Spray-cast Alloy Research Articles

Comparison of dendrite and dispersive structure in rapidly solidified Cu–Co immiscible alloy with different heat flow modes

Rapid solidification of Cu–Co immiscible alloy was investigated by glass-fluxing, spray casting and melt-spinning techniques. Both the transition from dendrite to dispersive structure and corresponding scale evolution were revealed and further elucidated in terms of the heat flow mode, nucleation and growth processes under different solidification conditions. With the increase of undercooling, columnar dendrite is replaced by dispersive structure due to the immiscible effect. In contrast, equiaxed dendrite forms in spray cast alloy due to multiple nucleation events and becomes thinner for the case of higher cooling rate. Ascribed to the enhanced non-equilibrium effect and insufficient period for collision and coagulation processes between separated droplets, fine globular dispersion appears upon the diameter of spray casting reaching 4 mm. As for the melt-spun ribbon with the highest cooling rate, a single-phase solid solution microstructure with refined grain of cellular morphology can be obtained, which is attributed to the suppression of liquid phase separation by instant solidification.

Dry Sliding Wear Behavior of Cold Rolled Spray Cast Al–6Si Alloy

In the present investigation, dry sliding wear behavior of spray cast Al–6Si alloy in both as-sprayed and different cold rolled reductions have been studied as a function of applied load. The spray cast preform was cold rolled to increase its densification. The preform showed equiaxed grain morphology containing some irregular pores and these were almost eliminated by the cold rolling. The improved wear properties of cold rolled spray cast alloy are discussed on the basis of microstructural features at different rolling reductions and the nature of debris particles collected from wear test.

Effect of Microstructure on Wear Characteristics of Spray Cast Al-Si Alloys

In the present study, the effect of microstructure on the wear characteristics of spray cast Al-15Si and Al-20Si alloys has been investigated and compared with that of vertical axis centrifugal cast and chill cast alloys. The spray cast alloys have been subjected to Hot Isostatic Pressing for porosity reduction. HIPing has reduced the porosity in the spray cast Al-15Si and Al-20Si alloys from 19% to 8% and 19% to 2% respectively. The microstructures of spray cast alloys consisted of finely divided globular shaped Si particles with sizes varying from 2–10 µm. The Si particles have been distributed uniformly throughout the Al matrix. On the contrary coarse and segregated microstructures have been observed in vertical axis centrifugal cast and chill cast alloys. The sizes of primary and eutectic Si in vertical axis centrifugal cast alloys are smaller than that in chill cast alloys. The wear rates are the lowest for spray cast alloys and highest for chill cast alloys over a wide range of sliding velocity. The reasons for better wear resistance of spray cast alloys have been discussed in the light of the microstructural features evolved during spray casting.

Evolution of microstructure and its effect on wear and mechanical properties of spray cast Al–12Si alloy

Abstract In the present study, the microstructural features, mechanical properties and wear characteristics of spray cast Al–12Si alloy have been investigated and compared with that of the chill cast alloy. The spray cast alloy was subjected to hot isostatic pressing for reducing the porosity of the deposit from 26% to 6%. The microstructure of the spray cast and hot isostatically pressed alloy consisted of finely divided globular shaped eutectic Si uniformly distributed in the Al matrix. The fine particles of Si were invariably observed in the bottom and top regions of the spray-deposit in contrast to relatively large size Si particles in the central region. The microstructure of the chill cast alloy was coarse and consisted of an eutectic mixture of α-Al and Si needles. The room temperature tensile tests and hardness measurements showed improved mechanical properties of spray cast alloy over that of the chill cast alloy. In addition, the dry sliding wear test indicated a superior wear resistance of the spray cast alloy over a wide range of sliding velocity. This behavior of the spray cast alloy is discussed in the light of its microstructural modification induced by spray casting.

Microstructural Features, Wear, and Corrosion Behaviour of Spray Cast Al—Si Alloys

In the present study, the microstructural features, wear characteristics, and corrosion behaviour of spray cast Al—12Si, Al—15Si, and Al—20Si alloys have been investigated. The alloys were spray cast and hot isostatically pressed. Hot isostatic pressing has considerably reduced the porosity in spray cast alloys. The microstructure of spray cast alloys showed fine and globular-shaped Si particles in the Al matrix in contrast to needle-type eutectic Si and blocky-type primary Si in the Al matrix in chill cast alloys. The dry sliding wear tests showed that the wear rates of spray cast alloys are invariably lower and the potentiodynamic polarization tests showed that the corrosion resistance of spray cast alloys is considerably higher than that of chill cast alloys. The high wear and corrosion resistance of spray cast alloys have been discussed in the light of the microstructural modification induced during the spray casting process.

Effect of processing techniques on the mechanical and wear properties of Al-20Si alloy

In the present investigation, the mechanical and wear properties of Al-20Si alloy processed by spray casting and vertical centrifugal casting processes have been evaluated and compared. In spray casting process the melt was gas atomized and the spray-deposited over a copper substrate. The spray-deposit exhibited considerable porosity and subjected to hot isostatic pressing to reduce the porosity from 19 to 2%. The centrifugal casting process provided cylindrical shaped preform with characteristically low porosity. The microstructure of spray cast alloy showed ultra fine and uniformly distributed primary and eutectic Si particles in the Al matrix. In contrast, a coarse polyhedral shaped morphology of the primary Si phase was observed in the microstructure of the centrifugal cast alloy. The wear rate of spray cast alloy was invariably lower than that of centrifugal cast alloy. The room temperature tensile and hardness tests of spray cast alloy showed considerable improvement in its strength, ductility and hardness over that of centrifugal cast alloy. The improvement in wear properties of spray cast alloy is discussed in the light of its microstructural modification induced by spray casting and nature of debris particles generated during wear testing.

가스분무주조 Cu-Sn-Ni-Si 합금의 미세조직 및 상온 인장성질

In this study, Cu-10Sn and Cu-10Sn-2Ni-0.2Si alloys have been manufactured by spray casting in order to achieve a fine scale microstructure and high tensile strength, and investigated in terms of microstructural evolution, aging characteristics and tensile properties. Spray cast alloys had a much lower microhardness than continuous cast billet because of an improved homogenization and an extended Sn solid solubility. Spray cast Cu-Sn-Ni-Si alloy was characterized by an equiaxed grain microstructure with a small-sized (Ni, Si)-rich precipitates. Cold rolling of Cu-Sn-Ni-Si alloy increased a tensile strength to 1220 MPa, but subsequent ageing treatment reduced a ultimate tensile strength to 780 MPa with an elongation of 18%.

Mechanical Properties of a Spray-Cast Aluminum Alloy Processed by Severe Plastic Deformation

Experiments were conducted to evaluate the mechanical properties of a spray-cast Al- 7034 alloy processed by severe plastic deformation. The alloy was received with an average grain size of ~2.1 μm and processed by equal-channel angular pressing (ECAP) at a temperature of 473 K to give a grain size of ~0.3 μm after 6 or 8 passes. Following ECAP, the mechanical properties were evaluated at room temperature (298 K) and at an elevated temperature of 673 K. In tensile testing at ambient temperature, the stress-strain curves show very short regions of strain hardening after ECAP and low values for the ultimate tensile strength by comparison with the unpressed alloy. This lack of strength is due to the high pressures imposed by ECAP and the consequent fragmentation and dissolution of the rod-like MgZn2 precipitates. It is shown that the strength may be restored by performing an appropriate ageing treatment after ECAP. Superplastic ductilities were recorded at a temperature of 673 K with tensile elongations exceeding 1000%. Careful inspection of the polished surfaces of samples pulled to fracture in the superplastic condition revealed the occurrence of extensive internal cavitation. Quantitative measurements showed the development of these internal cavities is consistent with conventional superplastic alloys.

A parametric study of the relationship between microstructure and wear resistance of Al–Si alloys

Wear tests were conducted on three commercial grade Al–Si alloys, a die cast 383 with 9.5 wt.% Si, a sand cast A390 with 18.5 wt.% Si and a spray cast alloy with 25 wt.% Si under dry sliding conditions. In addition, heat treatment processes were used to modify the microstructure and hardness of 383 alloy. This selection of materials and heat treatments provided a broad range of samples with different silicon weight percentage, morphology, size, and alloy hardness for comparison purposes. Using a method of pair-wise comparison, it was found that the effect of the individual contribution of each microstructural feature on the wear resistance could be isolated. Block-on-ring tests were performed under a controlled atmosphere of 5% relative humidity at a constant speed of 1 m/s. A mild to severe wear transition occurred at loads above 150 N irrespective of the alloy composition and microstructure. The mild wear regime consisted of two sub-regimes that were named as the first sub-regime of mild wear (MW-1) at low loads and the second sub-regime of mild wear (MW-2) at higher loads. The steady state wear rates ( W) in each sub-regime of the mild wear showed a power-law dependence to the applied load ( L) as W = CL n , where C is the wear coefficient, and n is the wear exponent. The wear coefficients, C 1 of MW-1 and C 2 of MW-2, and the transition loads L 1 and L 2, which denoted the start and end of the transition zone between MW-1 to MW-2, were sensitive to microstructure. Pair wise comparisons showed that an increase in the Si content from 9.5 to 25 wt.% increased the transition load L 1 by 140%, but had only a minor effect on C. An increase in alloy hardness from 31.6 to 53.5 kg/mm 2 provided a very significant increase in the transition loads (e.g., 400% increase in L 1), but did not have a notable effect on the values of C (only 3% increase). On the other hand, a decrease in the silicon particle aspect ratio from 3.75 to 1.98 increased L 1 by 25% and reduced C 1 by 25% (and C 2 by 31%). A decrease in the silicon particle size from 45.8 to 3.1 μm had the most significant effect on both wear parameters by reducing C 1 by 35% and C 2 by 58%, and increasing the transition load L 1 by 71%.

An Investigation of Cavitation in the Tensile Testing of a Spray-Cast Aluminum Alloy Processed by ECAP

A commercial aluminum 7034 alloy, produced by spray casting and having an initial grain size of ~2.1 μm, was subjected to equal-channel angular pressing (ECAP) through six passes at 473 K. In the as-pressed condition, the microstructure was reasonably homogeneous and the grain size was reduced to an ultrafine grain size of ~0.3 μm. This alloy contains MgZn2 and Al3Zr precipitates which restrict grain growth. In tensile testing at 673 K after processing by ECAP, an elongation of >1000% was achieved at a strain rate of 1.0 × 10-2 s-1 corresponding to high strain rate superplasticity. Quantitative cavity measurements were conducted on the specimens after tensile testing for both the as-received condition and after ECAP. These measurements reveal a significant number of small cavities in the samples and especially in the sample that exhibited a very high elongation. This paper describes the morphology of cavity development in the spray-cast aluminum alloy in both the as-received and as-pressed condition.

Creep and Mechanical Properties of a Commercial Aluminum Alloy Processed by ECAP

A commercial spray-cast aluminum alloy, having a composition of Al-11.5% Zn-2.5% Mg-0.9% Cu-0.2% Zr, was processed by equal-channel angular pressing (ECAP) to give an ultrafine-grained microstructure with a grain size of ~0.3 μm and a fracturing of the rod-shaped MgZn2 precipitates. The mechanical properties of the alloy were investigated using microhardness testing at ambient temperature and tensile testing at a temperature of 673 K. In addition, creep tests were conducted using a double-shear testing facility at temperatures from 473 to 673 K. The results show the microhardness decreases with increasing numbers of passes in ECAP and high strain rate superplasticity is achieved after processing by ECAP at 473 K for 6 or 8 passes. A maximum elongation of >1000% was recorded at a strain rate of 10-2 s-1 when testing at 673 K. The creep tests show the creep rates are faster in the as-pressed material than in the as-received material when testing at the same applied stress and testing temperature.

Using ECAP to achieve grain refinement, precipitate fragmentation and high strain rate superplasticity in a spray-cast aluminum alloy

An aluminum 7034 alloy, produced by spray casting and with an initial grain size of ~2.1 μm, was processed by equal-channel angular pressing (ECAP) at 473 K to produce an ultrafine grain size of ~0.3 μm. It is shown that the rod-like MgZn 2 precipitates present in the as-received alloy are broken into very small spherical particles during ECAP and these particles become distributed reasonably uniformly throughout the material. The presence of these fine MgZn 2 particles, combined with a distribution of fine Al 3Zr precipitates, is very effective in restricting grain growth so that submicrometer grains are retained at elevated temperatures up to at least ~670 K. Tensile testing of the pressed material revealed high elongations to failure, including elongations of >1000% when testing at a temperature of 673 K at initial strain rates at and above 10 −2 s −1. These results confirm the occurrence of high strain rate superplasticity in the spray-cast alloy.

Evolution of near-equiaxed microstructure in the semisolid state

The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

High-Strain Rate Superplasticity in Three Contrasting Fine Grained Aluminum Alloys

AbstractThe superplastic properties and microstructures of three contrasting fine grained aluminium alloys were investigated. These included (i) a powder metallurgy MMC, (ii) a severely deformed spray cast alloy, and (iii) the Zn-22%Al eutectoid alloy. The results showed some differences in the details of behaviour between the alloys. One of these was the presence of true work hardening, associated with dislocation activity, in the MMC, and its absence in the microduplex Zn-Al eutectoid alloy. In addition, the powder route MMC had the high threshold stress (up to 10MPa) commonly encountered in such materials, whereas this was not the case with the cast and severely deformed alloy, indicating that the threshold stress was associated with the presence of ceramic particles in the MMC (oxide + reinforcement). In all the alloys, however, the m value tended to increase with temperature, and this led to a corresponding increase in elongation with temperature until the microstructure became destabilised. In the two predominantly single phase alloys studied this destabilisation corresponded to the solvus temperature. The constantly changing m value indicates that there is no unique value of this parameter. The same was found to be true of the grain size exponent, p. The direct interpretation of apparent activation energies in terms of simple physical processes should be made with care in the light of the present results, as (i) the microstructures of two of the alloys were found to change continuously and significantly with small changes in temperature, and (ii) the fact that m is a function of temperature necessarily implies that the apparent activation energy is a function of stress.

Inhibited coarsening of solid-liquid microstructures in spray casting at high volume fractions of solid

Experimental studies on coarsening in fine grained solid-liquid microstructures at high volume fractions of solid ( f s ) have been carried out to determine if inhibited coarsening under these circumstances could account for the anomalous fine cell sizes observed in spray castings. The materials investigated included a chill-cast dendritic binary alloy of Al-Cu, two spray cast alloys—AA2014 and Cu-Ti, whose grain size was the segregate spacing, and a d.c.-cast alloy Al-4.5 wt% Cu-1.5 wt% Mg in both coarse-grain and grain-refined conditions. The observed segregate spacings after coarsening were smaller than that predicted by empirical correlations of dendrite arm spacing and freezing time. In all cases, the coarsening was found to become slower as the temperature was reduced and f s increased. Conventional coarsening theories and experiments predict the opposite, i.e. faster coarsening at higher volume fractions of solid. Two additional coarsening models were developed for the grain growth at high volume fractions of solid by processes whose rates are limited by migration of liquid at grain boundaries as liquid films on 2-grain surfaces or liquid rods on 3-grain triple points. In both models, the conventional diffusion-limited t 13 coarsening law was reproduced, but the rate constant K contained the term 11−f s and so also predicted accelerated coarsening as f s → 1 . Three possible explanations for the observed lower K values at increasing f s are proposed. The first is the effect of the increasing difference between the solute contents of solid and liquid as the temperature is reduced. This produces a1/X 1 dependence of the coarsening rate constant K. The second inhibiting effect, specific to dendritic structures, is in-grain coalescence of dendrite arms at high f s which produces isolated liquid particles within the grains. The final possibility is particle-inhibition of grain boundary migration by minority (impurity) particles at the grain boundaries. Such particles were seen, however, for only two of the alloys, viz. the grain defined d.c. cast Al-4.5 wt% Cu-1.5 wt% Mg and the spray case AA2014, but they or gas-filled pores are proposed as strong possibilities to account for the fine grain sizes observed in all spray cast microstructures.

Spray cast Al-Si base alloys for stiffness and fatigue strength requirements

Hypereutectic AlSiFe spray-cast alloys exhibit properties similar to those of metal-matrix composite (MMC's): high Young's modulus and a low coefficient of thermal expansion. These physical properties can be adjusted by changing the Si content of the alloy. The refinement of the microstructure is produced by formation of a large amount of nuclei in the spray, Consolidation done by extrusion (bars, tubes or profiles) and/or forging leads to high mechanical properties, especially very good dynamic properties. High fatigue properties coupled with high modulus, good high temperature behaviour and low thermal expansion, allow their use for applications in the automotive industry. In opposition to MMC's, these materials present the advantage of easy recycling and easy machinability as it is the case for the conventional AlSi alloys. The low oxygen content allows quality joining with conventional arc welding techniques