The effects of mischmetal, cooling rate and heat treatment on the eutectic Si particle characteristics of A319.1, A356.2 and A413.1 Al–Si casting alloys

The effects of mischmetal, cooling rate and heat treatment on the eutectic Si particle characteristics of A319.1, A356.2 and A413.1 Al–Si casting alloys were investigated and recorded for this study. Mischmetal was added to the alloys in the form of Al–20% mischmetal master alloy to produce four levels of mischmetal addition (0, 2, 4 and 6 wt%). The alloys were also modified with strontium (250 ppm) to study the combined modification effect of Sr and mischmetal at both high and low cooling rates corresponding to dendrite arm spacings of 40 and 120 μm, respectively. The alloys were subjected to solution heat treatment (495 °C/8 h for A319.1 and A413.1 alloys, and 540 °C/8 h for A356.2 alloy) to investigate its effect on the eutectic Si particle morphology.

An optical microscope-image analyzer system was used to measure the characteristics of eutectic Si particles such as area, length, roundness ratio and aspect ratio, in order to monitor the modifying effect of mischmetal, as well as the combined modification effect of mischmetal and Sr. For each alloy sample examined, the Si particle characteristics were measured over an area of 50 fields and the average particle characteristics were thus determined.

The eutectic Si particle measurements revealed that partial modification was obtained with the addition of mischmetal while full modification was achieved with the addition of Sr in the as-cast condition, at both high and low cooling rates. The interaction between Sr and mischmetal was observed to weaken the effectiveness of Sr as a Si particle-modifying agent. This effect was particularly evident at the low cooling rate.

During solution heat treatment, the eutectic Si particles in the non-modified alloys underwent rapid coarsening, otherwise known as Ostwald ripening, whereas those in the Sr-modified alloys exhibited a high spheroidization rate. The coarsening was evidenced by an increase in the thickness of the Si particles, clearly observed in the A356.2 alloy at both cooling rates. In the alloys containing mischmetal, the presence of this mixture of rare earth elements reduced the coarsening of the Si particles slightly.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.     

Influence of aging treatments and alloying additives on the hardness of Al–11Si–2.5Cu–Mg alloys

This study investigated the effects of cooling rate, heat treatment as well as additions of Mn and Sr on hardness and hardening characteristics in Al–11Si–2.5Cu–Mg alloys. The results of scanning electron microscopy reveal that the age-hardening behaviour is related to the precipitation sequence of alloy. An energy dispersive spectroscopy analysis was used to identify the precipitated phases. The results also show that the hardness of the solution heat-treated samples is higher in air-cooled alloys than in furnace-cooled ones. Furthermore, the hardness observed in solution heat-treated samples is higher than in as-cast samples for air-cooled alloys, with the highest hardness level in the non-modified alloys. The highest hardness levels among the artificially aged samples were observed in the non-modified, air-cooled alloys. These levels occur after aging for longer times at lower temperatures (e.g. 30 h at 155 °C). The alloys studied did not display any softening after 44 h at 155 °C, whereas at 180 °C, softening was noted to occur after 10–15 h. At short aging times of 5–10 h, high hardness values may be obtained by aging at 180 °C. At aging temperatures of 200 °C, 220 °C and 240 °C, softening began after 2 h had elapsed. The cooling rate during solidification does not appear to have any significant effect on the precipitation characteristics and hardness of the Sr-modified alloys at certain aging temperatures. On the other hand, the effects of cooling rate may be clearly observed in the non-modified alloys. Manganese has a minimal effect on the hardness of the aged samples as it diminishes the potential action of age-hardening, while strontium lessens the hardness of the artificially aged samples. The effect of strontium, however, is more pronounced in the air-cooled alloys than in the furnace-cooled alloys. Strontium also has a noticeable effect on the reduction of hardness in aged Mg-containing Al–Si–Cu alloys, in that it affects the precipitates containing Cu and Mg.     

Effect of solution heat treatment and additives on the hardness, tensile properties and fracture behaviour of Al-Si (A413.1) automotive alloys

A study was carried out to determine the role of Mg, Cu, Be, Ag, Ni, and Zn additives during the solution heat treatment of grain refined, Sr-modified eutectic A413.1 (Al-11.7% Si) alloy, and their consequent effect on mechanical properties. For comparison purposes, some of the alloys were also studied in the non-modified condition. The alloys were cast in the form of test bars using a steel permanent mold preheated at 425°C that provided a microstructure with an average dendrite arm spacing (DAS) of 22 m. The test bars were solution heat treated at 500 ± 2°C for times up to 24 h, followed by artificial aging at 155°C for 5 h (T6 treatment). Tensile and hardness tests were carried out on the heat-treated test bars. Details of the microstructural evaluation are reported in a previous article [1].With respect to the mechanical properties, it is found that the hardness and strength (YS, UTS) of Mg-containing alloys decrease with the addition of Sr due to the sluggish dissolution of the Al₅Cu₂Mg₈Si₆ phase during solution treatment, and a delay in the precipitation of Mg₂Si or Al₂MgCu phases during artificial aging thereafter. The properties of the Cu-containing alloys, however, remain unaffected by the addition of Sr. With the exception of Ni, all alloying elements used improve hardness and strength, particularly after heat treatment. In the case of Ni, addition of up to 1.41% Ni is observed to decrease the mechanical properties in the T6 condition.Fracture of non-modified alloys takes place through crack initiation within the brittle acicular Si particles without the crack passing through the ductile Al matrix. In the Sr-modified alloys, the fracture is of ductile type, as evidenced by the pinpoint nature of the -Al dendrites on the fracture surface. The number of cracked Si particles and intermetallics beneath the fracture surface increases in proportion to the increase in alloy strength.     

Effects of Sr-modification,iron-based intermetallics and aging treatment on the impact toughness of 356 Al–Si–Mg alloy

Impact toughness as a property has been acquiring increased importance in recent years, since data regarding this property can provide a means for assessing alloy ductility under high rates of deformation. The main objective of this study is to investigate the effects of Sr-modification, Fe-based intermetallic phases and aging conditions on the impact toughness of widely used 356 alloys. The total absorbed energy was measured using a computer-aided instrumented Instron Charpy impact testing machine. Increasing the level of iron additions decreases the impact energy values of 356 alloys to a noticeable degree (~35–57%). The addition of 0.1 wt% Mn to non-modified 356 alloys seems to have no observable effect on the impact energy, while increasing the Mn addition to 0.4 wt% produces a slight improvement in the impact energy values for non-modified and Sr-modified 356 alloys compared to that of those containing only iron under the same conditions. The application of solution heat treatment in combination with Sr-modification was found to significantly improve the impact energy of as-cast 356 alloys, particularly at low iron additions. Artificial aging of non-modified and Sr-modified 356 alloys at 180 °C diminishes the impact energy values with an increase in the aging time up to 8 h compared to those obtained under the solution heat-treated conditions. On the other hand, aging at 220 °C for 12 h increases the impact energy values of Sr-modified 356 alloy containing 0.12 wt% Fe and combined 0.2 wt% Fe–0.1 wt% Mn to about 20 and 18 J, respectively. The fracture behavior of non-modified 356 alloys containing 0.12 wt% Fe is mainly controlled by the acicular eutectic Si particles whereas β-iron platelets act as crack initiation sites and provide further path for final crack propagation in non-modified 356 alloys containing 0.9 wt% Fe. The β-iron platelets and π-iron phase particles contribute largely to crack initiation and propagation in the Sr-modified 356 alloys containing 0.9 wt% Fe.     

First investigations on twin-rolled Zr59Cu20Al10Ni8Ti3 bulk amorphous alloy by mechanical spectroscopy

Zr₅₉Cu₂₀Al₁₀Ni₈Ti₃ is one among compositions of ZrCu-based alloys giving bulk amorphous material by cooling from the melt. Twin-roll casting enabling samples suitable for our inverted torsion pendulum has been processed in strips of about 0.60 mm thick.

Low temperature IF measurements have been conducted on a specimen from room temperature to −120 °C at different heating and cooling rates. IF spectra exhibit peaks at around −40 °C (cooling) and −10 °C (heating) which are sensitive to heating rates and to the number of cycle (heating and cooling). DSC measurements have also been performed to help interpret the phenomena linked to the IF peaks.     

Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy

Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of magnesium matrix and Mg₁₂Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO₁₉ structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.     

Effect of microstructure on the mechanical properties and fracture of commercial hypoeutectic Al-Si alloy modified with Na, Sb and Sr

The microstructure, hardness, tensile properties and fracture have been studied for the non-modified and modified aluminium (Al) silicon (Si) commercial hypoeutectic alloy. Three modifiers were used being sodium (Na), antimony (Sb) and strontium (Sr). The Sb-modified structure revealed small plate-like Si morphology. The Na and Sr-modified structures exhibited fibrous Si. A slight increase in the hardness values (HV) due to modification was observed. A general increase in the tensile properties was observed due to modification. The tensile properties of the sand mould Sr-modified alloy were significantly higher than those of the Na-modified alloy by 12.7% in proof stress, 16.3% in ductility and 33.3% in toughness. For the metal mould ingots the increase in tensile properties of Sr-modified alloy were respectively: 16.7%, 32.5% and 41.7% compared to a Na-modified alloy. Optical fractography on longitudinal sections near the fracture surfaces of the modified alloys revealed that the crack propagates in the eutectic thus, circumventing the Al-dendrites. The dimple and smooth ripple patterns observed by scanning electron microscope (SEM) on the fracture surface of the Na and Sr-modified alloys suggest a transgranular type of fracture across the grains of the eutectic matrix.     

Impact toughness and fractography of Al–Si–Cu–Mg base alloys

Critical automotive applications using heat-treatable alloys are designed for high impact toughness which can be improved using a specified heat treatment. The alloy toughness and fracture behavior are influenced by the alloy composition and the solidification conditions applied. The mechanical properties of alloys containing Cu and Mg can also be enhanced through heat treatment. The present study was undertaken to investigate the effects of Mg content, aging and cooling rate on the impact toughness and fractography of both non-modified and Sr-modified Al–Si–Cu–Mg base alloys. Castings were prepared from both experimental and industrial 319 alloy melts containing 0–0.6wt% Mg. Test bars were cast in two different cooling rate molds, a star-like permanent mold and an L-shaped permanent mold, with dendrite arm spacing (DAS) values of 24 and 50 μm, respectively. Test bars were aged at 180 °C and 220 °C for 2–48 h. Charpy Impact test was used to provide the impact energy. It was observed that high cooling rates improve the impact toughness whereas the presence of Cu significantly lowers the impact properties which are determined mainly by the Al₂Cu phase and not by the eutectic Si particles. The addition of Mg and Sr were also seen to decrease the impact toughness. The crack initiation energy in these alloys is greater than the crack propagation energy, reflecting the high ductility of Al–Si–Cu–Mg base alloys.     

Aging behaviour of Al–Cu–Mg alloy–SiC composites

The age-hardening kinetics of powder metallurgy processed Al–Cu–Mg alloy and composites with 5, 15 or 25 vol.% SiC reinforcements, subjected to solution treatment at 495 °C for 0.5 h or at 504 °C for 4 h followed by aging at 191 °C, have been studied. The Al–SiC interfaces in composites show undissolved, coarse intermetallic precipitates rich in Cu, Fe, and Mg, with its extent varying with processing conditions. Examination of aging kinetics indicates that the peak-age hardness values are higher, and the time taken for peak aging is an hour longer on solutionizing at 504 °C for 4 h, due to greater solute dissolution. Contrary to the accepted view, the composites have taken longer time to peak-age than the alloy, probably due to lower vacancy concentration, large-scale interfacial segregation of alloying elements, and inadequate density of dislocations in matrix. The composite with 5 vol.% SiC with the lowest inter-particle spacing has shown the highest hardness.     

Fracture behavior and mechanical properties of Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) alloy at room temperature

The microstructure, mechanical properties and fracture behavior of gravity die cast Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) (WNG421) alloy are studied at room temperature in different thermal conditions, including as-cast, solution-treated and different aging-treated (both isothermal and two-step aging) conditions. The results indicate that WNG421 alloy shows different behaviors of crack initiation and propagation in different thermal conditions during tensile test at room temperature. After pre-aged at 200 °C for 5 h, the hardness of WNG421 alloy first reduces and then increases when secondary aged at 250 °C (two-step aging). The peak hardness and corresponding tensile strength of the two-step aged alloy both increases compared with those in 250 °C isothermal peak-aged condition. Tensile strength of WNG421 alloy at room temperature in low temperature (200 °C) isothermal peak-aged condition is much higher than that in high temperature (250 °C) isothermal peak-aged condition.     

Influence of fluidized sand bed heat treatment on the performance of Al–Si cast alloys

The present study was undertaken to arrive at a better understanding of the effects of solution heat-treatment time and melt treatment on the microstructure and tensile properties of T6-tempered A356.2 and B319.2 cast alloys heat treated using a fluidized sand bed furnace (FB) as opposed to a conventional convection furnace (CF). The alloys investigated were subjected to solution heat treatment at 530 °C and 495 °C, respectively, for times ranging from 0.5 to 24 h, warm water quenching, and conventional T6 aging at 155 °C and 180 °C, respectively, for times of 0.5, 1, 5, 8 and 12 h. The results revealed that the tensile strength of these alloys is more responsive to an FB heat treatment than to a CF treatment for solution treatment times of up to 8 h. A significant increase in strength is observed in the FB heat-treated samples after short aging times of 0.5 h and 1 h, the trend continuing up to 5 h. Analysis of the tensile properties in terms of quality index charts showed that both modified and non-modified 319 and 356 alloys display the same quality, or better, after only 2 h treatment in a fluidized bed compared to 10 h using a CF treatment. The 319 alloys show signs of overaging after 8 h of aging using a CF, whereas with an FB, overaging occurs only after 12 h. The Si particle characteristics of the alloys investigated show that the smallest particle size is obtained after solution heat treatment using a fluidized sand bed, the optimum solution heat-treatment time being 0.5 h for modified alloys, and up to 5 h for non-modified alloys.     

The characteristics of serrated flow in superalloy IN738LC

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10⁻⁵ to 8.77 × 10⁻³) s⁻¹. Activation energy for this process was calculated to be 0.69–0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy.     

Effect of Erodent Particle Hardness on Velocity Exponent in Erosion of Steels and Cast Irons

This article reports the effect of hardness of erodent particles on velocity exponent of some weld deposited alloys. Three steels and two alloy cast irons were selected for the present investigation. The bulk hardness of the alloys was in the range of 300 to 800 HV, whereas erodnet particles were having hardness in the range of 400 to 1875 HV. Erosion tests were conducted with 125-150 µm cement clinker, 125-150 µm blast furnace sinter, 100-150 µm silica sand, and 125-150 µm alumina particles and at impingement angles of 30° and 90° and with impingement velocities in the range of 25 to 120 m sec^-1. The erosion rate showed power-law dependence on impingement velocity, E = kVⁿ, where kis a constant and nis the velocity exponent. The velocity exponents obtained in the present work were in the range of 1.91 to 2.52. The velocity exponent showed an increasing trend with increasing hardness of the alloys irrespective of the hardness of the erodent particles and the impingement angle. The velocity exponent increased with increasing hardness of erodent particles, irrespective of the impingement angle and hardness of the alloys. The velocity exponents obtained in the present work were rationalized with respect to erodent particle properties, material properties and erosion mechanisms.     

The effects of trace Sc and Zr on microstructure and internal friction of Zn–Al eutectoid alloy

The damping properties of Zn–22 wt.% Al alloys without and with Sc (0.55 wt.%) and Zr (0.26 wt.%) were investigated. The internal friction of the determined by the microstructure has been measured in terms of logarithmic decrement (δ) using a low frequency inverted torsion pendulum over the temperature region of 10–230 °C. An internal friction peak was separately observed at about 218 °C in the Zn–Al alloy and at about 195 °C in Zn–Al–Sc–Zr alloy. The shift of the δ peak was found to be directly attributed to the precipitation of Al₃(Sc, Zr) phases from the alloy matrix. We consider that the both internal friction peak in the alloy originates from grain boundary (GB) relaxation, but the grain boundary relaxation can also be affected by Al–Sc–Zr intermetallics at the grain boundaries, which will impede grain boundary sliding. In addition, Al–Sc–Zr intermetallics at the grain boundaries can pin grain boundaries, and inhibit the growth of grains in aging, which increases the damping stability of Zn–22 wt.% Al alloy.     

Pre-ageing of AlSiCuMg alloys in relation to structure and mechanical properties

The effect of magnesium addition to the AlSi9Cu3.5 alloy on the hardening and precipitates morphology during ageing at RT, 160°C or two stage ageing (TSA) was studied using TEM and XSAS methods. It was found that only alloys with Mg addition harden during RT ageing and they also attain the highest hardness maximum at 160°C or during TSA. Two types of precipitates (starting from 0.4 and 1.2 nm) were identified during ageing at RT using XSAS method. They cause streaks in the electron diffraction patterns. In alloys aged at 160°C with Mg addition the S′ phase was identified using lattice imaging technique in addition to the θ′ plates formed during ageing of the ternary AlSiCu alloy.     

Continuous cooling transformation behavior of Alloy 718

The transformation behavior of Alloy 718 is affected significantly by the cooling rate. The γ″-phase appears at cooling rates less than 20 °C/min and δ-phase appears at grain boundaries as well as the MC type carbides at cooling rates below 5 °C/min. The δ-phase nucleates and grows preferentially at grain boundaries, and less preferentially at the MC carbides. The size of the γ″ and δ-precipitates increases consistently with decreasing cooling rate for the given conditions. The hardness varies with the transformation behavior. A hardness peak was noticed for a cooling rate of 5 °C/min. The hardness peak corresponded to the maximum volume fraction of γ″ which in turn was strongly affected by the presence of the δ-phase.     

Effect of grain refining and Sr-modification interactions on the impact toughness of Al–Si–Mg cast alloys

The present work investigates the effects of various types of grain refiners on the impact properties of Sr-modified A356.2 alloys in both the as-cast and heated-treated conditions. The results showed that the addition of Ti and B greatly improves the alloy toughness, but only when the alloy was in a fully modified state; moreover, the right type of master alloy and addition levels must be used. The highest values of the total absorbed energy recorded for T6-tempered alloys were obtained using Al–5%Ti–1%B and Al–10%Ti master alloys in addition to 0.04%Ti. A significant deterioration in the impact properties is observed due to the Sr–B interaction (in some cases). The improvements in toughness may be attributed to the change in Si particle morphology as well as to the dissolution and fragmentation of a number of the intermetallics formed during the T6 temper.     

Comparative study of martensitic transformation behavior of NiAlMn and NiAlMnFe high temperature shape memory alloys

A comparative study of microstructure and martensitic transformation (MT) behavior of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ high temperature shape memory alloys (SMAs) has been performed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), optical microscopy, and micro-hardness testing. The MT temperature (MTT) of Ni₅₉Al₁₁Mn₃₀ alloy is higher than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy, and both alloys’ MTT increases with increasing annealing temperature. The temperature hysteresis and hardness of Ni₅₉Al₁₁Mn₃₀ alloy are smaller than that of Ni₆₀Al₁₉Mn₁₆Fe₅ alloy. The MT behavior of Ni₆₀Al₁₉Mn₁₆Fe₅ is sensitive to aging temperature and its MTT and hysteresis decrease with increasing aging temperature. However, the MT behavior of Ni₅₉Al₁₁Mn₃₀ alloy is not sensitive to aging temperature. The MT stabilization effects appear in both alloys during thermal cycles. This stabilization effect vanishes from the second thermal cycle. The quenched microstructure of Ni₅₉Al₁₁Mn₃₀ and Ni₆₀Al₁₉Mn₁₆Fe₅ alloys is M plus gamma phase, in which the volume fraction of gamma phase is about 40 and 20%, respectively, and the microhardness of M is higher than that of gamma phase. No aging effects were found in both alloys after aging at 400 °C.     

A Superplastic Al-Li-Cu-Mg-Zr Powder Alloy with High Hardness and Modulus

Structure/property studies were made on an experimental Al-3.18% Li-4.29% Cu-1.17% Mg-0.18% Zr powder alloy, which is of the low density/high modulus type. Alloy powder was made by the P&W/GPD rapid solidification rate (RSR) process, canned, and extruded to bar. The density was 2.458 × 10⁶ g/m³. The material was solution-treated, and aged at 149°C (300°F), 171°C (340°F), and 193°C (380°F), using hardness tests to determine the aging curves. Testpieces solution-treated at 516°C (961°F) showed an average yield strength (0.2% offset) of 43.3 ksi (299 MPa) and ultimate tensile strength of 50.0 ksi (345 MPa), with 1% elongation, which increased to 73.0 ksi (503 MPa) and 73.1 ksi (504 MPa), respectively, with only 0.2% elongation, on peak aging at 193°C (380°F), with a modulus of elasticity of 11.4 × 10⁶ psi (78.3 GPa). Hardness values reached 90–92 R_B on aging at 149–193°C (300–380°F). The as-extruded alloy showed superplastic behavior at 400–500°C (752–932°F) with elongations of 80–185% on 25.6 mm, peaking at 450°C (842°F). An RSR Al-2.53% Li-2.82% Mn-0.02% Zr extruded alloy showed only 18–23% elongation at 400–500°C (752–932°F).