Al-Si Piston Alloy Research Articles

Dry sliding wear behaviour of hypereutectic Al–Si piston alloys containing iron-rich intermetallics

The effect of iron-rich intermetallics on the wear behaviour of Al–Si hypereutectic alloys has been studied. Dry sliding wear tests have been conducted using a pin-on-disk machine under different normal loads of 18, 51, 74 and 100 N and at a constant sliding speed of 0.3 m/s. The addition of 1.2 wt.% Fe to the LM28 alloy increased the wear rate due to the formation of needle beta intermetallics. Introducing 0.6 wt.% Mn to the iron-rich alloy changed the beta intermetallics into the modified alpha phases, and therefore reduced the detrimental effect of iron. TIG welding method as a surface melting process was applied on the iron and manganese containing alloy and led to a fine microstructure and increased the wear resistance.

Effect of Fe-rich intermetallics on the wear behavior of eutectic Al–Si piston alloy (LM13)

In the present study, the effect of Fe-rich intermetallics has been investigated on the wear behavior of eutectic Al–Si alloy (LM13). Dry sliding wear tests have been conducted using a pin-on-disk machine under different normal loads of 18, 51, 74 and 100 N at a constant sliding speed of 0.3 m/s. Addition of 1.2% Fe to the LM13 alloy leads to the formation of the flake like β-intermetallic compounds. These hard compounds initiate micro cracks and can reduce the wear resistance of the alloy. The addition of Mn converts the flake like β-intermetallic compounds to the star-like α-intermetallics and decreases the detrimental effect of iron. Applying high cooling rate during solidification of the alloy containing Fe and Mn, resulted to the formation of finer α-intermetallic compounds and improved the wear behavior of the alloy to a great extent.

The combined use of EBSD and EDX analyses for the identification of complex intermetallic phases in multicomponent Al–Si piston alloys

Multicomponent Al–Si based casting alloys are used for a variety of engineering applications, including for example, piston alloys. Properties include good castability, high strength, light weight, good wear resistance and low thermal expansion. In order for such alloys to continue operation to increasingly higher temperatures, alloy element modifications are continually being made to further enhance the properties. Improved mechanical and physical properties are strongly dependent upon the morphologies, type and distribution of the second phases, which are in turn a function of alloy composition and cooling rate. The presence of additional elements in the Al–Si alloy system allows many complex intermetallic phases to form, which make characterisation non-trivial. These include, for example, CuAl2, Al3Ni2, Al7Cu4Ni, Al9FeNi and Al5Cu2Mg8Si6 phases, all of which may have some solubility for additional elements. Identification is often non-trivial due to the fact that some of the phases have either similar crystal structures or only subtle changes in their chemistries. A combination of electron backscatter diffraction (EBSD) and energy dispersive X-ray analysis (EDX) has therefore been used for the identification of the various phases. This paper will present comparisons of phase identification methodologies using EBSD alone, and in combination with chemical information, either directly or through post processing.

Effects of trace Mn addition on the elevated temperature tensile strength and microstructure of a low-iron Al–Si piston alloy

The effects of different amount of trace Mn addition on the elevated temperature tensile strength and microstructure of a low-iron Al–Si piston alloy were investigated by tensile testing, EPMA, SEM and EDX. It has been found that with the increase of Mn addition, the elevated temperature tensile strength of the examined alloy exhibits an interesting change: from primary 67.07 MPa, increased to 75.62 MPa and then decreased to 69.67 MPa, 63.72 MPa; finally increased again to 71.92 MPa. Through microstructural analysis, it is found that there occurs an evolution of Mn-bearing intermetallic phases in alloy with the increase of the Mn addition into the alloy. When the amount is 0.15 wt.%, most of manganese exists in the dendritic Al 9FeNi phase; further addition of Mn can lead to the formation of plate-like Mn-bearing phases, whose strengthening effect is poor.

Al–Si/B 4C composite coatings on Al–Si substrate by plasma spray technique

Plasma-sprayed coatings of Al–Si/B 4C have been prepared on Al–Si piston alloys for diesel engine motors. The Al–Si/B 4C composite powders including 5–25 wt% B 4C were prepared by mixing and ball-milling processes. These powders were deposited on Al–Si substrate using an atmospheric plasma spray technique. The coatings have been characterised with respect to phase composition, microstructure, microhardness, bond strength and thermal expansion. It was found that Al, Si, B 4C and Al 2O 3 phases were determined in the coatings with approximately 600 μm thick by using X-ray diffraction analysis. Scanning electron microscope observation revealed that boron carbide particles were uniformly distributed in composite coatings and B 4C particles were fully wetted by Al–Si alloy. Also, no reaction products were observed in Al–Si/B 4C composite coatings. It was found that surface roughness, porosity, bond strength and thermal expansion coefficient of composite coatings decreased with increasing fraction of the boron carbide particle. It was demonstrated that the higher the B 4C content, the higher the hardness of coatings because the hardness of B 4C is higher than that of Al–Si.

Relationship between microstructures and contents of Ca/P in near-eutectic Al–Si piston alloys

The relationship between microstructures and contents of Ca/P in near-eutectic Al–Si piston alloy (ZL109) was investigated by optical microscopy and electron probe microanalyzer (EPMA). Ca was found to have an obvious influence on the microstructures of ZL109 alloys. It is difficult to present phosphorus refinement effect when the Ca content in the ZL109 alloy exceeds 100 ppm. In order to obtain a good phosphorus refinement effect, the content of Ca impurity in the alloy should be under 50 ppm. It has also been shown that Ca can modify eutectic Si and enhance the formation of α-Al dendrites and fibrous eutectic Si in the alloy. Controlling the content of Ca impurity by the addition of C 2Cl 6 to the melt resulted in the appearance of phosphorus refinement effect. With the decrease of Ca content from 100 ppm to 12 ppm, the average primary silicon particle size is reduced to 16.2 μm from 31.7 μm.

The nucleation sites of primary Si in Al–Si alloys after addition of boron and phosphorus

The influence of boron (B) and phosphorus (P) on the solidification structures of near eutectic Al–Si piston alloys (ZL109) was investigated. It has been found that the addition of B can promote the precipitation of primary Si crystals, not only enhance the modification efficiency of phosphorus (P) but also reduce the addition level of P. It has also been shown that there are several kinds of heterogeneous nucleation sites of primary Si particles in the alloys modified by boron and phosphorus, respectively, AlP, Al–B–P–Mg and Al–B–Mg compounds. The tensile strength of the piston alloy with 0.2% B addition exceeds that with 0.05% P addition; moreover, owing to form more kinds of the heterogeneous nuclei of primary Si, the solidification structures and tensile strength of the alloys modified with B and P are improved. Compared with the traditional modifying method, the addition of B and P together may be a new and better way to improve the structures and mechanical properties of the ZL109 alloys

EPMA analysis of calcium-rich compounds in near eutectic Al–Si alloys

The effect of Ca impurity on solidification structure of near eutectic Al–Si piston alloy (ZL109), the origin of the presence of calcium (Ca) and the transformation of Ca compounds in the alloy were investigated. It has been found that many kinds of Ca compounds exist in commercial pure silicon, which are the main source of the Ca impurity in ZL109 alloy. It has also been shown that calcium can modify eutectic Si and enhance the formation of α-Al dendrites and fibrous eutectic Si in the alloy, even retaining the structural characteristics after addition of sodium. The abnormal phenomenon is due to the formation of Ca x Si y P z compounds which are more stable than AlP phases in the melt. However, the Ca level can be reduced from 216 ppm to 30 ppm and then the P modification efficiency can be recovered after a refinement treatment by C 2Cl 6 because of the reaction: Cl 2 + Ca → CaCl 2(s), and the transformation: Al + Mg + Ca x Si y P z (s) + Cl 2 → (Al 5, Mg 0.85 + Ca 0.15) P(s) + CaCl 2(s) + MgCl 2(s), the (Al 5, Mg 0.85, Ca 0.15) P compounds serving as heterogeneous nucleating sites of the primary Si phases.

Elevated Temperature Fatigue of Al-Si Piston Alloys

Long crack fatigue tests were carried out on two cast Al-Si alloys used in the manufacture of pistons for small automotive engines. These were performed at a frequency of 15Hz at room temperature, 200 degreesC and 350 degreesC, in addition further tests were performed at 50Hz at the highest temperature. The fatigue performance of both alloys was found to improve with increasing temperature. This was linked to a change in fatigue crack propagation mechanism in both alloys with respect to the primary Si phase. It was seen that the transition between a given Si phase decohering or fracturing ahead of a fatigue crack occurred at a temperature and strain rate dependent critical stress intensity factor. The relation between critical stress intensity factor, temperature and strain rate differentiated the two materials studied.

A comparison of as-cast and heat treated properties in short fiber reinforced Al-Si piston alloys

In recent years, there has been a surge of interest in the use of discontinuously reinforced Metal Matrix Composites (MMCs) as structural materials. Reduction in the cost of constituent materials coupled with improvements in the processing methods have resulted in the ability to produce these materials at a competitive cost, when compared with more traditional structural metals. Most mechanical property data for discontinuously reinforced MMCs have been determined in tension and a relatively small amount of data has been obtained in compression. This is surprising since compression testing has certain advantages over tensile testing. The aim of this work is to investigate the effect of short alumina (Saffil) fibers to the hardness and compressive strength in the Al-Si (LM 13) piston alloy. Particular attention is given to the influence of the heat treatment and the fiber volume percent on the properties.

Vickers hardness measurements of unidirectionally solidified Al-Si eutectic alloy grown at different rates

An important commercial material is the Al-Si piston alloy. It consists mainly of the Al-Si eutectic modified with small additions of Cu and Mg to improve the strength by metastable precipitation, and Ni to improve the high temperature strength. A disadvantage of these precipitation hardening alloys is the significant decrease in strength in the 150 to 250/sup 0/C temperature range due to dissolution of the precipitates. Other means to improve high temperature properties of aluminum alloys, such as addition of oxide dispersions in the Al matrix, have been successful, but require expensive sintering processes. It is of interest to determine if directional solidification can improve the mechanical properties of the Al-Si eutectic alloy. The Al-Si eutectic is of the faceted-nonfaceted type, according to the classification of Hunt and Jackson. In conventional casting at high freezing rates, it is found that the Si phase grows independently into the melt ahead of the Al phase with no noticeable coupling to it. This results in a microstructure in which the eutectic Si occurs as more or less randomly oriented needles or irregular plates in the Al matrix. Directional solidification at low rates produces ''fiber'' alignment, but the fibers have a complicated cross sectionmore » first described by Day and Hellawell. In this work studies are reported of the microstructures and hardness of the unmodified Al-Si eutectic which has been directionally solidified at different growth rates.« less