Mechanical Properties and Conductivity of Low-Pressure Die-Cast 319 Aluminum Prepared with Hot Isostatic Pressing, Thermal Treatment, or Chemical Treatment

Abstract

Low-pressure die-cast (LPDC) 319 Al alloy is commonly processed with hot isostatic pressing (HIP), heat treatment, and chemical eutectic silicon modification for advanced automotive applications. Yet, the microstructure can be similarly influenced by each of these processes, which may obscure their isolated benefits. Hence, this study aimed to differentiate their effects by processing step blocks of LPDC 319 alloy with either a HIP or ambient-pressure heat treatment, each for 2 h at 500 °C, or with strontium additions up to 150 ppm. Both chemical and thermal modifications of eutectic silicon were found to effectively improve alloy conductivity. However, the potential benefits of strontium to alloy strength and hardness were offset by the decreases in density associated with the addition. In contrast, the as-cast, unmodified samples featured relatively high densities, although this limited the ability of the HIP treatment to reduce porosity. With identical temperatures and heating times, both the HIP and heat treatment promoted comparable Al₂Cu dissolution and silicon spheroidization. Accordingly, both treatments generally improved hardness and strength, and increased thermal and electrical conductivities by up to 12.5%. Thus, these processing parameters can each contribute to the development of enhanced engineering components individually, thereby reducing the need for their combined application.

Introduction

Aluminum die castings compose a major proportion of the global automotive manufacturing industry. By injecting the molten metal into a permanent mold at high pressure, components with smooth surface finishes, thin wall thicknesses, and close dimensional tolerances can be produced. However, the melt typically experiences significant turbulence during mold filling, followed by rapid solidification rates. As a result, die castings commonly contain internal defects such as porosity, due to the presence of entrapped gases or oxides. This is especially an issue for high-pressure die casting (HPDC), which applies pressures up to 350 MPa during mold filling and solidification (Ref 1). Porosity is known to be detrimental to mechanical and thermal properties, both of which are needed for sustaining the thermomechanical loads present in powertrain components (Ref 2). As well, the expansion of internal gas porosity at elevated temperatures can cause surface blistering. Hence, due to the dimensional instability associated with plastic deformation at the surface, it can be difficult to heat treat die castings (Ref 3, 4), which is often necessary for the enhancement of material properties for industrial use.

This issue can be alleviated by modifying the process parameters of the castings. For example, in low-pressure die casting (LPDC), melt turbulence can be reduced while achieving rapid mold fill rates, by using pressures on the order of 0.1 MPa to fill the mold from beneath. Therefore, there is much more flexibility in the heat treatment of alloys produced by this method, at the expense of minimum wall thickness and the production rate of castings (Ref 5). Furthermore, hot isostatic pressing (HIP) simultaneously applies high pressure via an inert gas and high temperature to effectively reduce porosity (Ref 6). This widely used technique induces small-scale plastic flow to collapse pores, given the higher diffusion rates and lower alloy yield strengths associated with high temperatures. By counteracting the detriment of internal porosity, HIP treatments have been used successfully to improve the mechanical properties of aluminum and other alloys (Ref 7,8,9,10,11,12). Accordingly, die-cast aluminum alloys are often subjected to a HIP treatment to ensure high integrity. Of note, HIP is commonly applied to parts cast of 319 alloy, which is a lightweight and age-hardenable Al-Si-Cu alloy, widely used in industry due to its high strength and conductivity. For instance, Boileau and Allison (Ref 13) found that the fatigue strength of W319-T6 alloy cast with an average secondary dendrite arm spacing of 23 µm increased from 90 to 130 MPa due to a successful HIP treatment.

However, the combination of pressure, temperature, and time can influence several other microstructural features in this alloy besides porosity, including the spheroidization, dissolution, precipitation, or redistribution of secondary phases. These same phenomena are typically produced by other processing parameters selected to achieve optimal 319 alloy properties. For example, the solution heat treatment of this alloy at temperatures between 480 and 520 °C can promote appreciable fragmentation and spheroidization of the eutectic Si particles (Ref 14,15,16) as well as dissolution of the Al₂Cu phases (Ref 17,18,19). Since the Si particles are naturally coarse and acicular, this thermal modification can improve strength and ductility by alleviating the stress concentrations at their edges. As well, it can improve thermal and electrical conductivities, since the spheroidized Si particles promote higher free electron mobility than in the as-cast state (Ref 15, 20). Conversely, the dissolution of Cu atoms into the Al matrix during solution heat treatment causes two competing effects on properties: The volume fraction of Al₂Cu secondary phases is reduced, yet the dissolution introduces strains into the Al lattice. Hence, some dispersion strengthening of the alloy is lost in lieu of solid solution strengthening. Moreover, although decreasing the amount of secondary phases is beneficial for electron flow, both thermal and electrical conductivities are diminished by increased solute concentrations in the Al solid solution (Ref 21, 22). Given that HIP treatments often apply similar temperatures to solution heat treatments, it is expected that comparable phenomena will occur in both treatments. Consequently, there is a need to differentiate the high-temperature effects from the high-pressure effects that occur during a HIP treatment.

By subjecting samples of 319 alloy to either a HIP treatment or an ambient-pressure heat treatment with the same temperature and time, the influence of each parameter on microstructure can be clarified. Yet, high-temperature treatments are not the only technique used in industry to modify the Si particle morphology. Typically, chemical modification of Si is achieved via microalloying with strontium (Sr). The trace Sr content effectively transforms the coarse and acicular eutectic Si particles to have a fine, fibrous morphology, by depressing the eutectic reaction temperature during solidification (Ref 23, 24). Similar to spheroidization at high temperatures, chemical modification has been found to improve both mechanical properties and conductivity (Ref 25,26,27,28,29), but it does so without dissolution of the Al₂Cu phases associated with solution heat treatment. For example, Hafiz and Kobayashi (Ref 26) found that adding 170 ppm Sr to Al-8 wt.% Si cast in a steel mold improved its tensile strength from 171 to 217 MPa. As well, Vandersluis et al. (Ref 30) recently demonstrated that adding 150 ppm Sr to B319 alloy improved its electrical conductivity by up to 3% IACS. Therefore, it is instructive to compare samples subjected to HIP and ambient-pressure heat treatments with those modified with Sr, so as to further differentiate between the microstructural processes that evolve at high temperature.

A study by the present authors characterizing commercial 319 Al alloy automotive cylinder heads reported significant influences of porosity, heat treatment, and eutectic Si modification to material properties (Ref 2). In the current research, the underlying microstructural phenomena were investigated, via the isolation of the effects of HIP, heat treatment, and Sr additions on the mechanical properties and conductivity of LPDC 319 alloy. By improving the current understanding of the specific contributions of high pressure, thermal and chemical Si modification, as well as of the dissolution of Al₂Cu on the alloy, the HIP and other processing conditions can be effectively manipulated for optimal alloy performance, without unreasonable equipment costs. These findings contribute to the development of microstructure–properties relationships, for both mechanical properties and conductivity, and provide insights for the production of enhanced die-cast alloys.

Materials and Methods

Alloy Processing

The material processing conditions used in this study are summarized in Table 1. The effects of solidification rate, heat treatment, and eutectic Si modification on low-pressure die-cast 319 alloy were evaluated by producing a series of step block castings in a Kurtz Al 7-5 system, using a positive holding pressure of 0.065 MPa. As shown in Fig. 1, the castings featured steps with thicknesses of 25.4, 12.7, 6.4 and 3.2 mm, where the reductions in size resulted in progressive increases in solidification rates. For all conditions, approximately 150 kg of the alloy was melted and maintained between 720 and 730 °C before pouring into a gray iron mold, preheated to 420 °C.

Table 1 Material processing conditions for LPDC

Fig. 1

Dimensions of the step block castings

Three sets of castings were produced with the nominal 319 alloy composition to evaluate a baseline as-cast condition, denoted “Base,” as well as two heat treatment conditions. Of the latter, one casting set, denoted “HIP,” was subjected to hot isostatic pressing in an AIP10-30H system for 2 h at 500 °C with a pressure of 138 MPa. This was done in efforts to minimize porosity and spheroidize the secondary phase particles. The HIP parameters were selected to comply with the standard guide for Al alloy castings in ASTM B998, as well as to correspond with the heat treatment temperature used by automotive 319 component manufacturers (Ref 2). The other casting set, denoted “HT,” was subjected to a 2-h heat treatment at 500 °C at atmospheric pressure in a Pyradia aluminum heat treatment furnace. Accordingly, any effects of pressure-assisted diffusion during the HIP treatment were separated from the diffusion that can occur due to the elevated temperature. For the HIP and HT treatments, the metal was cooled to room temperature via forced Ar and forced air cooling, respectively. Furthermore, two additional casting sets were produced to evaluate the influence of chemical eutectic Si modification. For the castings denoted “50Sr” and “150Sr,” 50 and 150 ppm Sr was added 1 h prior to pouring to the molten 319 alloy, respectively, in the form of 25 mm bars of Al-10 wt.% Sr. These two casting sets were not subjected to any heat treatments.

For each of the five casting conditions, three step block castings were produced for microstructural and material property evaluation. However, the 3.2 mm steps were not characterized, due to their relatively high concentration of defects. The average chemical compositions of the castings, as measured using a ARL 3460 Thermo Scientific OES Metals Analyzer optical emission spectrometer, are presented in Table 2. Additionally, the average Na concentration for all casting conditions was less than 10 ppm, and the P concentration was below the resolution of the spectrometer (30 ppm).

Table 2 Average measured chemical composition of 319 alloy castings, in wt.%

Characterization

Samples were extracted from the center of each step for microstructural analysis. The thicknesses of the steps were maintained in the samples, whereas their cross-sectional areas were each machined to 20 × 20 mm². These samples were subjected to progressive grinding processes with SiC papers, followed by polishing steps with colloidal silica. Optical microscopy with a Nikon Eclipse ME600 microscope was used to observe the sample dendritic structure and the Si particle morphology, whereas a Hitachi SU3500 scanning electron microscope (SEM) was used to observe the intermetallic phases, via backscattered electrons and high contrast. The average secondary dendrite arm spacing (SDAS) for each sample was calculated using 70 measurements with optical micrographs at 200 × magnification, according to the recommended linear intercept method (Ref 31). The area percentage of intermetallic phase was calculated from five backscattered electron micrographs at 350x magnification, using Clemex Vision quantitative image analysis software. The method used thresholds of gray scale for each pixel to differentiate between the phases.

Three 30-mm-long cylindrical samples were machined from each step of each condition, with diameters of 13.3, 6.9 and 4.5 mm for the 25.4, 12.7, and 6.4 mm steps, respectively. Sample density (ρ) was evaluated using the hydrostatic displacement technique (Ref 32):

$$ \rho = (m\rho_{\text{w}} )/\left( {m_{\text{bs}} {-}m_{\text{b}} } \right) $$

(1)

where m is the mass of the sample in air, ρ_w is the density of water, m_bs is the mass of the beaker with water and with the suspended sample, and m_b is the mass of the beaker with water. This was done for all cylindrical samples (three per step size per condition) except those from the 6.4 mm steps, since their lightweight prevented accurate measurement. For mechanical property testing, Brinell hardness measurements were taken with a DTLC-3000 Brinell tester using a 500 kgf load on samples with the full cross sections of each step. At least four hardness measurements were taken per condition for all but the 6.4 mm steps, as their small thicknesses prevented accurate testing. Yet, all steps were evaluated with tensile testing, which was performed on eight samples per step per condition using a Tinius Olsen Super L 398 Universal Tensile Testing system, with geometry and loading conditions specified in the ASTM E8 standard.

Ambient-temperature (22 °C) thermal and electrical conductivities were only measured for the 25.4 mm steps, due to geometric constraints of the equipment. For thermal conductivity, the transient plane source (TPS) method was utilized with a ThermTest Hot Disk TPS 2500S thermal constants analyzer (Ref 33, 34). One end of each 13.3-mm-diameter cylindrical sample, discussed above, was ground until plane for testing. Using the single-sided, one-dimensional module, a 6.403-mm-radius nickel double-spiral sensor with Kapton insulation was fitted between the sample face and insulating extruded polystyrene. The assembly was placed under vertical pressure via a thumbscrew for good thermal contact. Three tests were conducted per sample, using 1 W transient heating power and 3 s measuring times. Each test was separated by a 15 min holding time to reach thermal stability, as confirmed by a 40-s temperature drift measurement prior to testing. The accuracy of these measurements was improved by separately analyzing the ambient-temperature specific heat of the samples, using the TPS specific heat module. A 5-mm-thick disk was cut from the end of each cylindrical sample and placed into a 19-mm-diameter, 8-mm-high gold reference cell, which was then surrounded by insulation material. A reference measurement of the empty gold cell utilized 135 mW of heating power and a 40 s measurement time, whereas the measurement of each sample in the cell utilized 50 mW and 40 s. In either case, a temperature increase of approximately 1 K was achieved. Each test was separated by 75 min holding time to ensure thermal stability.

Electrical conductivity measurements were taken on three 5-mm-thick disks per condition, which were extracted from the cylindrical samples and ground flat on one surface. Analysis was performed using an ETher NDE SigmaCheck 2 eddy current conductivity meter, instrumented with a 13-mm-diameter probe operating at 60 kHz, in accordance with the ASTM E1004 standard. Measured values were expressed as a percentage of the International Annealed Copper Standard (% IACS), which defines 100% IACS as 1.7241 µΩ-cm at 20 °C. The device was calibrated with three known standards immediately prior to testing, with certified electrical conductivities of 8.372, 28.03, and 62.45% IACS.

Results

Microstructure

A progressive refinement in alloy microstructure was observed with decreasing step thickness for all casting and heat treatment conditions. As demonstrated in Fig. 2, the 6.4 mm steps featured finer dendritic structures and secondary phases than the 25.4 mm steps. This refinement was caused by an increase in solidification rate, which is typically present in the thinner sections of castings. As shown in Fig. 3, the average SDAS of the 25.4, 12.7 and 6.4 mm steps were 40, 27 and 19 µm, respectively. For a given step size, there was no considerable difference in SDAS between each alloy condition, with any minimal changes attributable to casting variation. As displayed by the optical micrographs in Fig. 2, the observed Base microstructure was typical for 319 alloy (Ref 2, 35). In addition to the α-Al matrix, the eutectic Si, α-Al₁₅(Fe,Mn)₃Si₂, β-Al₅FeSi, Al₂Cu, and Al₅Mg₈Cu₂Si₆ phases were observed. For all alloy conditions, the α-Al₁₅(Fe,Mn)₃Si₂ phases appeared coarse with Chinese script morphologies, whereas the β-Al₅FeSi phases appeared as needles. The Al₂Cu and Al₅Mg₈Cu₂Si₆ phases were present in complex eutectic structures. The morphologies of the Si and Al-₂Cu phases were influenced by the Sr content and heat treatment condition.

Fig. 2

Optical micrographs of the Base castings, demonstrating the refinement of microstructure between the (a) 25.4 mm and (b) 6.4 mm steps

Fig. 3

Casting secondary dendrite arm spacing measurements, where error bars represent 95% confidence intervals

In the Base castings (Fig. 2), the eutectic Si particles, appearing darkest in the optical micrographs, were coarse and acicular, as typically observed in the unmodified condition (Ref 36). However, the sharp edges of the particles were significantly rounded in both the HIP and HT conditions, as depicted in Fig. 4(a) and (b). Spheroidization of the Si particles during heat treatment has been reported in the literature for various Al-Si alloys, and its effectiveness was found to be enhanced as both the temperature and treatment time were increased (Ref 14,15,16). Evidently, the 2-h treatment at 500 °C conducted on both conditions was sufficient to achieve a similar result, albeit at an early stage before complete spheroidization. In contrast, Sr additions were highly effective at transforming the Si particles into a fine, fibrous morphology, as shown in Fig. 4(c) and (d). While the Si particles in the 50Sr condition could be characterized as partially modified, fine lamellae, the 150Sr condition featured a well-modified, fibrous eutectic structure. The other secondary phases were not noticeably affected by the Sr additions.

Fig. 4

Optical micrographs of the 12.7 mm steps, depicting modification of the Si particle morphology for the (a) HIP, (b) HT, (c) 50Sr, and (d) 150Sr conditions

Furthermore, the Al-₂Cu particles were considerably influenced by the 500 °C heat treatments in both the HIP and HT conditions. The Base condition featured relatively large, blocky Al₂Cu particles and some eutectic Al₂Cu lamellar structures (Fig. 5a). However, these phases were largely dissolved during the heat treatments, as indicated by their severely fragmented morphologies in Fig. 5(b). This dissolution was quantified by Al₂Cu phase area percentage measurements, as shown in Fig. 6. The Al₂Cu area percentage in the microstructure did not vary much with decreasing step thickness, despite the change in solidification rate. As well, Sr content did not have much effect on the amount of Al₂Cu present relative to the Base castings. All three of these conditions had an average area percentage of approximately 2%. In contrast, both the HIP and HT conditions equivalently featured significant reductions in Al₂Cu area percentage to an average of approximately 0.6%, which is characteristic for the solution heat treatment of this alloy (Ref 2). Given the similarity between these two conditions, it is evident that the Al₂Cu dissolution was not affected by the high pressure during the HIP treatment, but rather was caused by the elevated temperature, itself.

Fig. 5

Backscattered electron micrographs of the 6.4 mm steps, depicting dissolution of the eutectic Al₂Cu particles from the (a) Base to (b) HIP conditions

Fig. 6

Casting Al₂Cu area percent measurements

Physical and Mechanical Properties

The density measurements of the samples are shown in Fig. 7. The average density of the Base casting was approximately 2.773 g/cm³, without considerable difference between the step thicknesses. This corresponded well to the pore-free density of 319 alloy, typically listed in the literature as 2.79 g/cm³ (Ref 32), which indicated approximate porosity levels of only 0.6%. The 12.7 mm step solidified faster than the 25.4 mm step of the casting, and interdendritic shrinkage pores tend to be smaller and more uniformly distributed in finer microstructures (Ref 35, 37). Nonetheless, the change in SDAS between these two steps (27 to 40 µm) was likely too minimal to achieve an appreciable change in density.

Fig. 7

Casting density measurements, where error bars represent twice the standard deviation on the sample mean

Furthermore, density did not vary much between the Base and the HT condition, but high-temperature treatments at 500 °C were not expected to influence porosity (Ref 2). However, the HIP treatment increased the density of the 25.4 mm step to 2.777 g/cm³, which corresponded to 0.1-0.2% less porosity than the Base condition. It was unusual that the high pressure during the HIP treatment did not promote an increase in density in the 12.7 mm step, and that the improvement in density for the larger step thickness was relatively small. This may have been caused by several factors. The HIP treatment was designed to simultaneously apply high temperature and pressure to collapse internal pores by small-scale plastic flow (Ref 6). This works effectively for removing hydrogen gas porosity, as the hydrogen is soluble in the Al matrix and can diffuse out of the casting. Yet, pores filled with nitrogen, oxygen, or other relatively insoluble gases are generally more difficult to collapse using a HIP treatment. Additionally, surface pores and those interconnected to the surface can be infiltrated by the inert gas at high pressure in the HIP environment, which can prevent porosity reductions (Ref 11). Moreover, given that the porosity levels in the Base castings were already relatively low, at less than 1%, it is possible that HIP treatment was unable to promote considerable further improvements in density.

In contrast, density was noticeably decreased by increasing the Sr content, and the effects were more pronounced for the slower solidifying samples. For example, when increasing Sr content from 50 to 150 ppm, the density of the 25.4 mm step was reduced to approximately 2.766 and 2.746 g/cm³, respectively. Yet, the 12.7 mm step only experienced a significant decrease in density in the 150Sr condition, which featured 2.754 g/cm³. Eutectic Si modification has been reported to be associated with increases in porosity levels or changes in its dispersion (Ref 23, 38,39,40,41). This was attributed to increases in hydrogen solubility in molten Al, reductions in the surface tension of the melt, decreases in the required hydrogen concentration for the nucleation of pores, or modification of the characteristics of the solidification growth mode. Since the 50Sr condition was only partially modified, it follows that an intermediate density level was observed. Nonetheless, the reduction in porosity associated with increasing solidification rate for microstructural refinement was evident for both of the Sr-containing alloys.

The mechanical properties of the samples are shown in Fig. 8. For all conditions, there was a general increase in hardness, ultimate tensile strength (UTS), and ductility with decreasing step thickness. Yet, yield strength (YS) was relatively unaffected. For example, the UTS and ductility of the Base casting increased considerably from approximately 144 MPa and 1.5% to 210 MPa and 4.5% between the 25.4 mm and 6.4 mm steps, respectively. This improvement in mechanical properties can be attributed to the refinement in microstructure which is associated with increasing solidification rate in thinner steps, as discussed in Section 3.1. The finer SDAS values and more uniform distribution of finer secondary phases effectively impeded dislocation motion through the material and caused strengthening (Ref 20, 35, 42).

Fig. 8

Mechanical properties of the castings: (a) Brinell hardness, (b) ultimate tensile strength, (c) yield strength, and (d) ductility, where error bars represent twice the standard deviation on the sample mean

Hardness and UTS were generally increased in the HT condition. For example, the hardness of the 12.7 mm step was improved from approximately 75 to 81 HB, relative to the Base condition. At high temperature, significant Al₂Cu dissolution was observed, and the Si particles were found to spheroidize, as discussed in the previous section. Both of these effects influenced mechanical properties. The former promoted solid solution strengthening, but it may have been balanced by weakening due to the loss of stable Al₂Cu secondary phase particles (Ref 20, 43). As well, the samples were brought to room temperature after heat treatment with forced air, which may have enabled the formation of some nano-sized Al-Cu precipitates during cooling. Further strengthening likely occurred during precipitation at ambient temperatures following the heat treatment, via natural aging. Moreover, spheroidization of the Si particles can mitigate the stress concentrations promoted by their coarse and acicular morphology. Nonetheless, the heat treatment at 500 °C was only 2 h long, compared to the recommended solution heat treatment time at this temperature of approximately 12 h (Ref 18, 32). Thus, the full effect of a solution heat treatment was likely unrealized. Accordingly, since heat treatments are more effective on finer phases with larger surface area-to-volume ratios, the 25.4 mm steps were not appreciably influenced by the HT condition.

The HIP treatment produced similar results to the HT condition, featuring generally higher hardness, YS and UTS values than the Base condition. Given the equivalent heating temperature and time as the HT condition, the dissolution of Al₂Cu and spheroidization of Si occurred in the same way, as presented in the preceding section. Yet, the HIP produced some additional benefits due to the applied high pressure. These were predominantly observed for the 25.4 mm step. As presented in Fig. 7, the densities of the HIP samples were found to be higher than the Base or HT samples for the 25.4 mm step, which was associated with reduced porosity. It is well known that porosity is detrimental to the mechanical properties of a material. Hence, the improvement in the HIP condition relative to the HT condition may be attributed to the decrease in porosity. Furthermore, after high-temperature treatment, the HIP samples were cooled via Ar, whereas the HT samples were cooled with forced air. Consequently, a difference in cooling rates may have promoted a greater supersaturation of Cu following the former treatment, thereby enabling strengthening via natural aging to occur more efficiently.

Porosity also significantly affected the mechanical properties of the Sr-containing alloys. As demonstrated in Fig. 8, increasing Sr content progressively decreased the hardness of the samples, down to approximately 71 HB for the 150Sr, 25.3 mm step. Despite the expected benefits to mechanical properties from transforming the Si particles to a fine, fibrous structure, the associated increase in porosity shown in Fig. 7 was likely sufficient to counteract the advantage. Otherwise, the unaffected secondary phases like α-Al₁₅(Fe,Mn)₃Si₂ may have still acted as fracture initiation sites, promoting the insensitivity of mechanical properties to Si modification observed in this alloy. Nevertheless, compared to the Base condition, the increased ductility of the larger two steps of both Sr-containing alloys to approximately 3% elongation was similar to the two heat treatment conditions. This indicated that the modification of the Si particles, whether thermally or chemically, can effectively enhance alloy ductility. Yet, for optimal results, Sr additions should be combined with porosity removal techniques, such as HIP treatments.

Thermal and Electrical Conductivities

The thermal and electrical conductivities of the 25.4 mm steps are displayed in Fig. 9. The two conductivities corresponded very well to each other for every condition, in accordance with the Wiedemann–Franz law (Ref 20). The thermal and electrical conductivities of the Base casting were found to be approximately 120 W/m K and 28.7% IACS, respectively. These values were close to the standard quantities listed for 319 alloy in reference textbooks, 109 W/m K and 27% IACS (Ref 32). However, standard 319 alloy specifies 5.5-6.5 wt.% Si and 3.0-4.0 wt.% Cu, whereas the composition of the present alloy was on average approximately 5.6 wt.% Si and 2.8 wt.% Cu (Table 2). The higher conductivity can therefore be attributed to the smaller fraction of secondary phases in the microstructure and the reduced concentration of solute in the Al matrix, both of which impede the flow of free electrons.

Fig. 9

(a) Thermal and (b) electrical conductivity measurements of the 25.4 mm steps, where error bars represent twice the standard deviation on the sample mean

The conductivity of both the HIP and HT samples increased to approximately 135 W/m K and 32.3% IACS, which was a 12.5% improvement compared to the Base condition. However, the conductivity was nearly identical between these two heat treatment conditions. This indicates that the higher conductivity resulted from the high-temperature treatment itself, with no significant influence from the high pressure applied during the HIP treatment. Even though heat treatment at 500 °C dissolved Cu atoms into the Al solid solution (Fig. 6), which increased the concentration of electron scattering centers, this effect was evidently outweighed by the benefits of Si spheroidization to conductivity. As a result of the rounder Si particles and the coarsening that likely occurred at the expense of smaller particles, the mean free path of the electrons was increased, thereby enhancing both heat and electron transport (Ref 15). Also, some of the dissolved Cu likely precipitated during cooling from 500 °C as well as during natural aging at ambient temperatures prior to the conductivity measurements. Additionally, porosity is known to be detrimental to conductivity (Ref 37, 44), and porosity was somewhat decreased during the HIP treatment (Fig. 7). Nonetheless, the approximate 0.1-0.2% reduction in porosity was likely too minimal to cause a significant change in conductivity.

Conductivity was also progressively improved by increasing the Sr content in the alloy, up to 128 W/m K and 30.7% IACS for the 150Sr condition. This corresponded to almost a 7% improvement in conductivity compared to the Base condition. The addition of Sr was found to modify the Si particles to a fibrous morphology, as discussed in Section 3.1. Similar to high-temperature spheroidization, this modification improved thermal and electrical conductivities by increasing the free electron mobility (Ref 27, 45, 46). The benefits to conductivity increase progressively with the Si modification level, associated with the Sr content. However, at excessive Sr contents, over-modification can occur, causing particle coarsening or reversion to a platelike morphology. For 319 alloy, the optimal Sr addition level was reported to be on the order of 100-200 ppm (Ref 47, 48), which is consistent with the 150Sr condition.

Due to geometric constraints, the effects of casting step thickness on conductivity could not be evaluated in the present study. However, recent work in the literature has shown that conductivity is not affected by solidification rate in the as-cast, unmodified condition (Ref 35, 37), nor in the thermal sand reclamation (TSR) or overaged (T7) heat-treated conditions for a Sr-containing 319 alloy (Ref 2). However, solidification rate has been found to work synergistically with Sr content in modifying the eutectic Si phase (Ref 36). Therefore, it is expected that the 50Sr and 150Sr castings featured slightly higher thermal and electrical conductivities in the smaller step sizes. Similar conductivity results combining solidification rate and Sr content have been reported in the literature (Ref 27, 49).

Discussion

Although there are many ways to process LPDC 319 alloy, this study revealed that many of them similarly influenced various features in the microstructure. For instance, with the exclusion of the Base condition, modification of the eutectic Si particles was observed for all treatments, whether through thermal or chemical means. With increasing Sr content up to 150 ppm, the morphology of the particles was transformed to a fibrous structure, whereas with high- or ambient-pressure treatments at 500 °C, the edges of the acicular particles were rounded. By either method, the modification of the Si particles contributed to enhanced thermal and electrical conductivities, and it affected strength and hardness. Yet, chemical modification with relatively high concentrations of Sr was associated with significant decreases in density, which can be detrimental to material properties. In contrast, lengthy heat treatments can be expensive. Accordingly, the findings of this study suggest opportunities for the combination of thermal treatments with smaller Sr additions for optimal results. By subjecting a partially modified alloy to a thermal treatment, a modified eutectic structure could be attained without compromising the casting integrity. As well, given that the high-temperature treatment time in this study was only 2 h, it is likely that further Si particle spheroidization would be produced with longer treatments. This would enable further enhancement of the alloy properties. These parameters can be optimized to best suit the application for maximum alloy performance.

Furthermore, the juxtaposition of the HIP treatment with the ambient-pressure treatment with identical heating parameters usefully differentiated the effects of temperature from those of pressure. In addition to Si particle rounding, treatment at 500 °C for 2 h effectively fragmented and dissolved the Al₂Cu phases, independent of pressure. The equivalent microstructures led to similar improvements in mechanical properties and conductivity. It follows that HIP treatments can be incorporated as part of a typical precipitation hardening heat treatment schedule, thereby reducing the time required for the solution heat treatment step. Yet, the major benefits of the HIP treatment can be realized in relatively porous castings, provided that the pores are internal and not connected with the surface. In such materials, the high pressure can effectively collapse the porosity, and the higher density can further improve the material properties that are already enhanced by the high temperature. This was seen to some extent in the present study, as the HIP condition produced the highest performing alloy. Nonetheless, since the porosity in the Base condition was already relatively low, at approximately 0.6%, the effectiveness of the HIP treatment was likely limited for these samples. However, the combination of HIP treatment with Sr additions or other porosity-inducing processes could be very successful for mitigating the decreases in density and achieving superior results. Given the understanding of the effects of the individual processes on microstructure and properties that was currently developed, these processing parameters can be optimized. Yet, for castings with relatively low porosity contents, this study elucidated that it may not be necessary to use specialized and expensive HIP equipment during the manufacturing process. For such castings, heat treatment at ambient pressures can be an advantageous, simple, and effective solution for producing enhanced alloys for many engineering applications.

Conclusions

Step blocks of low-pressure die-cast 319 Al alloy were processed with hot isostatic pressing (HIP), heat treatment, and Sr additions up to 150 ppm to determine their individual effects on mechanical, thermal, and electrical properties. For all conditions, thinner step sizes in the castings produced finer microstructures, due to their faster solidification rates, which improved alloy hardness, ultimate tensile strength, and ductility. Additions of Sr effectively transformed the acicular Si particles into a fine, fibrous structure, but excessive amounts promoted a significant decrease in material density. Hence, the Sr-modified castings were found to be more ductile, but weaker than the unmodified casting. As well, the transformation of the Si particle morphology improved thermal and electrical conductivities by almost 7% for the 150 ppm Sr condition.

Both the HIP treatment and the heat treatment, which were applied with an identical temperature and time, produced similar fragmentation and dissolution of the Al₂Cu particles as well as partial spheroidization of the Si particles. As a result, the hardness and ultimate tensile strength of both conditions generally improved, and there was a 12.5% increase in their thermal and electrical conductivities compared to the Base, as-cast condition. Accordingly, modification of the Si particles, either chemically or thermally, was found to effectively improve alloy conductivity. Although the porosity levels in the casting were already low, the applied pressure during the HIP treatment slightly improved mechanical properties compared to the heat treatment condition. Therefore, given low-pressure die castings with large internal porosity levels, HIP treatments can be very successful in enhancing alloy mechanical properties and conductivity. Yet, given that the microstructural evolution was predominantly caused by the high temperature itself, ambient-pressure heat treatment can be just as effective for many engineering applications. Otherwise, there is potential for incorporating a HIP treatment as a part of a longer or more complex heat treatment procedure, such as precipitation hardening.