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The present study was performed on 356 and 413 alloys solidified at 8 °C/s. The samples from 413 alloys were solutionized at 510 °C, whereas samples from 356 alloy were solution heat treated at 555 °C, for times up to 1200 h. All polished samples were deeply etched using Keller’s etchant to dissolve the aluminum matrix and expose the eutectic silicon particles. Etched samples were investigated using field emission scanning electron microscopy (FESEM). It was observed that Sr-modified 356 alloy can nearly reach complete spheroidization in that all eutectic Si particles will be individually distributed throughout the matrix after a solutionizing time of 1200 h. Non-modified 356 alloy, under the same condition, still revealed partial fragmentation similar to Sr-modified 413 alloy. There was no complete spheroidization in non-modified alloys which explains the reported large standard deviation when polished samples are examined using optical microscopy. In other words, observations made from deeply etched samples (3D) disagree with those obtained from polished samples (2D). In addition, it is impossible to achieve coarsening of the spheroidized Si particles under industrial conditions. The present study was supported by statistical analysis as well as tensile testing.

In considering the critical role of crystal orientation and working temperature in shaping overall performance of piezoresistive pressure microsensors, this study investigates the effect of wafer orientation (i.e., crystal orientation) and working temperature on thermomechanical and fracture behaviour of a P-type silicon wafer used in pressure sensors. The study was carried out using analytical analysis, based on Kirchhoff–Love plate theory, and numerical analyses side by side with experimental investigations for validation and comparison purposes. The study addresses the following objectives: (1) to investigate the effects of working temperature, wafer positioning and crystal orientation on deflection and fracture behaviour of a silicon membrane; (2) to compare wafer deflections obtained experimentally to those obtained from simulations and numerical approaches; and (3) to determine the effect of maximum wafer deflection on crack initiation and fracture mode through the application of an experimental thermomechanical technique. An agreement between experimental and simulated deflection values of the Si wafer within a temperature range of 25–80 °C was concluded. Moreover, when working temperatures are below 100 °C, the fracture mode is highly brittle when a wafer is oriented at 0° and 45° with respect to [110] directions, whereas the wafer tends to render some ductility at temperatures above 120 °C. It was also found that at room temperature fracture occurs at a deflection of about 389 µm and 463 µm when the crystal is oriented at 45° and 0°, respectively.

In this study, the results pertaining to the influence of various metallurgical parameters on the tensile properties of Al–Si–Cu–Mg 354-type casting alloys are presented. This is discussed with an emphasis on acquiring an insight regarding the mechanical limits of Al–Si–Cu–Mg 354-type alloys in the context of these parameters, and to obtain an understanding of the effects of the addition of nickel and/or zirconium on alloy 354 with respect to the mechanical properties obtained at high temperature (155 °C and 300 °C for holding times at temperature ranging from 10 to 100 h). Based on an analysis of the results obtained, the following conclusions may be drawn. The results show that additions of Zr and Zr + Ni increase the high-temperature tensile properties, in particular for the alloy containing 0.2 wt% Zr + 0.2 wt% Ni, which exhibits an increase of more than 30% in the tensile properties at 300 °C compared with the base 354 alloy. In other words, the addition of 0.4 wt% Ni + 0.4 wt% Zr to the 354 alloy is not sufficient to resist softening at 300 °C/100 h (cf. UTS: 52 MPa, YS: 45 MPa, %El: 23% with UTS: 361 MPa, YS: 343 MPa, %El 1.23%, observed after T6 treatment). The addition of 0.4 wt% Ni to alloy 354 leads to a decrease in the tensile properties, compared to the base alloy. This decrease may be attributed to a Ni–Cu reaction which could interfere with the formation of the Al2Cu strengthening precipitates, thereby affecting the age hardening process. As a consequence, percentage elongation increases with the testing temperature, from as-cast to T6-treated samples tested at high temperature (155 °C and 300 °C, respectively). In summary, the best high-temperature tensile properties are displayed by the alloy containing 0.2 wt% Zr + 0.2 wt% Ni.

The present study was carried out to investigate the effects of Ni and Zr additions, individually or in combination, on the room-temperature tensile properties of 354 casting alloy (Al–9wt%Si–1.8wt%Cu–0.5wt%Mg) which was isothermally treated at temperatures in the range of 155–350 °C and aging times up to 1000 h. Tensile tests were carried out in the as-cast, solution heat-treated, and aged conditions using different aging times up to 1000 h. Quality charts were used as an evaluation tool for selecting the optimum conditions to achieve superior tensile properties and optimum quality in 354-type alloys. Zirconium reacts only with Ti, Si, and Al to form (Al,Si)2(Zr,Ti), (Al,Si)3(Zr,Ti), and Al3Zr phases. The beneficial effects of Zr and Ti additions appear in the refining of the α-Al grain size which reduces the size of the Al2Cu and α-Fe particles. Tensile test results at ambient temperature show a slight increase in alloys with Zr and Zr/Ni additions, particularly at aging temperatures above 240 oC. It is suggested that the maximum obtainable quality index values by means of heat treatment are the difference between the quality index values for the as-cast and solution heat treatment conditions.

The present work was undertaken with the aim of studying the microstructural changes and intermetallic phases in Al-Si base 413.0 alloys as well as variations in tensile properties, including ultimate stress, yield strength and elongation at rupture, resulting from the addition of alloyed elements: strontium (Sr), magnesium (Mg), copper (Cu), silver (Ag), zinc (Zn) and nickel (Ni), to the base alloy 413.0, under different conditions of heat treatments, i.e., solutionizing and artificial aging (in the temperature range of 155 °C–240 °C). The results obtained in relation to microstructural observations and tensile tests reveal that the addition of alloying elements, in particular Mg, Cu, Ag, Ni, Zn and Sr, leads to an increase in the ultimate stress and yield strength values with a decrease in the strain levels of the base alloy 413.0, following hardening during artificial aging. Both Ag and Zn do not form specific phases during solidification at a slow rate of ~ 0.8 °C/s but only segregate to the α-aluminum cell boundaries. Addition of 0.73% Ag improves the values of yield strength at all aging temperatures. Also, it enhances the alloy resistance to softening during aging in the temperature range 200–240 °C. Similar behavior was exhibited by the addition of 2.5% Zn.

The present work was carried out on Al–Si-Cu-Mg-354 alloy to investigate the role of transition metals addition (mainly Zr, Ni and Mn) as well as heat treatment on its performance at ambient/room temperature (25 °C). Mechanical properties were studied using tensile and impact testing, whereas the microstructure was investigated using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (STEM). The fracture surfaces of selected samples prepared from impact-tested bars were examined applying FESEM. The results show that the proposed additions enhanced the alloy tensile and impact properties. The T6-tempered alloys revealed higher tensile strength than T5-tempered alloys. The impact properties of the alloys are highly influenced by the Al2Cu phase particles rather than the eutectic silicon particles. Thus, the impact properties show no significant variations with respect to the condition studied, i.e., the total energies absorbed by the five alloys in the as-cast condition are close in their values, as is the case for the T5- and T6-treated conditions. Due to the precipitation of a fairly large amount of insoluble intermetallics, all fractures were brittle in nature.

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.

The tensile data showed that strength and percent elongation of the Zr-containing 354 (Al–9%Si–1.8%Cu–0.5%Mg) alloys increased in the one-step solution heat-treated condition compared to the as-cast case. The multi-step solution heat treatment displayed higher tensile properties than those achieved with single-step solution heat treatment. The use of the T6 treatment, incorporating the multiple solution heat treatment, allows for maximum dissolution of the copper phases in the multiple stages of solution treatment, resulting in the greatest improvement in the alloy strength. The best tensile properties of alloys tested at room temperature after stabilization at 250 °C for 200 h are obtained with the T6 heat treatment. The addition of Zr, Ni, and Mn to Al–Si alloys improves the high-temperature tensile properties of the 354 alloy. Alloys containing 2%Ni or 0.75%Mn perform better in case of high-temperature conditions, with 1-h stabilization at 250 °C. After 200-h stabilization at 250 °C, the strength of the T6-treated alloys—regardless of the type of solution heat treatment process, is reduced considerably, while the ductility is increased, with base alloy showing the highest percent elongation, ~ 19%. The reduction in strength may be attributed to the alloy softening which occurs after such long stabilization time at the high testing temperature (250 °C).

The main objective of the present work is to explore the morphological changes in the eutectic silicon particles associated with the increase in the solution treatment time up to 400 h by quantifying the characteristics of the eutectic silicon particles at different durations of solution heat treatment. 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-type and 356 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 diffusion of small particles into larger particles.

The present work was carried out on an Al–Cu alloy containing 6.5%Cu, 0.1%Si, 0.4%Mn, and 0.18%Zr (coded HT200). The investigations included hot-tearing tests, microstructural characterization, and tensile properties at room temperature (25 °C). The results were interpreted in terms of application of grain refiner, solidification rate, and phase precipitation. Grain refining using TiBor (0.15%Ti) coupled with high solidification rate (~ 8 °C/s) is proven to be very effective in reducing the alloy hot-tearing sensitivity to be equal to that of A319 alloy with an average grain size of ~ 50–80 µm. In contrast to A319 alloy, the present HT200 alloy required a single step solutionizing treatment (4 h at 520 °C) followed by water quenching to achieve maximum dissolution of the Cu phase. The alloy also exhibited a fast response to aging treatment, resulting in a significant improvement in the alloy tensile strength and quality.

The present study was performed on A356 and A413 alloys in non-modified and Sr-modified conditions. To these alloys, pure rare earth (RE) metals, mainly lanthanum (La) and cerium (Ce), were added in the amount of 1 wt% each, individually or combined. The results showed that the addition of 2% RE leads to a significant reduction in the grain size (~ 60%) compared to the base alloy. However, it also leads to precipitation of a large amount of insoluble intermetallics (~ 10% in volume fraction). These intermetallics cause a significant decrease in the alloy ductility and hence deteriorate the tensile properties and impact energies. In addition, Sr may also cause destabilization of the RE intermetallics with possibility of partial dissolution during solution heat treatment. The RE metals have an affinity to react with Sr leading to partial demodification of the eutectic Si. They also react with Cu, diminishing the alloy response to age hardening. The RE intermetallics act as crack initiation sites, leading to premature failure of the sample. The combined occurrence of these various effects complicates the prediction of the alloy behavior.

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 present study focuses on the porosity formation in three Al-Si cast alloys widely used in automotive industries viz. A319.0, A356.0, and A413.0 alloys under various conditions: stirring, degassing. Sr level, amount of grain refining, combined modification and grain refining, as well as hydrogen level. The solidification rate was the same for all alloys in terms of the mold used and its temperature. The microstructural investigations were carried out quantitatively using an optical microscope-image analyzer system scanning systematically over a polished sample area of 25 mm × 25 mm and qualitatively using an electron probe microanalzer equipped with EDS and WDS systems. The results were coupled with hardness measurements.

The present study was carried out on three Al-Si cast alloys viz., 319, 356, and 413 alloys, solidified at 8°C/s. Samples from 319 and 413 alloys were solution heat-treated at 510°C, whereas samples from 356 alloy were solutionized at 550°C, for up to 1200 h. The results reveal that complete spheroidization of eutectic Si particles in terms of achieving individual spherical particles cannot be achieved in most Al-Si-Cu-Mg alloys even after a solutionizing time of 1200 h which contradicts with the existing theory. Addition of Sr to Cu-free 356 alloy could lead to complete spheroidization after 1200 h at 550°C if the alloy was solidified at 8°C/s. Besides the dissolution theory of Ostwald, coarsening of Si particles can as well take place by impingement, fusion, and agglomeration. Increasing the Si content makes it difficult to achieve spheroidization, i.e., fragmentation and coarsening. Results obtained from observations of deeply etched samples (3D) contradict those obtained from polished samples (2D).

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.

A study of the effects of the casting parameters on the microstructure and mechanical properties of the A356 alloy, obtained from lost foam casting, was carried out using various testing techniques. In light of the results obtained, it was observed that the chemical composition of the strontium master alloys used had no effect on the microstructure or the mechanical properties of the alloys studied. The value of the secondary dendrite arm spacing for the bolt boss section is ~52 μm compared to 86 μm for the combustion chamber section in the same cylinder head. This difference demonstrates the existence of different solidification rates inside the same casting. Rapid solidification of the metal brings a reduction in the size of the eutectic silicon particles in the as-cast condition. However, the T6 heat treatment can cancel this effect if the silicon particles enter the growth phase. The growth of these particles in the bolt boss section is not as rapid as in the combustion chamber section, indicating that the Si particle size depends greatly on the geometry of the section of the casting in question. Due to the increased formation of porosity in the presence of H2, the alloy ductility is severely affected as fracture occurs more easily. Gains of 17 and 24 % were observed for the hardness and elastic limit, respectively, for the bolt boss samples compared to the combustion chamber samples.