Sintering behaviour of Al-6061 powder produced by rapid solidification process

Abstract

Complex aluminium alloy components fabricated by powder metallurgy (P/M) offer the promise of a low cost and high strength-to-weight ratio, which meets the demands of the automotive sector. This paper describes the die compaction and sintering response of an atomised Al-6061 alloy powder containing Mg and Si produced by rapid solidification. A design of experiments is used involving three levels for each of the die compaction pressure, sintering temperature, peak temperature hold time and heating rate. Three trials were used to obtain the optimum press sinter processing conditions. Besides the mechanical properties, phase transformation and microstructure are investigated. Supplemental insight is gained through thermogravimetric analysis, differential scanning calorimetry and SEM with energy dispersive spectroscopy. Analysis of variation is used to quantify the contribution of each design variable to the mechanical properties.     

The unidirectional solidification of Al-4 wt pct Cu ingots in microgravity

Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during the flight of a sounding rocket, with solidification rates of about 1.6×10⁻⁴ m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger. The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities. The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion of the alloy during solidification, which resulted in decreased growth velocities. K.N. TANDON, formerly Associate Professor, Materials Engineering Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba     

The unidirectional solidification of Al-4 Wt pct Cu ingots in microgravity

Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during the flight of a sounding rocket, with solidification rates of about 1.6 × 10⁻⁴ m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger. The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities. The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion of the alloy during solidification, which resulted in decreased growth velocities.     

Numerical analysis of the rapid solidification of gas-atomized Al-8 Wt Pct Fe droplets

A numerical analysis of the microstructural evolution of microcellular and cellular α-Al phase in gas-atomized Al-8 wt pct Fe droplets was represented. The two-dimensional (2-D) non-Newtonian heat transfer and the dendritic growth theory in the undercooled melt were combined, assuming a point nucleation on the droplet surface and the macroscopically smooth solid-liquid interface enveloping the cell tips. It reproduced the main characteristic features of the reported microstructures quite well and predicted a considerable volume fraction of thermal dendritic growth region in a droplet smaller than 10 μm if an initial undercooling was larger than 100 K. The volume fractions of the microcellular region, g_A, and the sum of the microcellular and cellular region, g_α, were predicted as functions of the heat-transfer coefficient, h, and the initial undercooling, ΔT. It was shown that g_A and g_α, in the typical atomization processes with h = 0.1 to 1.0 W/C.M²K, are dominated by ΔT and h, respectively, but for h larger than 4.0 W/C.M² K, a fully microcellular structure can be obtained irrespective of the initial undercooling.     

Estimation of droplet solidification temperature in rapid solidification using in-situ measurements

Copper and D2 tool steel powders were produced using a drop tube-impulse atomization technique. In order to measure the radiant energy and droplet size of atomised D2 steel droplets, DPV-2000 (Tecnar Automation Ltée, St. Hubert Quebec, Canada) was utilised. In-situ velocity and droplet size of the atomised droplets were also measured using shadowgraphy technique (Sizing Master Shadow from LaVision GmbH in Gottingen, Germany). A 3D translation stage was designed, constructed and installed inside the drop tube system. DPV-2000 and shadowgraph were then mounted on the translation stage. The Cu droplets were primarily used to calibrate to particle size and velocity measurements between both instruments. Using this stage, online measurements were conducted at 4?cm, 18?cm and 28?cm distances for D2 droplets below the crucible. Using liquid (fully undercooled) and semi-solid behaviour of droplets, it was possible to estimate the droplet size and temperature at which recalescence ends. These values were then confirmed by the thermal model using experimentally estimated primary phase undercooling values.     

Microstructural and compositional transients during accelerated directional solidification of Al-4.5 wt pct Cu

The transient behavior of mushy-zone velocities, primary dendrite arm spacings, and microsegregation effects have been investigated for an Al-4.5 wt pct Cu alloy by instantaneous velocity changes in a standard Bridgman furnace. After suddenly imposed velocity changes, the mushy-zone velocities, dendrite arm spacings, and compositions exponentially adjust to new steady-state values. Good agreement was found between the transient mushy-zone positions and velocities and predictions from the theoretical model of Saitou and Hirata. The primary dendrite arm spacings appear to adjust to changed velocity conditions about as rapidly as the mushy-zone velocity adjusts. Steady-state arm spacings agree very well with corresponding steady-state data from the literature. However, the observed composition profiles in the dendrite core and the interdendritic liquid appear to adjust more slowly than the corresponding adjustment of the mushy-zone velocity and arm spacings. Our observation of the sluggish response of the compositional profiles is consistent with an estimated Lewis number of 9.4 × 10³ for the aluminum-copper system. The diffusivity of heat, thus, greatly exceeds the diffusivity of solute in this system. These results indicate that testing for the steady state during directional solidification experiments by looking for constant primary dendrite arm spacings can lead to errors, since the microsegregation profiles adjust more slowly than the spacings. It is suggested that constancy of composition also be tested for critical experiments investigating steady-state microsegregation effects.     

High-energy X-ray computed tomography of the progression of the solidification front in pure aluminum

An X-ray computed tomography (CT) system was developed for monitoring the solidification front in metal casting. The X-ray source was a 6 MeV linear accelerator (linac) emitting photons in 5 μs pulses at a rate of 180 Hz. The source intensity was 300 R/min at 1 m. The X-ray beam was collimated in a 30 deg fan shape with a 10-mm height. A detector array comprising 128 elements was located 845 mm from the source. Pure aluminum in a clay-graphite crucible (178-mm o.d., 146-mm i.d.) was melted in a resistance heater furnace, and a cooling tube at the center of the crucible solidified the molten aluminum to simulate the casting process. A solidification front formed around the tube and progressed outward over an hour until the aluminum was completely solidified. X-ray attenuation measurements were taken every minute during this time. Density images were later reconstructed from these measurements using CT. From these images, the progression of the solidification front was determined with a planar resolution of 1.3 mm and a sensitivity of 3.7 pct. The density maps agree with expected values and correlate well with temperature measurements obtained independently by thermocouples.     

High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

Numerical treatment of rapid solidification

Techniques are reviewed for describing crystal nucleation and subsequent advance of the crystallization front in metallic melts under the range of conditions generated during rapid solidification processing. The selection of appropriate boundary conditions in these cases is discussed, with particular reference to coupling between the undercooling at and the velocity of the growth front. It is emphasized that, although solute redistribution may occur, the process is normally under heat flow control and techniques for treating the associated phenomena are described. Some illustrative theoretical curves are presented for unidimensional heat flow toward a massive substrate, while the relevance to other processes, such as gas atomization, is periodically noted. Some attention is devoted to recalescence (removal of the undercooling) of the growth front as a result of latent heat evolution and the degree to which this might be counteracted by external cooling. A rationale is presented enabling the conditions under which recalescence could be arrested (thus permitting strongly enhanced crystal growth rates) to be estimated for different base metals. Mention is made of solute redistribution phenomena during recalescence of alloys. Finally, some implications of the data presented are briefly discussed in practical terms.     

Ductilization of a powder metallurgy Al-17 wt pct Cu by means of channel-die compression and extrusion

Metal powders always contain a surface oxide layer, which is particularly tenacious in aluminum alloys. After hot pressing, this oxide coats the particle boundaries and reduces the ductility. In this article, a study of the Al-17 wt pct Cu alloy densified from rapidly solidified powder is presented. Different thermomechanical treatments were investigated to improve the ductility of this material. Channel-die (CD) forging was performed at two temperatures (430 °C and 500 °C). Eight compression runs were applied to the samples in each CD treatment. At 430 °C, three strain values per run were investigated (35, 50 and 70 pct). A bar was also extruded with a 40:1 ratio. Because of the small size of the samples, the ductility was assessed by means of the ring expansion test and analyzed by post mortem (fracture surface and cross section) observations. No ductility was measured after CD compression at 430 °C, although it appears from the fracture surface observations that increasing the strain per run has a beneficial effect. The CD compression at 500 °C and extrusion were both successful at promoting ductility, extrusion being more effective.     

Equiaxed dendritic solidification with convection: Part II. Numerical simulations for an Al-4 Wt pct Cu alloy

The multiphase model developed in part I for equiaxed dendritic solidification with melt convection and solid-phase transport is applied to numerically predict structural and compositional development in an Al-4 wt pct Cu alloy solidifying in a rectangular cavity. A numerical technique combining a fully implicit control-volume-based finite difference method with a multiple time-step scheme is developed for accurate and efficient simulations of both micro- and macroscale phenomena. Quantitative results for the dendritic microstructure evolution in the presence of melt convection and solid movement are obtained. The remarkable effects of the solid-liquid multiphase flow pattern on macrosegregation as well as the grain size distribution are illustrated.     

Molecular-dynamics simulation of rapid solidification of aluminum

A constant pressure molecular-dynamics simulation is used to examine changes in the structural characteristics of aluminum during rapid solidification. The glass formation and the crystallization from the liquid are predicted. The structure transformations which occur during solidification are described using the mean atomic volume, the internal energy, the radial distribution function and the surface tension. Changes in these parameters calculated as function of temperature display anomalies associated with a change in the structural ordering and change in the phase transformation.     

Theory of microstructural development during rapid solidification

A theoretical model is presented for describing columnar (directional) growth of dendrites including growth rates in the range of the limit of absolute stability. A maximum growth rate for dendrites is predicted which is slightly below the limit of absolute stability of a planar interface. The changes in microstructures which may occur at high growth rates are discussed. Both the effect of the temperature dependant diffusion coefficient and the velocity and temperature dependant partition coefficient on microstructure characteristics are considered. The model is applied to the Ag-Cu system where detailed microstructural analysis has been published. The theoretical results are found to be in reasonable agreement with experimental data obtained from the literature.     

Modeling of crystal growth during rapid solidification

Models for computer simulation of solidification of alloys in two different geometries have been developed. These models allow us to study the roles of crystal nucleation and heat transfer and the effects of different modes of growth on the evolution of the microstructure during solidification. In particular, by applying the powerful and versatile tool comprised of the solute-drag model and the thermodynamic modeling of the alloy system, we are able to predict the formation of a microstructure consisting of alternating layers parallel to the growth front. This type of banded microstructure, experimentally observed in Al-Cu, Al-Fe, and Ag-Cu alloys, has not been satisfactorily explained before.