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The main objective of this study was to evaluate the performance of 354-type Al-Si-Cu-Mg cast alloys at both ambient (25 °C) and elevated temperatures (250 °C) through the addition of zirconium (Zr), nickel (Ni) and manganese (Mn). The motive behind these additions was to develop thermally stable alloys capable of resisting softening instigated by prolonged exposure at elevated temperatures and hence preserve acceptable mechanical properties. Another parameter affecting performance is geometry. Four different notch geometries were employed: two hole-type geometries and two types of V-notches (symmetric and asymmetric for each). The results reveal that the presence of asymmetric notches in the tensile test bars proved to be more deleterious than symmetric notches to the notch tensile strength and percentage elongation to fracture at both 25 °C and at 250 °C. The softening that took place during tensile testing at 250 °C rendered the alloys some ductility (in particular, the M1S (354 + 0.3 wt.% Zr), M2S (354 + 0.3 wt.% Zr + 2 wt.% Ni) and M3S alloys (354 + 0.3 wt.% Zr +0.75 wt.% Mn)), as a result, the notched tensile bars with asymmetric holes exhibited high strength values for these alloys compared to the tensile strength of unnotched bars subjected to similar heat treatment conditions owing to the presence of notches. The notch sensitivity ratios vary between 0.78 and 0.95 for asymmetric holes versus 0.7 and 0.85 for asymmetric V-notches at room temperature and 0.9 and 1.07 for asymmetric holes versus 0.7 and 0.92 for asymmetric V-notches at 250 °C.

In the present study, the morphological evolution of eutectic Si particles subjected to extended thermal modification and chemical treatment has been investigated for Zr containing A354 and A356 alloys along with its effect on the mechanical properties. Thermal modification of silicon particles proved to be more effective in the Sr-modified alloys rather than their Sr-free counterparts. The evolution of silicon particles during extended solution treatments followed the same trends and sequences for non-modified and Sr-modified 354- and 356-type alloys, at different evolution rates. The coarsening of eutectic Si particles occurred through particle coalescence and Ostwald ripening mechanisms. While both mechanisms were active at the same time, however, they operated independently and additively. The pinholes observed in the silicon particles derive from the impression or imprint left behind from the collision of small particles with, and their diffusion into, larger particles. With respect to the tensile test data obtained at room temperature, solution heat treatment improved the UTS and ductility values of both Sr-modified and non-modified alloys in the first 100 h of the treatment followed by reduction in the values as a result of the morphological changes in the Si particles; however, YS values remained almost unchanged. Morphological changes in the Si particles had a very limited effect on the high temperature tensile properties and, surprisingly, this limited effect extended to the ductility values as well.

This study investigated the effects of adding Zr, as a base alloying element, besides Ni and Mn in different amounts and combinations on the melting and solidification characteristics of 35...

The present work was undertaken with the aim of studying the microstructural changes as well as variations in the tensile properties of 413.0 alloy. The ultimate tensile strength (UTS), elastic limit (YS) and elongation to fracture (%El) resulting from the addition of alloying elements – strontium (Sr), magnesium (Mg), copper (Cu), silver (Ag), nickel (Ni), zinc (Zn), cerium (Ce) and lanthanum (La) to the base alloy, and heat treatment were measured. Furthermore, the effect of the addition of phosphorus (P) as well as heat treatment on the microstructure and properties of the base alloy 413.0 modified with Sr was studied from the point of view of the interaction between phosphorus and strontium during the solidification process. The findings revealed that the addition of Mg, Cu, Ag, Ni, Zn, and Sr cause an increase in the values of UTS and YS coupled with a decrease in the values of %El of the base alloy 413.0 following the heat treatment. The hardening effect produced by the addition of ∼0.4% Mg is more or less equal to that obtained from the addition of ∼3% Cu. Alloys modified with Sr show high tensile properties. In addition, the results demonstrate that alloys modified with Sr in which P was added possess ductility values of the order of 4–12%, which is much higher compared to the 2% obtained for the non-modified 413.0 base alloy. Keywords: Aluminum alloys, Heat treatment, Alloying elements, Q-charts, Tensile properties, P–Sr interaction

The effect of the addition of Zr, Ni, and Mn and the elevated temperature thermal exposure on the mechanical performance (tensile and hardness properties) of Al-Si-Cu-Mg cast alloy were investigated for this research study. The evolution of the strengthening precipitates during the prolonged thermal exposure were also investigated using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) in order to correlate the variation in the mechanical properties to the nature of the strengthening precipitates. Varying the thermal exposure technique from static into dynamic had little effect on the tensile and hardness data obtained at room temperature. This permitted using the static exposure (stabilization) technique to simulate the behavior of the material under dynamic thermal exposure conditions as in the case of the automotive engine components. Coarsening of the strengthening precipitates following prolonged exposure at 250 °C had a deleterious effect on the tensile properties and hardness values. Noticeable reduction in the strength values, particularly the yield strength, and a remarkable increase in the ductility values were observed. The coarsening kinetics of the precipitates decayed with time, due to the continuously increased distance between the precipitates with increase in the exposure time, causing the observed deterioration in the mechanical performance after exposure at 250 °C up to the first 100 h. However, further thermal exposure up to 200 h did not result in further reduction in the strength and hardness values.

The principal aim of the present research work was to investigate the effects of minor additions of nickel and zirconium on the strength of cast aluminum alloy 354 at room temperature (25°C). A decrease in tensile properties of ∼10% with the addition of 0.4 wt.% nickel is attributed to a nickel-copper reaction which interferes with the formation of Al2Cu precipitates. The platelet-like phases (Al,Si)3(Zr,Ni,Fe) and (Al,Si)3(Zr,Ti) are the main features observed in the microstructures of the tensile samples of alloys with Zr additions. The reduction in mechanical properties is due to the increase in the percentage of intermetallic phases formed during solidification; such particles would act as stress concentrators, decreasing the alloy ductility. The main effect of Zr addition lies in a significant reduction in the alloy grain size ∼40%, rather than an increase in the mechanical properties. Quality index charts could be used in assessing the effects of the Ni and Zr additions to the base alloy, as well as evaluating the heat treatment relationships to the alloy tensile properties, in particular when the system shows multiple precipitation reactions. Due to the high liquidus temperature of the Al-Zr binary phase diagram, addition of Zr beyond 0.2% is not recommended to avoid undissolved Zr.

The principal aim of the present research work was to compare the role of dispersoids Al2O3 (∼15 μm average particle size) and SiC (∼15 μm average particle size) with that offered by Zr- and Ni-based intermetallics (∼35–70 μm average particle size) on the hardening of cast aluminum 354 alloy (9.1% Si, 0.12% Fe, 1.8% Cu, 0.008% Mn, 0.6% Mg, and 87.6% Al) at ambient temperature. There is no observable poisoning effect on the refinement of grain size after the addition of Zr to the alloys investigated in this study. The tensile test results were examined in light of the microstructural features of the corresponding alloy samples. The contribution of the added dispersoids or Ni and Zr alloying elements on the tensile properties of the 354 alloys was determined employing ∆P plots (where P = Property, UTS, YS, or %El), using the base alloy (in the as-cast condition) as a reference point. The tensile results were supported by investigating the precipitation-hardening phases using scanning and transmission electron microscopy as well as examining the fracture surfaces of selected conditions applying field emission scanning electron microscopy. The results show that, in all cases, Al2Cu phase is the main hardening agent. The contribution of about 1.5 vol% of SiC or Al2O3 to the strength of the base alloy is higher than that offered by Zr- and Ni-based intermetallics, under the same aging treatment.

The present study focused on the tensile properties at ambient and high temperatures of alloy 354 without and with the addition of zirconium. Tensile tests were performed on alloy samples submitted to various aging treatments, with the aim of understanding the effects of the addition made on the tensile properties of the alloy. Zirconium reacts only with Ti, Si, and Al in the alloys examined to form the phases (Al,Si)2(Zr,Ti) and (Al,Si)3(Zr,Ti). Testing at 25°C reveals that the minimum and maximum quality index values, 259 and 459 MPa, are observed for the as-cast and solution heat-treated conditions, respectively. The yield strength shows a maximum of 345 MPa and a minimum of 80 MPa within the whole range of aging treatments applied. The ultimate tensile and yield strength values obtained at room temperature for T5-treated samples stabilized at 250°C for 200 h are comparable to those of T6-treated samples stabilized under the same conditions, and higher in the case of elevated-temperature (250°C) tensile testing. Coarsening of the strengthening precipitates following such prolonged exposure at 250°C led to noticeable reduction in the strength values, particularly the yield strength, and a remarkable increase in the ductility values.

This study investigated the fracture behavior of 0.3% Zr-containing 354 alloy following the addition of 0.75% Mn or 2%Ni. The examined surfaces were obtained from tensile testing of the bars at 250°C. For each alloy, the test bar samples were examined in the T6-treated conditions after being exposed to 250°C for 1 and 200 hours. The fracture surface of the base alloy (0.3% Zr-containing 354 alloy) after stabilization for one hour at 250°C reveals a dimpled structure throughout, indicating the ductile nature of the fracture mode. The Alx (Zr, Ti)Si complex compound is observed with star-like and blocky morphologies, with cracks appearing in various particles of this compound. By increasing the stabilization time up to 200 hours, coarser and deeper dimples are formed, highlighting the increased ductility of the alloy due to the softening behavior associated with the prolonged exposure at 250°C. In the one-hour stabilized T6-treated condition alloy containing 2% Ni tested at 250°C, the appearance of microcracks in the Ni-rich phases and the lower density of dimples on the fracture surface compared to those observed in the base alloy emphasize the low ductility of alloy due to Ni-containing intermetallics. Examination of the fracture surface of Mn-containing alloys revealed the advantageous role of sludge particles in resisting the propagation of cracks that developed in many intermetallic phases.

In this article, the volume fraction of intermetallic compounds in Zr-containing 354-type Al-Si-Cu-Mg alloys, characteristics of eutectic Si particles, and tensile, hardness, and impact properties have been evaluated with varying Ni and Mn contents and combination. The results revealed that additions of Ni and Mn in different amounts and combinations increased the volume fraction of intermetallic compounds in the tailored alloys, compared to the base alloy (cf. 12.21% for 4% Ni-containing alloy with 2.5% for base alloy), producing a significant effect on the mechanical performance. The proposed additions enhanced the mechanical performance of the alloys, namely, the ambient- and elevated-temperature tensile properties, hardness values, and impact properties. For the Mn-containing alloys, the improvement in properties was attributed to the formation of sludge particles in the form of blocky α-Al15(Fe,Mn)3Si2 alongside the script-like α-iron phase that resisted crack propagation. The precipitation of Ni-bearing phases such as Al9FeNi, Al3CuNi, and Al3Ni in the Ni-containing alloys improved the mechanical properties through hindering cracks propagation. Interestingly, addition of 0.75 wt.% Mn to the base alloy proved to be competitive in strength values to the addition of 2 and 4 wt.% Ni, and better in terms of ductility values. The investigations showed that the variations in hardness and impact values follow the same trend as variations in the percentage volume fraction of intermetallic compounds, i.e., maximum property value is associated to the alloy with highest volume fraction of intermetallic compounds. Furthermore, the impact properties showed higher dependency on Al2Cu phase particles rather than the eutectic Si particles.

The present study was performed on the 354 (Al–9wt%Si–1.8wt%Cu–0.5wt%Mg)-based alloy to which measured amounts of Zr, Ni, Mn and Sc were added. Tensile test bars were prepared from the different 354 alloys using an ASTM B-108 permanent mold. The test bars were solution heat-treated using a one-step or a multi-step solution heat treatment, followed by quenching in warm water, and then artificial aging employing different aging treatments. Tensile bars were thereafter tested at 25 and 250 °C. Analysis of the tensile data was carried out using quality charts and color contour maps. The results show that at room temperature, Alloy R (354 + 0.25%Zr) showed the best quality index values after T6 treatment, i.e., 411 and 418.64 MPa without, and with 200-h stabilization at 250 °C, respectively. Alloy U (354 + 0.25%Zr + 0.75%Mn) displayed a Q-value of 397.84 MPa after T6 treatment (with no stabilization) and 404.62 MPa following T6 treatment and 200-h stabilization at 250 °C before testing. At 250 °C testing temperature and after 200-h stabilization at 250 °C, the strength of all alloys exhibited a decrease in strength with an increase in ductility being related to the softening, which occurred due to the onset of overaging.

Alloys such as the 354.1 (Al–9 wt% Si–1.8 wt% Cu–0.5 wt% Mg) alloys show a greater response to heat treatment as a result of the presence of both Cu and Mg. These alloys display excellent strength values at both low and high temperatures. Additions of Zr, Ni and Mn are expected to maintain the alloy performance at still higher temperatures. The present study was carried out to investigate the effects of these additions on the tensile properties of 354.1 alloy castings at ambient and at high temperature (250 °C), using different holding times at testing temperature. Alloys were prepared using 0.2 wt% Ti grain-refined 354.1 alloy, comprising alloy R (354.1 + 0.25 wt% Zr) considered as the base or reference alloy, alloy S (354.1 + 0.25 wt% Zr + 2 wt% Ni) and alloy U (354.1 + 0.25 wt% Zr + 0.75 wt% Mn), containing Ni and Mn. The latter two alloys perform better under high-temperature conditions following 1-h stabilization at 250 °C, displaying strength values of 226.64/230 MPa UTS and 225/226 MPa YS, respectively. For a prolonged stabilization time (200 h at 250 °C), however, the strength values are reduced to 111/96 MPa UTS and 90/78 MPa YS. The decrease in strength and increase in ductility observed after prolonged stabilization are related to alloy softening, which occurs because of the overaging conditions during which the equilibrium precipitates form, leading to loss of coherency strain between the precipitates and the matrix.

This study investigated the active strengthening precipitates existing in the structure of 354-type Al–Si–Cu–Mg cast alloys following the addition of Zr as a primary alloying element in addition to Ni and Mn in different amounts and combinations. The investigation comprised three main parts: the first part simulated the formation and dissolution of strengthening precipitates and their precursors using differential scanning calorimetric (DSC) heating analysis of as-quenched samples, the second part presented transmission electron microscopy (TEM) observations of the fine precipitates existing in the structure of the alloys studied, and the third part examined the mechanical performance (tensile properties and hardness values) of the alloys studied. Interpretation of the DSC heating data in accordance to the available literature showed that the alloys are strengthened primarily by θ-Al2Cu and S–Al2CuMg precipitates and their precursors. Furthermore, some of the copper content was consumed in forming Al–Ni–Cu particles instead of the θ-Al2Cu strengthening phase and its precursors and hence the Ni-free alloys contain higher fractions of the θ-Al2Cu phase and its precursors than the Ni-containing alloys. TEM investigations confirmed the results obtained from the DSC heating analysis regarding the combined strengthening effect of θ-Al2Cu and S–Al2CuMg precipitates and their precursors as well as the fraction of θ-Al2Cu phase in Ni-free and Ni-containing alloys. TEM observations revealed, also, that the alloys studied have a secondary strengthening effect which is exerted by precipitates in the form of Alx (Zr,Ti)Si that formed following the addition of Zr. The Mechanical performance showed to be highly dependent on the chemical composition which controls alloys structure and response to the applied heat treatment and hence the formation of the fine precipitates which are mainly responsible to the strengthening effect.

This study investigated the decomposition of π-AlMgFeSi into β-Al5FeSi phase needles during extended periods of solution heat treatment and its effects on the mechanical properties of Al–7Si–0.55Mg–0.1Fe alloys. An analysis of the results obtained from the Charpy impact test using unnotched samples shows that the highest initiation and propagation energies are obtained for the as-cast and heat-treated alloys when these alloys are solidified at a high cooling rate and modified with strontium. An increase in the solution treatment time improves the impact properties of the alloys compared to the as-cast condition. In accordance with this finding, the recommended solution treatment time at which the maximum initiation and propagation energy values can be obtained is 20 h for all alloys studied. The results also show that the impact properties are more sensitive to the changes occurring in the microstructure which result from solution heat treatment and Sr modification, namely, the eutectic Si and π-phase morphologies, rather than those related to the tensile properties, i.e., to the Mg content in the matrix. Fracture analysis was carried out using a scanning electron microscope equipped with an EDX system for element analysis. The results obtained show that the tensile and impact fracture behavior of the Al–7Si–0.55Mg–0.1Fe alloys is controlled mainly by the morphology of the eutectic silicon. The π-phase iron intermetallics act as crack initiation sites and provide an easy path for crack propagation in both non-modified and Sr-modified alloys. The fracture analysis of the 80-h solution-treated sample shows the presence of newly formed β-phase needles which provide an additional source for crack initiation, and thus all the 80-h solution-treated alloys show the lowest energy values.

The present study was performed on Mg-based alloys containing Zn and Mn. The alloys were cast in a permanent metallic mold preheated to 200°C and with a protective atmosphere of dry air, CO2, and SF6. Two main phases are observed in the as-cast condition: Mg-Al-Zn and Mn-Al intermetallics. The size and morphology of the Mg-Al-Zn phase are significantly affected by the concentration of Al. Tensile properties, using standard ASTM B-108 samples, are directly related to the size, morphology, and density of the existing phase particles. The alloy ductility is reduced with increase in the Al concentration, whereas the ultimate tensile strength and the yield strength are more or less stable. The fracture surface of the tested tensile bars is mostly ductile for low Al-containing alloys and tends to be brittle with the increase in Al content as evidenced by an increase in the density of cleavage ruptured areas.

The present study was performed on castings obtained from A356 alloy melts subjected to different melt treatments (degassing, modification, grain refining). The castings were prepared using three different processes/molds: lost foam, ASTM standard metallic, and end-chill refractory molds. An examination of samples sectioned from the cylinder head/lost foam casting showed that the secondary dendrite arm spacing (SDAS) depends on the sample location in the cylinder head, with bolt boss and combustion chamber sections exhibiting SDAS values of 52 and 86 µm, respectively, versus 26 µm for the ASTM metallic mold samples, and from 25 to 85 µm with increasing distance from the end-chill, along the height of the refractory mold. Grain refining (Al-4 %B) leads to redistribution of porosity in the casting, as the AlB2 particles of the grain refiner act as sites for the precipitation of hydrogen gas bubbles, resulting in finer pores. This observation is independent of the mold type. In the presence of modifier, Sr–B interaction causes loss in the available Sr and B for modification and grain refining, resulting in a heterogeneous distribution of the eutectic silicon particles. Although increasing the solidification rate reduces the percentage porosity, it also leads to segregation of hydrogen at the solid/liquid interface in the case of directional solidification obtained with the end-chill mold, so that the porosity is the outcome of the solidification rate and the hydrogen content. The tensile properties of the A356 alloy are controlled by the size and morphology of the eutectic silicon particles more than by the addition of grain refiner. The difference in tensile properties obtained using the three molds is essentially attributed to % porosity (shrinkage, gas, and presence of residual pyrolysis) and SDAS. The use of proper melt treatments and the ASTM standard metallic (tensile test bar) mold could produce strength values that are 170 % higher than that obtained from the lost foam castings.