Environmental savings can be made by increasing the use of aluminium alloys in the automotive industry as the vehicles can be made lighter Increasing the knowledge about the heat treatment process 

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Heat treatment of Al-Si-Cu-Mg casting alloys

The heat treatment of Al–Si–Cu–Mg casting alloys

Excellent thermal conductivity and lower density make Al–Si alloys a suitable alternative for cast iron in the fabrication of engine components. The increase in the maximum operation temperature and pressure of engines necessitates improving the thermomechanical fatigue performance of Al–Si alloys. This paper has two major parts focussing on the use of Al–Si based alloys in cylinder heads and engine blocks.In the first part, the structural stress–strain and material property requirements of cylinder heads are discussed. In addition, the physical and mechanical properties of different competing materials used in the manufacture of engine components are reviewed. The physical metallurgy, solidification sequence and thermal conductivity of Al–Si based alloys are reviewed. Also discussed is the effect of microstructural features on thermomechanical fatigue lifetime. This part also includes an overview of the strengthening mechanisms of cast Al–Si alloys, by dispersed phases and heat treatment.Demands ...

Application of cast Al–Si alloys in internal combustion engine components

Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification The β-Al5FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied The volume fraction of β-phase formed was seen to increase with the decrease in cooling rate (ie with increasing distance from the chill) in the untreated alloy Sympathetic (preferential) nucleation of the β-iron needles was also observed, in which the branching of β-needles from a parent needle resulted in the formation of large β-needle entities that can cover distances of ∼ 1200 μm across the matrix surface The beneficial effect of modification, ie strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the β-needles The strontium “poisons” the sites where sympathetic nucleation takes place Dissolution was accelerated with increasing strontium content, the optimum level being ∼ 300 ppm Grain refining, on the other hand, negated the beneficial effect of modification, in that the β-needles underwent thickening and the sympathetic nucleation/branching also occurred The modified alloy was found to possess the lowest volume fraction of β-Al5FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

The dissolution and melting of Al2Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al2Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al2Cu phase. Dissolution of the (Al + Al2Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al2Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al2Cu). Unlike the eutectic, fragments of the blocky Al2Cu phase are still observed in the matrix, even after 24 hours at 540 °C.

J. Gauthier

Papers

Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment