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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

On the other hand, if

On the other hand, if the outlet of the preform is closed or the preform is enveloped within the melt pool prior to infiltration, the entrapped air determines whether an oxide film between the matrix alloy and reinforcement is formed or not. Depending on the specific surface area of the reinforcing phase, the calculations showed that, in preforms composed of more than about 50% by volume of fibres, oxide monolayers cannot be formed on all fibre surfaces. 2.5.3. High pressure die casting infiltration The main benefit of high pressure die casting (HPDC) is to obtain components with accurate near net-shape geometry due to the use of a high precision metal die. HPDC was found to have excellent performance with regard to control of casting dimensions, making it the most accurate casting method for light metal components (141) . The filling pressures in HPDC machines are high and commonly operate at metallic pressures of up to 100 MPa. Such pressures will force the liquid into radii of only 10 -8 m which is approaching atomic dimensions (6) . In the standard HPDC process, the high pressures are thus not needed to fill the cavity but to reduce the volume of the entrapped air. The air originates from turbulent filling of the die. In casting of film-forming metals, the turbulent filling process leads to formation of double oxide films (6) . These films decrease the mechanical properties, as they act as cracks inside the castings. A parameter to evaluate the turbulence is represented by the dimensionless Weber number We: 2 V rρ We = Equation 37 γ where V is the velocity of the liquid, ρ is the density of the liquid, r is the radius of the section thickness the liquid has to pass and γ is the surface tension of the liquid. As reported by Campbell (6) , if We ranges between 0.2 and 0.8, flow conditions free from surface turbulence might be assumed. If the Weber number reaches 100, surface turbulence can form double 57

layer oxide films. The Weber number in high pressure die castings can rise up to 100,000 which means that conditions are closer to atomisation than to fluid flow (6) . Fine grain microstructures of HPDC components result from rapid solidification. Nevertheless the material generally gives moderate strength and poor deformability. This is attributed to the large amount of internal defects (142) and the presence of iron in HPDC alloys. Iron is added to commercial HPDC alloys to prevent sticking of the metal to the uncoated die and results therefore in increased die life (143) . Iron is detrimental to material properties (144) as it forms an increased amount of iron-rich intermetallics of the AlxFey or the AlxFeySiz(Mn) (145) type which provoke crack initiation. Even though HPDC is widely used for large scale manufacturing of light metal components, it has not been used extensively for preform infiltration. The main reason is probably the lack of suitable preforms which endure the high dynamic pressure. The plunger velocities in standard HPDC are in the range of 1 to 5 m/s. Due to low ratios of gate area to plunger area, gate velocities are as high as 50 m/s. The dynamic pressure often leads to intensive deformation and cracking of the reinforcement. A measure of this deformation behaviour is the compression which is defined as the ratio of the MMC volume after infiltration to the initial overall preform volume, comprised of reinforcing phase plus porosity. The compression in the published HPDC infiltration experiments reaches 30% (50,51) . In compressed areas, the local volume fraction of reinforcing phase is increased by the factor of compression. Michaud et al. (122) investigated the preform compression behaviour when infiltrating fibre preforms with pure Al. Fluid flow starts only above the threshold pressure P0, which leads to compressive strain in the preform which is usually purely elastic. Assuming no premature solidification after full infiltration of the preform, complete relaxation is possible leaving no 58

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