Effect of Bi and Ca on the Solidification Parameters of Sr-Modified Al-S-Cu (Mg) Alloys.

The effect of tramp elements, mainly Bi and Ca, on the thermal characteristics of Sr-modified Al-Si-Cu and Al-Si-Cu-Mg alloys has been investigated using thermal analysis, X-ray radiography, and field emission scanning electron microscopy (FESEM) techniques. The high affinity of Bi to interact with Sr results in an increase in the Al-Si eutectic temperature, and hence an increase in the size of eutectic silicon particles. In contrast, the Ca-Sr interaction seems to have no significant effect on the alloy thermal behavior. The effect of these interactions on porosity formation has been discussed. Hot zones may be formed in thin cavities, in particular, near the bottom of the mold, leading to formation of unexpected coarse porosity, mostly shrinkage type. The study also highlights the significance of other parameters on porosity formation, such as no melt degassing, SrO, Al2O3 (strings or bifilms), as well as the presence of iron-based intermetallics.

Effect of Zr and Ti Addition and Aging Treatment on the Microstructure and Tensile Properties of Al-2%Cu-Based Alloys.

The present work investigated the effect of aging treatment on the microstructure and tensile properties of an Al-2%Cu base alloy containing various additions of Zr and other alloying elements. Aging was carried out at temperatures of 180-300 °C for different aging times at each temperature. The tensile properties indicated that Zr additions improved the strength of the base alloy, especially at high Zr levels at 180 °C. At the 220 °C aging temperature, however, while Zr addition did not have a beneficial effect on the alloy strength, the ductility was found to improve. Zr-Ti combined additions had a significant effect on the microstructure of the base alloy, as the morphology of the α-Al grains transformed into a non-dendritic morphology, and the grain size decreased sharply. These effects were at their maximum at 180 °C and 0.5 wt% Zr addition. Moreover, the Zr-containing alloys aged at higher temperatures, such as 220 °C and 240 °C, maintained a noticeably higher level of strength over the base alloy aged at the same temperatures. Quality index charts based on the tensile test data also reflected an improvement in alloy quality and strength with Zr-Ti combined additions.

Assessment of the Influence of Additives on the Mechanical Properties and Machinability of Al-11%Si Cast Alloys: Application of DOE and ANOVA Methods.

In the present study, the statistical design of experiments (DOE) method was applied to study and control the properties of near-eutectic Al-11%Si alloys. In this study, we developed regression equations between response variables, including hardness, yield stress, ultimate tensile stress, elongation, total cutting force, cutting power, and tool life, and varying factors which included the percentage of the alloying element in the composition and the modification level. These equations may be analyzed quantitatively to acquire an understating of the effects of the main variables and their interactions on the mechanical behavior and the machinability of the alloy under investigation. Analysis of variance (ANOVA) was performed to verify the fit and adequacy of the developed mathematical models. The results show that increasing the levels of Cu and Fe results in an increase in hardness, yield stress and ultimate tensile strength in both modified and non-modified alloys. On the other hand, both Cu and Fe appear to affect the elongation adversely, whereas the Sr level shows a positive effect on the elongation percentage. We found that the Sr level had the most significant effect on the cutting forces and cutting power, followed by Fe and Cu contents.

Mechanical Performance and Precipitation Behavior in Al-Si-Cu-Mg Cast Alloys: Effect of Prolonged Thermal Exposure.

Al-Si-Cu-Mg cast (354) alloys are used in the automotive sector owing to their remarkable properties which are achievable after applying appropriate thermal treatments. Zirconium, Nickel, and Manganese were added to this category of Al-alloys to preserve good mechanical properties while being exposed to elevated temperatures for long times. The ultimate and yield strength values obtained at room temperature for the stabilized (thermally-exposed) T5-treated condition were comparable to those of the stabilized T6-treated condition, whereas the same properties for T5-treated alloys were higher than those of T6-treated ones for elevated-temperature tensile testing. Interestingly, the results showed that the addition of 0.75 wt.% Mn was competitive with the addition of 2 and 4 wt.% Ni with respect to the elevated-temperature and ambient temperature strength values, respectively. In addition, the Mn-containing alloy M3S exhibited improved ductility values at ambient temperature and at 250 °C, compared to the Ni-containing alloys. Examination of the fracture surface of tested samples revealed the advantageous role of sludge particles in enhancing the performance of Mn-containing alloys through their resistance to the propagation of cracks that developed in many intermetallic phases. This finding is considered to be economically significant in view of the lower price of manganese compared to that of nickel.

Intermetallics Formation during Solidification of Al-Si-Cu-Mg Cast Alloys.

The present study was undertaken to examine the effect of iron, manganese, copper and magnesium on the microstructural characteristics of Al-11%Si-2%Cu-Mg-based alloy referred to as 396 under different working conditions. The results show that strontium (Sr) has high affinity to react with magnesium (Mg), resulting in reduced effectiveness as eutectic silicon modifier or age hardening agent. In addition, Sr alters the sequence of the precipitation of the α-AlFeMnSi phase from post-eutectic to pro-eutectic which would harden the soft α-Aluminum matrix. The mechanism is still under investigation. The interactions between iron (Fe) and Mg and Sr-Mg result in the formation of serval dissolvable intermetallics during the solutionizing treatment such as ß-AlFeSi, π-AlFeMgSi and Q-AlMgSiCu phases. The study also emphasizes the role of modification and grain refining as well as intermetallics in porosity formation and hardness of samples aged in the temperature range 155-240 °C.

Effect of Intermetallics and Drill Materials on the Machinability of Al-Si Cast Alloys.

The present study was conducted on the machinability of 396 alloy (containing approximately 11% Si) and B319.2 alloy mainly to emphasize the effects of Fe-intermetallics, i.e., α-Fe, ß-Fe, and sludge. The results demonstrate that the presence of sludge in the form of hard spots has a significant effect on cutting forces and tool life, in that it decreases drill life by 50% compared to the base alloy. The formation of the α-Fe phase in the M1 base alloy has a beneficial effect on tool life in that this alloy produces the highest number of holes drilled compared to alloys containing sludge or ß-Fe; this result may be explained by the fact that the formation of the α-Fe intermetallic, with its rounded Chinese script morphology and its presence within α-Al dendrites, is expected to improve matrix homogeneity via hardening of the soft α-Al dendrites. Increasing the Fe-content from 0.5% to 1% in the 396-T6 alloy containing 0.5% Mn produces a distinct improvement in alloy machinability in terms of cutting force and tool life. The addition of Fe and/or Mn appears to have no discernible effect on the build-up edge area (BUE) and chip shape.

Effects of Trace Elements on the Microstructural and Machinability Characteristics of Al-Si-Cu-Mg Castings.

This study was undertaken to emphasize the influence of Sn and Bi addition on the machinability of Sr-modified, grain-refined, and heat-treated Al-Si B319 and 396 alloys. Drilling and tapping tests were conducted to examine the cutting forces, tool life, tool wear, built-up edge evolution, and chip shape. Microstructures were examined using optical and electron microscopy. Drilling test results show that the B319.2 alloy with 0.15%Sn yields the longest drill life, i.e., twice that of the B319.2 alloy containing 0.5%Bi, and one-and-a-half times that of the B319.2 alloy containing 0.15%Sn + 0.5%Bi. The presence of 0.5%Bi in the B319.2 alloy causes a deterioration of drill life (cf., 1101 holes with 2100 holes drilled in the B319.2 alloy containing 0.15%Sn). The α-Fe phase in the 396 alloy produces the highest number of holes drilled compared with alloys containing sludge or ß-Fe. The presence of sludge decreases the drill life by 50%. Built-up edge (BUE) measurements and optical photographs show little change in the BUE width for different numbers of holes except for the B319.2 alloy containing 0.5%Bi, which shows a slightly lower width (0.166 mm) compared with that containing 0.15% Sn (0.184 mm) or 0.15%Sn + 0.5%Bi (0.170 mm).

The Effect of Bi-Sr and Ca-Sr Interactions on the Microstructure and Tensile Properties of Al-Si-Based Alloys.

The effect of bismuth and calcium additions on the microstructural characteristics and the tensile properties of the modified and grain-refined Al-Si based B319 alloys were studied in this paper. Based on the results obtained, it has been concluded that Bi reacts with both Sr and Mg, leading to severe demodification of the eutectic Si at Bi levels of 0.15%-0.6% Bi. Bismuth causes a decrease of the yield and tensile strengths for the as-cast and artificially aged conditions and an increase of yield strength in the solution heat-treated condition. The elongation increases with the Bi in the solution heat-treated condition. Based on this, Bi is found to be an efficient solid-solution strengthening element for these alloys. Thus, solution heat treatment, rather than the artificial aging, may be recommended for alloys containing about 1.0% Bi. Calcium has no significant demodification effect on the Sr-modified Si particles at 100-400 ppm Ca, and has a modifying effect at ~600 ppm Ca. The elongation increases with the Ca level at all conditions (as-cast, solution heat-treated, and artificially aged). A slight increase of the tensile strength in the heat-treated conditions was also observed. The lowest tensile properties either in the as-cast or the heat-treated conditions correspond to the most demodified-Si condition obtained at 408 ppm Ca. Calcium is, therefore, not as detrimental to the tensile properties as Bi.

Metallurgical Parameters Controlling the Eutectic Silicon Charateristics in Be-Treated Al-Si-Mg Alloys.

The present work was carried out on Al-7%Si-0.4%Mg-X alloy (where X = Mg, Fe, Sr or Be), where the effect of solidification rate on the eutectic silicon characteristics was investigated. Two solidification rates corresponding to dendrite arm spacings (DAS) of 24 and 65 µm were employed. Samples with 24 µm DAS were solution heat-treated at 540 °C for 5 and 12 h prior to quenching in warm water at 65 °C. Eutectic Si particle charateristics were measured using an image analyzer. The results show that the addition of 0.05% Be leads to partial modification of the Si particles. Full modification was only obtained when Sr was added in an amount of 150-200 ppm, depending on the applied solidification rate. Increasing the amount of Mg to 0.8% in Sr-modified alloys leads to a reduction in the effectiveness of Sr as the main modifier. Similar observations were made when the Fe content was increased in Be-treated alloys due to the Be-Fe interaction. Over-modification results in the precipitation of hard Sr-rich particles, mainly Al4SrSi2, whereas overheating causes incipient melting of the Al-Cu eutectic and hence the surrounding matrix. Both factors lead to a deterioration in the alloy mechanical properties. Furthermore, the presence of long, acicular Si particles accelerates the occurrence of fracture and, as a result, yields poor ductility. In low iron (less than 0.1 wt%) Al-Si-Mg alloys, the mechanical properties in the as cast, as well as heat treated conditions, are mainly controlled by the eutectic Si charatersitics. Increasing the iron content and, hence, the volume fraction of Fe-based intermetallics leads to a complex fracture mode.