Alloy Si Content Research Articles

Investigation on machinability in turning of as-cast and T6 heat-treated Al-(3, 7, 12%)Si-0.6%Mg alloys

The machinability in turning of Al-Si-Mg alloys under as-cast and heat-treated conditions has been investigated. Three alloys with 3%, 7% and 12%Si and 0.6%Mg contents (wt%) were prepared. Unsteady solidified ingots were obtained in a vertical upward directional solidification apparatus using an instrumented cylindrical water-cooled mold. Samples extracted from the ingots were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) examinations and by Brinell hardness tests, before and after T6 heat treatments (540 °C solutioning temperature for 10 h, water quenching at 25 °C and artificial aging at 155 °C for 5 h). Specimens in both the as-cast and heat-treated conditions were subjected to machinability tests as a function of cutting force in face turning operation using a CNC lathe machine, fixing parameters as cutting depth, feed rate, cutting speed, and cutting tool. The as-cast microstructure consists of a dendritic α-Al rich phase and an interdendritic eutectic microconstituent (α-Al + Si particles). As the alloy Si content increased, the solidification cooling rate increased, the amount and size of α-Al dendrites decreased while the amount of eutectic mixture increased. After heat treatment, Si particles were found to be more rounded and an increase in fraction of Mg2Si was observed. The hardness profile declines along the length of the as-cast ingots, however the values improved with both alloy Si content and heat treatment. The as-cast alloys showed maximum cutting force at the beginning of the process, decreasing during machining progress, whereas in the heat-treated condition the cutting forces showed values almost constant. The results suggest a more stable machinability behavior during turning of the heat-treated alloys as a consequence of a more homogenous microstructure and hardness.

Influences of alloying elements and dendritic spacing on the corrosion behavior of Al–Si–Ag alloys

The most important commercial casting alloys are those based on Al–Si. Their technical characteristics are highly connected to their as-cast microstructure, which is translated by the secondary dendrite arm spacing (SDAS). The alloy microstructure has been demonstrated to be changed by the adding of certain alloying elements, in the sense of boosting the desired properties and features. Search has been intensified to provide a positive balance of several properties, such as: high yield strength, high electrical conductivity, and high corrosion resistance, besides maintaining the ability for the alloy to be conventionally solidified. Under such context, both addition of Ag and the control of the fineness of the dendritic structure can be alternatives seeking better properties for Al–Si alloys. In addition to the possibility of application in current studies contributing to the growing development needs as engine materials, Al–Si–Ag alloys can emerge as an alternative to the manufacture of parts for electric vehicles, such as rotors and inverters, where high mechanical strength, corrosion resistance and electrical conductivity are required. Corrosion findings of Al–Si–Ag specimens having quite different SDAS of five alloys are presented here: Al-5wt.% Si, Al-5wt.% Si-0.1wt.% Ag, Al-10wt.% Si-0.1wt.% Ag, Al-5wt.% Si-2wt.% Ag, Al-10wt.%Si-2wt.%Ag alloys. A complete set of characterization with potentiodynamic polarization, electrochemical impedance spectroscopy, immersion tests, optical microscopy and scanning electron microscopy (SEM) was developed for these alloys. It has been demonstrated that increase in the alloy Si content diminishes corrosion resistance, and that increase in Ag content, considering Al-10% Si alloys, reduces corrosion resistance as well, despite creating thicker passive films. Furthermore, the alloys with the lowest Ag content (0.1 wt.%) had the greatest corrosion performance among the Ag-containing alloys, while the microstructure refinement tends to enhance the nobility of the samples.

Effects of solidification thermal parameters and Bi doping on silicon size, morphology and mechanical properties of Al-15wt.% Si-3.2wt.% Bi and Al-18wt.% Si-3.2wt.% Bi alloys

The use of bismuth (Bi) in aluminum (Al) industries is recognized as prime importance because of possible recycling as alloying element in special applications such as those enhancing machinability. The effects of Bi interaction on solidification progress as well as on the resulting microstructure of hypereutectic Al-Si alloys, i.e. the morphology and size of Si crystals are rare information. As such, the present research work investigates the characteristic parameters of eutectic and primary Si with addition of Bi for both ternary Al-15 wt.%Si- 3.2 wt.%Bi and Al-18 wt.%Si- 3.2 wt.%Bi alloys samples directionally solidified (DS) under various cooling rates. The alloy containing 15 wt.%Si showed that Bi acts in the suppression of the growth of primary Si crystals for samples solidified at cooling rates ranging from 2.0 to 31 K/s. For lower rates, a minor fraction of five-fold branched Si was recognized in transverse specimens. A mixture of globular-fibrous eutectic Si is shown to occur for eutectic cooling rates higher than 5 K/s. The Al-18 wt.%Si- 3.2 wt.%Bi alloy is characterized by a mixed structure of eutectic flaky Si and primary angular Si with intersecting spines at nearly 90º all over the entire range of experimental cooling rates (i.e., from 0.4 to 40 K/s). Despite the intricacy of the formed microstructures, it was possible to establish relationships between tensile properties and eutectic Si spacing in order to compare the properties between the tested alloys. Tensile strength and elongation decrease with increasing alloy Si content.

Recycling silicon from silicon cutting waste by Al–Si alloying

Abstract Large amounts of silicon cutting waste (SCW) will be generated as the photovoltaic (PV) industry grows, which not only wastes valuable resources, but also causes environmental issues if SCW is not addressed. In this study, a novel and effective method to recycle Si from SCW by Al–Si alloying with a cryolite additive is proposed. The influence of the smelting parameters including smelting temperature, holding time and cryolite/SCW ratio, on the Si content of alloys and the utilization ratio of SCW was investigated and optimized. The optimal conditions were a smelting temperature of 1150 °C, holding time of 30 min and cryolite/SCW ratio of 6.3. Under these optimized conditions, the Si content of alloys and the utilization ratio of SCW were 12.02 wt% and 64.25%, respectively. The characterization analysis results of the products showed that Al–Si alloys with a uniform distribution of Si were successfully prepared. Furthermore, the mechanism of the alloying process was studied. Cryolite additives can efficiently dissolve the SiO2 film on the surface of Si particles and hence contribute to the subsequent transfer of the exposed Si to molten Al. The study of the slag-metal interface demonstrates that Si is recycled with a highly efficiency from SCW. Despite the relatively large amount of cryolite, the generated slag can be recycled and used as an additive for the next batch of experiments. This method not only recycles silicon scrap for use as valuable raw materials, but also reduces the production cost of Al–Si alloys, and thus, this method is a feasible way to commercially recycle SCW in an economically and environmentally friendly manner.

Effect of Sr–Grain Refiner–Si Interactions on the Microstructure Characteristics of Al–Si Hypereutectic Alloys

The present study is part of a larger study where the effectiveness of different types of grain refiners in strontium-modified hypoeutectic alloys was evaluated. Silicon was shown to affect the mechanism of nucleation of α-aluminum. To explore this phenomenon, an investigation was carried out on hypereutectic A390.1 alloy containing 17.3%Si to establish the role of primary silicon crystallization on subsequent α-aluminum and silicon eutectic reactions with different grain refiner additions. The alloy was modified with 0.02%Sr as before, and the same three-grain refiners: Al–10%Ti, Al–5%Ti–1%B, and Al–4%B master alloys, were used at controlled addition levels. The liquid metal was held at 750 °C for periods up to 120 min during which the molten metal was continuously stirred. The results show that addition of 0.1%Ti (using Al–10%Ti) to A390.1 alloy modified with 0.02%Sr results in reducing the initial grain size by about 20% only, due to transformation of Al3Ti phase to (Al,Si)2Ti phase which is a poor grain refiner. The stability of the (Al,Si)2Ti phase increases with the increase in the alloy Si content. The formation of (Al,Si)2Ti also reduces the undercooling caused by Sr addition, reducing the degree of morphology modification. Additionally, boron has a strong affinity to react with Sr, forming the compound SrB6, resulting in no grain refining or “poisoning,” regardless of the added amount of B or the holding time. Combined addition of Sr and B, however, improves the modification of the eutectic Si. In general, due to the high Si content of A390.1 alloy, it is difficult to achieve an appreciable degree of grain refining.

Effect of alloy composition and heat treatment on mechanical performance of 6xxx aluminum alloys

The effect of alloy composition and heat treatment, including natural ageing and pre-ageing, on the mechanical performance of eight 6xxx alloys designed with systematically varying Si, Mg and Cu contents was studied. The results show that not only the alloy composition and heat treatment before forming influence the formability, but also they have an effect on the paint bake response of the alloys. Increasing the alloy Si content, decreasing Mg/Si ratio and adding 0.3% Cu (mass fraction) were generally found to improve the tensile ductility and formability of the alloys studied, while pre-ageing was found to decrease these properties. A full property profile of these alloys in terms of strength, tensile ductility, work hardening, strain rate sensitivity, forming limit and paint bake response was presented.

Effects of Fe intermetallics on the machinability of heat-treated Al–(7–11)% Si alloys

The need for bridging the divide between the casting process and the machining process provides a strong motivation for examining the various aspects affecting the machinability of Al–Si casting alloys, given that these alloys constitute about 85% of all aluminum castings produced. Strontium-modified, grain-refined, T6 heat-treated 396 alloys (containing ∼11% Si), and B319.2 and A356.2 alloys (containing ∼7% Si) were selected, with a view to studying their machinability characteristics. Three 396 alloy compositions were selected (M1, M3, M4) such that different iron intermetallic phases were obtained in each case. Drilling and tapping operations were both carried out using a Makino A88E machine under fixed machining conditions. The machinability criteria relates to forces and moment analysis as well as to tool life, chip configuration, and built-up edge (BUE) evolution. The effects of Fe-intermetallics (α-Fe, β-Fe, and sludge) on the machining characteristics of these alloys were investigated and a comparison was made between the three 396 alloys in terms of mean total drilling forces, mean total tapping forces, tool life, and chip configuration. The results demonstrate that the presence of sludge has a significant effect on cutting forces and tool life. The tool life of the 396-M3 alloy (containing sludge) decreased by 50% compared to the base alloy 396-M1 (containing α-Fe intermetallics). Increasing the Fe-content from 0.5% to 1% in the M1 alloy (i.e., M4 alloy) produces a distinct improvement in the alloy machinability in terms of cutting forces and tool life. The addition of Fe and Mn appears to have no discernible effect on the built-up-edge (BUE) width and chip configuration as compared to the base alloy. The dominant type of wear which leads to drill failure and breakage is outer corner wear; there is, however, no evidence of crater-wear. Fan-shaped chips were obtained during machining of the 396, B319.2 and A356.2 alloys, where the latter alloy yielded the largest chips. As far as the alloy Si content was concerned, it was found that the 396 alloys produce drilling results similar to those of the B319.2 and A356.2 alloys, in terms of the number of holes drilled (cf. 2160 with 2100 and 2285 holes drilled in the B319.2, and A356.2 alloys, respectively). During tapping tests, however, the B319.2 alloy yielded the longest tool life, i.e., more than twice that of 396 alloy and four-and-half-times that of the A356.2 alloy.

Effects of Si content on effective magnetic anisotropy in Fe/sub 87-x/Cu/sub 1/Nb/sub 3/Si/sub x/B/sub 9/ nanocrystalline alloys

The effective magnetic anisotropy K/sub eff/ of Fe/sub 87-x/Cu/sub 1/Nb/sub 3/Si/sub x/B/sub 9/ nanocrystalline alloys was studied by means of the law of approach to saturation. The results show that the effective magnetic anisotropy K/sub eff/ decreases with the increasing Si content of the alloys. It results from a decrease in the magnetocrystalline anisotropy K/sub 1/ of the /spl alpha/-Fe(Si) phase in the alloys, and the variation of the effective magnetic anisotropy K/sub eff/ with the magnetocrystalline anisotropy K/sub 1/ is linear. The technical magnetic properties are improved by increasing the Si content of the nanocrystalline alloys.

Precipitation behavior of MnS during δ/γ transformation in Fe−Si alloys

The precipitation behavior of MnS after solidification was analyzed with low-carbon Fe−Si alloys. This system was chosen since it has a wide temperature range for the δ/γ transformation. Experimental results showed that the amount of MnS precipitates increased drastically between 1300°C and 1100°C, and MnS precipitates were segregated almost entirely in the δ phase. This result was interpreted quantitatively by a mathematical model, taking into account the diffusion and the redistribution of solute elements and also the solubility product limit of Mn and S in both phases. Mathematical analysis shows that the precipitation of MnS starts first in the γ phase, but its growth will be very limited because of slow diffusion of Mn in the γ phase. The effects of some factors such as cooling rate and Si content of alloys on the rate of precipitation were discussed, and the degree of contributions of diffusion of Mn and the redistribution of S were estimated.

Oxidation of iron-silicon alloys

The isothermal and cyclic oxidation behavior of Fe-Si alloys (5, 10, 14, and 20 wt.% Si) was studied between 900 and 1100°C. The oxidation rate in air decreased with increasing Si content at all temperatures. Alloys containing 10% Si or more oxidized slower than a typical Cr2O3-forming alloy due to the formation of an SiO2 film. This film may have initially been vitreous but was crystalline after short times. The oxidation kinetics, although slow, were linear due to the outward transport of Fe through the slowly growing SiO2 film. It is hypothesized that the Fe transport involved atoms rather than ions. Cyclic oxidation behavior varied with the alloy Si content.