L. H. Chen

SiO2 particle erosion of A356.2 aluminum alloy and the related microstructural changes

Abstract A356-series AlSiMg alloys are subjected to SiO 2 sand erosion when they are used as the molding die in automatic molding to prepare sand mold for casting. In the current investigation, SiO 2 sand erosion of the as-cast and T6-treated A356.2 alloy was studied. To simulate the erosion during the sand-filling process in automatic molding, two test conditions of about 36 and 66 ms −1 average particle velocities were chosen. The results show typical ductile erosion behavior with maximum erosion rate at oblique impact, although brittle cracking of eutectic Si particles occurs. An ultrafine-grained surface layer which is mixed with fine SiO 2 particles can be observed on top of the strain-hardened dislocation cell region. Erosion of the T6 specimens leads to dissolution of the Mg 2 Si precipitates. Owing to this softening effect, the T6 treatment does not improve the erosion resistance.

The concept of effective hardness in the abrasion of coarse two-phase materials with hard second-phase particles

During the abrasion of a coarse two-phase material which contains hard second-phase particles, a brittleness mechanism can frequently coexist with plastic grooving. By taking R(X) as the abrasion resistance of given material with X as its second-phase volume fraction, the concept of effective hardness, Heff, gives rise to a modification of the linear rule of mixtures as R(X) ∫ (1-X)Hm + αXHs, where Hm and Hs are the hardness of pure matrix material and pure second phase, respectively. The parameter α decreases with increasing severity of the brittleness effect. By defining αc=Hm/Hs, dR/dX<0 provided that α<αc. The SiC abrasion data of a series of Al-Si alloys (with the volume fractions of pro-eutectic Si particles ranging from 0.023 to 0.219) can be rationalized by the above model since R(X) shows a nice linear fit with X. The αc value of the test alloys is close to zero. Therefore, as indicated from the wear surface, extensive brittle fracture can occur without deteriorating the abrasion resistance. For those with dR/dX<0, subsurface fracture is also pronounced.

Analysis on the amplitude of serrated flow associated with the Portevin-LeChatelier effect of substitutional fcc alloys

The serration of flow curve is a common feature of the Portevin-LeChatelier effect of substitutional fcc alloys. Under the condition that the onset strain of serration decreases with increasing temperature, previous experimental results have shown that the serration stress amplitude, Δσ, increases with increasing strain (ε) and temperature (T); it decreases with increasing grain size (d) and strain rate (ε). A conventional rationalization assumes that Δσ is proportional to the number of solute atoms, N, in dislocation atmospheres, where N is a function of the diffusion coefficient, aging time, and temperature. By adopting this approach and taking into account the effects of strain rate, strain, and grain size, the relation Δ α[ε−1 εβ(1/2 + γ) d−n(1/2 + γ) T-1 exp (-Q/kT)]2/3 is proposed, where Q is the activation energy associated with substitutional diffusion. The proposed model fits well with the experimental data of an Al-3.7 wt pct Mg alloy.

The effect of particles on the critical strain associated with the Portevin-LeChatelier effect in aluminium alloys

AbstractThe effect of particles on the critical strain, ɛc, associated with the Portevin-LeChatelier (PL) effect of aluminium alloys is studied using Al-Mg-Ni and Al-Si alloys. Al-Mg-Ni and Al-Si alloy matrixes are composed of Al3Ni and Si particles, respectively. Tensile tests were performed in the temperature range 223–273 K in which the critical strain decreases with increasing temperature, and strain rates between 10−5 and 10−2 s−1 were chosen. According to the apparent activation energies, Q, Mg and Si solute atoms are responsible for the flow instability in Al-Mg-Ni and Al-Si alloys, respectively. The experimental results also show that the critical strain decreases with decreasing particle spacing, dp. Since the particle spacing is small compared to the corresponding grain size, the decrease in critical strain should be ascribed to the effect of particles. Considering that the dislocation density is increased by the particles, a modified model showing the critical strain, ɛc, as a function of particle spacing, dp, is proposed as

A study of the texture hardening of pure aluminum and AlAl33Ni eutectic alloy with [111] fiber texture

\dot \varepsilon \propto \varepsilon _c^{\beta (\gamma + 1/2)} d_p^{ - n (\gamma + 1/2)}

A study on the subgrain superplasticity of extruded Al-Al3Ni eutectic alloy

T−1 exp (−Q/kT), in which

Distribution of the Inclusion Particles in Ferritic Spheroidal Graphite Cast Irons

\dot \varepsilon