Microstructure and mechanical properties of spray co-deposited Al–8.9 wt.% Si–3.2 wt.% Cu–0.9 wt.% Fe + (Al–3 wt.% Mn–4 wt.% Si)p composite
Introduction
The hypoeutectic Al–Si alloys such as the 380 (Al–7.5–9.5 wt.% Si–2–4 wt.% Cu–∼1 wt.% Fe) series play an important role in the recycling chain of the aluminium alloys and represent the most widely used system for the production of aluminium-based cast parts such as engine blocks/heads and gearboxes [1]. The 380 alloys can be heat treated by ageing if the magnesium content is above 2%, in order to allow the formation of Mg2Si precipitates, which have a hardening effect. However, their use as structural materials has been limited due to lack of ductility [2] caused by a microstructure composed of plate-like silicon particles and coarse, needle-like intermetallics, embedded in an Al matrix, and therefore there is no meaning in heat treat the 380 alloys in normal casting operation.
The spray forming process refers to the energetic disintegration of molten metal into micron-size droplets by high velocity gas jets. The subsequent deposition of these droplets, which are a mixture of solid, liquid and partially solidified particles, onto a substrate forms a dense deposit. Spray forming presents features of rapid solidification techniques and thus produces fine-grained microstructures, increased solid solubility, non-equilibrium phases and refined intermetallics [3]. These features allow the mechanical processing of hypoeutectic Al–Si alloys, which then are suitable to be used as structural materials. Prior researches aiming to determine the mechanical properties of spray formed Al–Si 380 alloy showed almost 150% increase in elongation when comparing with the values obtained by sand cast processing [4]. However, the absolute value attained (3.74%) was not enough for structural applications yet. This limitation was ascribed to the high porosity levels (4–7%) of the spray formed deposits, which is highly harmful to the mechanical properties [5]. The stress concentration due to the presence of pores is responsible to premature fracture when the material is loaded, impairing both ultimate tensile strength and ductility [6]. Therefore, the application of spray formed Al–Si 380 alloy as structural parts requires further processing such as extrusion to fully densification of the billet [7], [8], [9]. During extrusion the most important parameters are the temperature of billet and ram speed [10], whose careful control leads to a material with improved mechanical properties. Co-deposition of particles containing well selected pre-nuclei [11], [12], [13] changes the morphology and chemistry of harmful intermetallics, and therefore is another way to improve the mechanical properties.
In the present work we investigated the effect of co-deposition of particles containing AlMnSi-phases on the microstructure and mechanical properties of a widely used, hypoeutectic Al–Si alloy with a composition similar to the Al–Si 380 alloy. The spray formed deposits were extruded and heat treated. Moreover, the effect of Mg addition (only 0.3 wt.%) was also evaluated on the response to the heat treatment and on the mechanical properties.
Section snippets
Experimental procedure
The composition of the material used in this work, as determined by atomic absorption spectroscopy-AAS, was Al–8.9 wt.% Si–3.2 wt.% Cu–0.9 wt.% Fe–0.2 wt.% Mn–0.04 wt.% Mg and it will be hereinafter referred to as alloy 380. In some heats 0.3 wt.% Mg was added and this modified alloy will be referred to as 380 + Mg. SEM back-scattered electron images (BEI) coupled with EDS were used to distinguish the presence of different phases in the microstructure, which were later more precisely identified with
Results and discussion
Fig. 1 shows the microstructure of the powder particles that were used for co-deposition. It was observed the formation of a homogeneously distributed, round, α-AlMnSi phase embebed in an Al-matrix. XRD diffraction pattern of the powder, Fig. 2, confirms the presence of α-AlMnSi cubic phase, the Al and Si phases. The α-AlMnSi was predicted as a primary phase by the equilibrium diagram, as shown in Fig. 3. It must be pointed out that the phases present in XDR analyses are those predicted in the
Conclusions
Room temperature tensile tests of the spray formed and extruded/heat treated 380 alloy showed significant increase of elongation to fracture when compared with the values observed for the as-spray formed deposits, >10% and <4%, respectively. This result can be ascribed to the porosity elimination promoted by the extrusion process and to the lower aspect ratio of the silicon and intermetallic particles. Moreover, the co-deposition of AlMnSi-containing particles and the addition of Mg resulted in
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