Mechanical properties and thermal stability of Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn alloys prepared by powder metallurgy

Abstract

Al–23Si–8Fe–1Cr and Al–23Si–8Fe–5Mn (in wt%) alloys were prepared via melt centrifugal atomization following powder compaction by uni-axial pressing at 6 GPa and 450 °C for 60 min. The obtained materials were porosity-free with good particle-to-particle contact. The “composite-like” microstructures were very fine, showing silicon particles distributed in an α-Al matrix and displaying nearly equiaxed intermetallic phases. The intermetallic phases were identified as β-Al₅FeSi and α-Al_9.3FeMn_1.4Si_1.8 in the Al–23Si–8Fe–1Cr and Al–23Si–8Fe–5Mn alloys, respectively. The Vickers hardness and compressive yield strength were 182 HV5 and 540 MPa, respectively, for the Al–23Si–8Fe–1Cr alloy and 197 HV5 and 650 MPa, respectively, for the Al–23Si–8Fe–5Mn alloy. The thermal stability of the alloys was studied by mechanical testing after long-term annealing at 300–400 °C, by compressive testing at 200–300 °C and by creep testing at 300 °C and 250 MPa. The thermal stability of both alloys was significantly better when compared to the “thermally stable” casting Al–12Si–1Cu–1Mg–1Ni alloy that is commonly utilized in the automotive industry for thermally loaded components such as pistons for combustion or diesel engines.

Introduction

Al–Si-based alloys exhibit a high strength-to-weight ratio, high thermal conductivity and excellent castability. Thus, Al–Si-based alloys have potential for the fabrication of various lightweight components in the automotive and aerospace industries, such as engine blocks, cylinder heads and pistons. Millions of these components are produced every year by gravity or die casting. Adding elements that are slow diffusers in Al to these alloys improves their thermal stability, which is important for the fabrication of thermally loaded parts, such as pistons and turbocharger rotors. Several studies have shown that transition metals (TMs) such as Fe, Cr and Mn are very effective in this way [1], [2], [3]. However, the concentration of transition metals in casting and wrought Al alloys is strongly limited because of their low solid solubility in Al. Generally, only a few weight percent of TMs may be utilized in these alloys because higher concentrations produce excessive fractions of hard and brittle intermetallic phases in the microstructure, which deteriorate the material's mechanical properties.

In the industrial practice of recycling aluminium alloys, scraps commonly contain high fractions of Fe, which must currently be removed. Iron is commonly removed by magnetic separation, careful control of the melting process and/or the addition of pure aluminium to a molten alloy. However, all of these approaches increase the cost of recycled aluminium. The negative effect of iron in Al–Si casting alloys can also be partially reduced by appropriate additions of manganese.

Powder metallurgy (PM) is a promising alternative to the common casting and ingot metallurgy for processing Al–Si–Fe-based alloys containing iron and other transition metals in concentrations far exceeding those in common casting and wrought alloys [4]. PM Al–Si–Fe–X (X=Cr, Mn, Cu, Ni and other) alloys containing several wt% Fe were studied and shown to have a good combination of strength and thermal stability [5], [6], [7], [8]. Therefore, Al–Si–Fe-based alloys are promising for future structural applications, specifically because they are inexpensive. Rapid solidification of a melt, which generally occurs in powder preparation, significantly refines the microstructure, reduces the volume fraction of intermetallic phases and forms new metastable crystalline, quasi-crystalline and amorphous phases [4]. All of these structural features are beneficial for achieving desirable combinations of strength, ductility and thermal stability. Powder compaction is generally performed by hot extrusion [9], but high extrusion temperatures may negatively affect the resulting mechanical properties of a material, as they may encourage the growth of the intermetallic particles and Al grains. For these reasons, extrusion temperatures must be minimized, which is limited by the press facility used for the compaction. Alternatively, other widely used compaction processes, such as compression and sintering or hot isostatic pressing (HIP), generally operate at moderate pressures of approximately 100 MPa, which is not suitable for aluminium alloys because oxide layers formed on the powder particles prevent diffusion bonding. Recently, however, compression occurring at significantly higher pressures (on the order of ∼1 GPa) was shown to produce sufficiently compact and pore-free materials [10], [11].

In this study, we investigated PM alloys containing 23 wt% of Si and 8 wt% of Fe because PM offers a way to directly process Fe-containing wastes. Additions of Cr and Mn were used to modify the morphology of the intermetallic phases and to positively influence the thermal stability. The rapidly solidified alloys are compacted by uni-axial pressing at an ultra-high-pressure of 6 GPa, which provides sufficiently compact materials [11]. We demonstrate that such highly alloyed materials, when processed by PM, exhibit remarkable structural refining, hardness, strength, thermal stability and creep resistance. To compare the elevated temperature performance of the investigated PM alloys with currently used thermally stable Al-based materials, the Al–12Si–1Cu–1Mg–1Ni alloy was also studied as a reference material. This alloy is commonly applied to thermally cast and mechanically loaded components such as pistons for combustion or diesel engines.