Hypereutectic Aluminum-silicon Alloy Research Articles

Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools

Hypereutectic aluminium silicon alloys, e.g. casted AlSi17Cu4Mg, are commonly used in the automotive and aeronautical industries. These alloys consist of hard, abrasive silicon particles in a soft aluminium matrix and thus place high mechanical loads on the tool during machining processes. Polycrystalline Diamond or CVD (chemical vapour deposition) diamond based cutting tools can be used for the high speed machining of these alloys due to their high hardness and wear resistance. Diamond thin film coatings of different film morphologies are commonly applied on cemented carbide tools using Hot Filament CVD. The distinguishing characteristic to other coatings is utmost hardness resulting in high resistance to abrasion, low tendency to adhesion and low friction coefficient. The manufacturing of CVD diamond coated shaft type cutting tools is challenging due to the complex design of the cutting edges and the demanding stress behaviour during tool application. The influencing parameters of substrate type, chemical and mechanical substrate pre-treatment as well as diamond film modification on the tool cutting performance are discussed. The manufacturing route of CVD diamond coated thread milling drills is analysed with the use of material and tribological tests. The complex thread manufacturing tools are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.

Deformation behavior of spray-formed hypereutectic Al–Si alloys

The deformation behavior of spray-formed hypereutectic aluminum–silicon alloys—AlSix (x = 18, 25, and 35 wt%)—has been studied by means of compression test at various temperatures and strain rates. The flow stress of the spray-formed Al–Si alloys increases with decreasing compression temperature and increasing strain rate. Higher silicon content in the alloys also leads to higher flow stress during deformation. The flow curves determined from the compression tests exhibit that the deformation of the materials is controlled by two competing mechanisms: strain hardening, and flow softening. Particle damage during the deformation may have an influence on the flow curves of the alloys with large silicon particles. Based on the flow curves obtained from the compression tests and knowledge of aluminum extrusion, the spray-formed hypereutectic Al–Si alloy billets have been hot extruded into wires with a high area reduction ratio around 189. Since primary silicon particles were greatly refined and uniformly distributed in the spray-formed materials, the heavy deformations of the spray-formed Al–Si alloys containing high amount of silicon were successfully performed.

Optimal holding temperatures and phosphorus additions for primary silicon refinement in Al–high Si alloys

The refinement of primary silicon particles in hypereutectic aluminium–silicon alloys with exceptionally high Si contents (Al–high Si alloys) has been studied using optical microscopy, scanning electron microscopy and electron probe microanalysis. A new Si–20P master alloy has been developed which was used to study the optimal melt treating technology for achieving a satisfactory refinement of primary silicon. The experimental results show that the optimal superheating temperature and the amount of phosphorus addition increase abruptly when silicon content reaches about 50 wt-%. A few of experiential formulae are presented for the optimal superheating temperature and the amount of phosphorus addition. The refinement mechanism of the primary silicon is also discussed.

Metallurgical assessment of a hypereutectic aluminum–silicon P/M alloy

The objective of this study was to perform a comprehensive assessment on the press-and-sinter P/M processing of a novel hypereutectic aluminum–silicon alloy known as Alumix-231 (Al–15Si–2.5Cu–0.5Mg). As this patented commercial product is relatively new, the amount of scientific data available in the literature is highly limited. To address this issue sintered products were evaluated by analyzing the density before and after sintering, apparent hardness, microstructural features, tensile properties, and finally the response to heat treatment. It was determined that the optimum processing route for Alumix-231 involved compaction at a pressure of 600 MPa followed by de-lubrication at 400 °C for 20 min before being sintered at 560 °C for a period of 60 min. The optimum heat treatment involved solutionizing the samples at 520 °C for 1 h, followed by water quenching, and artificially aging the samples at 160 °C for 8 h. The system proved highly responsive to this manner of processing attaining a sintered density on the order of 98% of theoretical and a UTS of 330 MPa. Based on a combination of DSC, XRD, and EPMA analyses it was concluded that θ-type phases were the dominate precipitates formed during heat treatment.

Surface interactions and tribochemistry in boundary lubrication of hypereutectic aluminium—silicon alloys

The tribofilms formed on a hypereutectic Al—Si alloy after sliding against AISI 52100 steel pins in a reciprocating pin-on-plate tribometer with polyalpha olefin (PAO) base oil containing zinc dialkyldithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives have been studied. Surface-sensitive analytical techniques (X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), Raman spectroscopy, and nanoindentation) have been used for tribofilm chemical and mechanical characterization. The thickness of the tribofilms has been evaluated using depth profiling and focus ion beam (FIB) milling. This is the only reported work to analyse this kind of tribofilm on Al—Si alloys in this detail. From the analysis obtained from miniSIMS, differences in the composition of the top layer of the tribofilms formed from ZDDP and ZDDP+MoDTC lubricants have been observed. When using only ZDDP, this top layer is rich in Zn and Fe; with the addition of MoDTC the top layer contains phosphates, probably from the decomposition of the ZDDP, and MoS2. The effect of adding MoDTC can also be observed in the overall tribofilm thickness and coverage of the samples.

Effect of DH treatment on microstructure and mechanical properties of an LM28–0.3wt.% Nd hypereutectic aluminum silicon alloy

In this article, a novel heat treatment method, deformation heat (DH) treatment (extrusion + aging), was employed on a hypereutectic aluminum silicon alloy (LM28–0.3wt.%Nd), and its effect on microstructure and mechanical properties of this alloy was investigated. It can be concluded that the eutectic microstructure disappears after the DH treatment, in the meantime the morphology of α(Al) solid solution was transformed from fir-tree and cellular to equiaxed, Si was refined with grain sizes of about 1–10 μm and distributed dispersedly, even some of them became globular shape, which can reduce stress accumulation effectively. Mechanical properties test indicated that the DH treatment could significantly improve the room-temperature strength and have little effect on the high-temperature strength of LM28–0.3wt.%Nd alloy. The fracture mechanism is ductile fracture by fracture morphology analysis.

Analytical Method for the Inverse Determination of Interfacial Wear Temperatures

Frictionally generated interfacial temperatures during a pin-on-ring wear test were inversely determined using a single thermocouple placed near the wear surface of the pin. Once captured by the thermocouple, the transient temperature history was used with a finite-element-based solution for a unit response and a least-squares fitting of the data to a generalized equation to determine the interfacial temperatures (boundary condition). In order to verify the results, the predicted boundary condition was then applied to the same finite-element model and solved directly with the numerical predictions for the thermocouple location showing excellent agreement with the experimental data. To avoid excessive wear and therefore maintain linearity of the problem, a wear resistant hypereutectic aluminum-silicon alloy (B390) was used as the pin on a hardened steel disk. Based on specific wear-test conditions, it was found that the interfacial temperatures asymptotically approached a maximum of 140°C in approximately 15 s with 80-90% of the temperature achieved in approximately 5 s.

In-Situ Si/Al Composite Produced by Semisolid Metal Processing

Hypereutectic aluminium-silicon alloy can be considered as an in-situ composite with primary silicon acting as the reinforcing phase. In this paper, we use semisolid metal processing technique to produce such in-situ Si/Al composite. Hypereutectic Al–Si alloy, AlSi17Cu5Mg0.55, was cast into a permanent mould with controlled solidification conditions. In the resulted composite, primary Si particles were observed to co-exist with primary α-Al particles, uniformly dispersing in Al–Si eutectic matrix. The structure of the Si particle can be controlled by AlP refinement and thermal refinement, and the α-Al particle can be controlled by semisolid partial remelting.

Refinement of primary Si in hypereutectic Al–Si alloy by electromagnetic stirring

Abstract The effect of electromagnetic stirring on the morphology of primary Si particles in a hypereutectic aluminum–silicon alloy was investigated in the semi-solid temperature region. It was found that with the stirring current decreasing from 32 to 16 A, the primary Si particles congregated severely, formed large primary Si particles. When the current was less than 12 A, however, the particle distribution became uniform and there was no any particle congregation. The primary Si particles reached a minimum equivalent diameter of 44.8 μm when the stirring current was 8 A. Nevertheless, when the stirring current was less than 8 A, the primary Si particles coarsened again gradually. The congregating of primary Si particles is attributed to the presence of centrifugal force field caused by electromagnetic stirring, which forced the Si particles to move in radial direction, distribute in grade and congregate. The diameter of the Si particles is decided by the cooperation of the congregation effect and refining effect of electromagnetic stirring. The present research shows that a relatively weak electromagnetic stirring is more effective in refining the primary Si particles than a strong electromagnetic stirring.

The Dry Sliding Behavior of Al2O3 Transformed, Hypereutectic, 2xxx, and 7xxx, Aluminum Alloys Under Simulated Wire–Rope Induced Wear

The dry sliding wear behavior of various 2xxx and 7xxx aluminum alloys along with an aluminum alloy with the surface transformed to alumina (Al 2 O 3 ) and a hypereutectic aluminum silicon alloy (B390) were evaluated with a pin-on-disk wear apparatus to simulate wire-rope against sheave wheel interactions for a unique Naval application. Simulation of a steel cable on an aluminum sheave wheel was accomplished by using a 387 steel (RC 58) disk containing 45° cuts to mimic individual strands of the wire-rope. Using system dictated conditions, the ring was rotated at 1500 RPM to obtain a sliding velocity of 9.42 m/s, with the contact pressure of the candidate aluminum pins adjusted to maintain a steady 0.69 MPa (100 psi). Under the prescribed conditions, the transformed alumina exhibited a superior wear resistance when compared to the 2xxx, 7xxx, and B390 alloys. However, the brittle material did fracture on one occasion, thereby indicating the potential for deleterious abrasive debris. Wear mode analyses of the various materials using optical and electron microscopy revealed wear mechanisms that included adhesion, abrasion, and fracture. Final manuscript approved May 22, 2006 Review led by Emmanuel Ezugwu

Effect of solidification cooling rate and phosphorus inoculation on number per unit volume of primary silicon particles in hypereutectic aluminium—silicon alloys

Collected data on derived number per unit volume $$\bar N_V$$ of primary silicon particles in hypereutectic Al-Si alloys show a power relationship with solidification cooling rate $$\dot T$$ of the form $$\bar N_V=A\dot T^n$$ where typically $$n \sim 1$$ and $$A \simeq 130$$ mm-3 (K/s)-1 in the absence of phosphorus and $$A \simeq 720$$ mm-3 (K/s)-1 in its prescence. Significantly lower apparent values of $$\bar N_V$$ from one set of results appear to stem from measurement of a mean long dimension rather than diameter of particle sections as well as lower measured undercoolings than in Bridgman experiments at similar $$\dot T$$ .

Inhibited coarsening of a spray-formed and extruded hypereutectic aluminum-silicon alloy in the semisolid state

The microstructural evolution of a spray-formed and extruded hypereutectic aluminum-30 pct silicon-5 pct copper-2 pct magnesium alloy heated into the semisolid state has been investigated. Liquid is formed initially by a quaternary eutectic reaction and then by a ternary melt reaction. These reactions occur relatively quickly. However, the binary Al-Si eutectic melt reaction takes a significant time—around several hours depending on the temperature. The coarsening rate constants (K) for the growth of the silicon particles are approximately three to four orders of magnitude lower than those for the majority of metal spray-formed alloys. This may be associated with difficulties in addition or removal of atoms from the low index silicon facets. Where growth does occur, agglomeration of silicon particles may play a large role, especially at higher liquid contents. Electron backscatter diffraction (EBSD) gives evidence of agglomeration, and furthermore shows that high-angle silicon-silicon boundaries are not wetted with liquid.

Hydrogen content and porosity behavior of hypereutectic aluminum-silicon alloy with phosphorus

By making castings that pick up gas from moisture in red sand molds, the porosity generated at different cooling rates was discussed during solidification of hypereutectic Al−25% Si alloy without and with phosphorus additions. The effect of phosphorus addition on hydrogen content in the melt was also studied. It was observed that the phosphorus addition made hydrogen content in alloy melts present a “see-saw” tendency. In addition to primary silicon refinement, the phosphorus promoted gas porosity formed not only in slowly cooled sections, but also in rapidly cooled sections. There was a small difference in density of full dense sample between P-refined and unrefined castings, with a larger density associated with phosphorous addition. The change of the surface tension seemed more reasonable to explain the mechanism of porosity behavior.

Sliding wear and friction behaviour of Al–18% Si–0.5% Mg alloy

In present paper, the influence of copper on wear-friction behaviour of hypereutectic Al–Si alloy (Al–18% Si–0.5% Mg) has been investigated. Sliding tests were conducted under dry sliding conditions against hardened steel En-31 counter surface over a range of sliding speed from 2.0 to 7.0 m/s and contact load from 10 to 80 N. It was observed that the wear rate is a function of contact load, sliding speed, composition and thermal softening characteristics of sliding metal. Wear of hypereutectic aluminium–silicon alloy is not appreciably affected with the addition of copper. However, addition of copper increases the transition load at 2.0 m/s sliding speed. Copper addition more than 2%, did not show any effect on the transition load.

Solidification mechanisms of unmodified and strontium-modified hypereutectic aluminium–silicon alloys

The effects of strontium on the solidification mode of hypereutectic aluminium–silicon alloys have been studied. Samples were prepared from an aluminium–17 wt% silicon-based alloy and strontium was added at several different concentrations. The development of the microstructure was investigated by cooling curve analysis, interrupted solidification experiments and optical and scanning electron microscopy. It was found that nucleation of primary silicon is suppressed by additions of strontium. The suppressed nucleation results in supersaturation of the liquid prior to nucleation, and an increased growth rate after nucleation. As a result, the silicon crystals become less faceted and more dendritic with increasing strontium additions. Increasing the strontium concentration slightly refined the eutectic spacing and introduced a small amount of fibrous silicon. Electron back-scatter diffraction measurements were performed to determine the crystallographic relation between the primary and eutectic silicon phases. The eutectic silicon in the unmodified alloy does not have any crystallographic relationship with the primary silicon crystals. In contrast, the eutectic silicon crystals in the strontium-modified alloys often share an identical or twin relationship with nearby primary silicon crystals. The incidence of twinning within primary silicon crystals was relatively low and did not appear to increase with strontium additions.

Experimental simulation of impact and sliding wear in the top piston ring groove of a gasoline engine

The influence of normal impact and sliding on wear and surface damage at the lubricated interface between the top piston ring and the piston was investigated using a specially adapted tribometer. Standard production components were tested, the piston rings being nitrocarburized steel with a production lapped finish and the pistons a hypereutectic aluminium-silicon alloy, either tin-plated or hard anodized. Experiments were undertaken at loads, velocities and temperatures comparable with a modern automotive, four-stroke, gasoline engine, either dry or lubricated with a fully formulated SAE 10W30 engine oil. Lubricant starvation was found to be the most critical operational parameter leading to surface damage and wear. An important phenomenon observed was the tendency of piston material to transfer and adhere to the ring surface, so-called ‘microwelding’. Anodizing was shown to be a very effective treatment to resist microwelding, although it is regarded as an expensive solution by the automotive industry. The experimental observations are consistent with fatigue wear of the piston groove, followed by material transfer to the piston ring flank and accelerated wear during sliding.

Dispersion strengthening in a hypereutectic Al-Si alloy prepared by extrusion of rapidly solidified powder

When a hypereutectic aluminum-silicon alloy containing 16 wt pct silicon was rapidly solidified into powder using the spinning water atomization process, the individual powder grains were predominantly aluminum that was supersaturated with silicon and also contained well-dispersed 0.02-µm silicon particles. Although the silicon particles grew when the powder was extruded into a bar at temperatures from 673 to 803 K at an extrusion ratio of 4.3 and an extrusion speed of 0.9 mm/s, the average diameter was maintained on a submicron level. When the extrusion temperature was decreased from 803 to 673 K, the average diameter of the silicon particles in the extruded bar decreased from 0.8 to 0.5 µm, while the Vickers hardness (HV) and the ultimate tensile strength of the extruded bar increased from 120 to 160 (HV) and from 330 to 500 MPa, respectively. Both the hardness and the tensile strength of the extruded bars were several times higher than those of conventionally cast bars of the same alloy with cooling rates from 10−1 to 102 K/s. On the other hand, the elongation decreased from 5.5 to 3.1 pct when the extrusion temperature was decreased from 803 to 673 K.

A high-fe aluminum matrix welding filler metal for hardfacing aluminum-silicon alloys

A high-Fe containing aluminum matrix filler metal for hardfacing aluminum-silicon alloys has been developed by using iron, nickel, and silicon as the major strengthening elements, and by measuring mechanical properties, room temperature and high temperature wear tests, and microstructural analysis. The filler metal, which contains 3.0%–5.0% Fe and 11.0%–13.0% Si, exhibits an excellent weldobility. The as-cast and as-welded microstructures for the filler metal are of uniformly distribution and its dispersed network of hard phase is enriched with Al-Si-Fe-Ni. The filler metal shows high mechanical properties and wear resistance at both room temperature and high temperatures. The deposited metal has a better resistance to impact wear at 220°C than that of substrate Al-Si-Mg-Cu piston alloy; at room temperature, the deposited metal has an equivalent resistance to slide wear with lubrication as that of a hyper-eutectic aluminum-silicon alloy with 27% Si and 1% Ni.

Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy

Hypereutectic aluminium–silicon alloys offer the possibility of an in situ natural composite (the silicon acting as the reinforcing phase) with properties that make them attractive for a number of automotive applications. However, conventional casting techniques result in excessive growth of the silicon particles in the melt, which adversely affect the mechanical properties. Thixoforming allows hypereutectic Al–Si alloys containing 40–50% fraction liquid to be shaped into complex near net shape components, whilst keeping the silicon particle size quite fine. This paper describes the development of a series of hypereutectic alloys based on the A390 composition (17%Si, 5%Cu, 0.5%Mg), their thixoforming, their resulting microstructures and mechanical properties. Finally the thixoforming of an automotive component using an A390 alloy is also described.

Evaluation of the Reciprocating-Wear Behavior of Unlubricated Hypereutectic Al-Si Alloys

The reduction of abrasive and scuffing wear between aluminum and cast-iron wear-pairs is an important goal for the automotive industry given the implications for improved engine performance and reliability. Hypereutectic aluminum-silicon alloys such as B390 may help lead the way towards this goal because of their potential for wear resistance and durability. Yet, despite this potential, B390 has not been evaluated under the severe reciprocating conditions typical of automotive applications. Moreover, the influence of various manufacturing and processing steps on the resulting wear resistance of the alloy has not been studied at all. To help fill this void, a series of unlubricated tests were conducted using cast, spray-formed, spray-formed then extruded, and semi-solid formed variations of B390 reciprocated against gray cast-iron under a constant contact-stress. Originally, the weight-loss per reciprocating distance was measured and converted to volume-loss to determine the steady-state wear rates for each alloy variation. However, it was determined that debris from the mating cast-iron surface was adhering to the B390 and obscuring the actual material lost to wear. To compensate for the trapped iron debris, the volume-loss was directly calculated from the changing contact area or “flat” originally measured for the loading adjustments. After this correction, the data indicated that the spray-formed, spray-formed then extruded, and semi-solid formed all experienced measurable, albeit modest decreases in their wear rates relative to the cast B390. However, there were not any significant disparities in the observed wear rates between the spray-formed, spray-formed then extruded, and semi-solid formed hypereutectic alloy despite their processing and microstructural differences.