Tensile and fully reversed bending fatigue tests (R=-1) have been performed and the results have been compared to EN AC 46000 HPDC values. Samples in heat treated conditions and with Cu addition exhibited superior tensile properties than the base alloy in as-cast state for both casting processes. Yield strength and elongation values for the sand cast T5 treated and with Cu addition samples were similar to the HPDC ones. In terms of fatigue behaviour, T6 treated and with Cu addition samples exhibited strength improvements for plaster cast samples, while no changes were observed for sand cast samples. Only sand cast samples exhibited similar fatigue behaviour to the HPDC samples. The sand cast T5 treated samples were found to produce the most similar overall mechanical behaviour to EN AC 46000 HPDC

The effects of Fe-particles on the tensile properties of Al-Si-Cu alloys

The effect of Fe-rich particles has been a topic for discussion in the aluminum casting industry because of the negative impact they exert on the mechanical properties. However, there are still contradictions on the effects of various morphologies of Fe-particles. In this study, microstructural characterization of tensile tested samples has been performed to reveal how unmodified and modified Fe-rich particles impact on the tensile behavior. Analysis of additions of Fe modifiers such as Mn and Cr, showed higher amounts of primary Fe-rich particles (sludge) with increased porosity and, as result, degraded tensile properties. From the fracture analysis of tensile tested HIPed samples it could be concluded that the mechanical properties were mainly governed by the Fe-rich particles, which were fracturing through cleavage, not by the porosity

Effect of cooling rate and eutectic modification on texture and grain structure of Al-Si-Cu-Mg die cast alloy

The effect of cooling rate and eutectic modification on texture evolution and grain structure of an Al-Si-Cu-Mg die cast alloy were investigated using optical microscopy (OM) and electron backscatter diffraction (EBSD) techniques. Directional solidification technique was utilized to produce as-cast specimens having low level of casting defects with controlled microstructural scale: specimens with average SDAS of 10 and 25 µm. Mode of solidification, cooling rate and eutectic modification did not induce any significant texture in the microstructure. An increase in cooling rate resulted in reduction grain size. High degree of grains orientation randomness was found in high cooling rate regardless of modification treatment

Influence of Quench Rate on the Artificial Ageing Response of an Al-8Si-0.4Mg Cast Alloy

The aim of the study is to present the influence of quench rate on the artificial ageing response of Al-8%Si-0.4%Mg cast alloy in terms of Brinell hardness and yield strength. The investigated material was produced by a gradient solidification technique and exhibited a microstructure that corresponds to the one of gravity die castings, with a dendrite arm spacing of approximately 25 μm. The study comprises two solution treatment temperatures, five quench rates and artificial ageingtimes exceeding 100 hours at 170 and 220 ⁰C. The microstructure and concentration profiles of Mgand Si were evaluated using energy and wavelength dispersive spectroscopy. Microstructural examination reveals an increment of solutes in the Al-matrix when higher solution treatment temperatures accompanied with high quench rates are applied and shows how both Si and Mgatoms have diffused towards the eutectic during quenching. Consequently, i.e. by increasing the levels of solutes and vacancies, the highest strength levels were realized. The study confirmed that quench rates above 2 ⁰C /s do not offer substantial strength improvement while quenching at lower rates resulted in a lower peak hardness and longer times to peak

High Temperature Tensile Deformation Behaviour and Failure Process of an Al-Si-Cu-Mg Cast Alloy : The Microstructural Scale Effect

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10− 4 s− 1 to 10− 1 s− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure

Effect of Co and Ni Addition on the Microstructure and Mechanical Properties at Room and Elevated Temperature of an Al–7%Si Alloy

Increasing environmental demands are forcing the automotive industry to reduce vehicle emissions by producing more light-weight and fuel efficient vehicles. Al–Si alloys are commonly used in automotive applications because of excellent castability, high thermal conductivity, good wear properties and high strength-to-weight ratio. However, most of the aluminium alloys on the market exhibit significantly reduced strength at temperatures above 200 °C. This paper presents results of a study of the effects of Co and Ni in a hypoeutectic Al–Si alloy on microstructure and mechanical properties at room and elevated temperature. Tensile test specimens with microstructures comparable to those obtained in high-pressure die casting, i.e. SDAS ~ 10 µm, were produced by directional solidification in a Bridgman furnace. The results show an improvement in tensile properties up to 230 °C