Al-Si Piston Alloy Research Articles

Microstructural Evolution and Tensile Properties of Al-Si Piston Alloys during Long-Term Thermal Exposure

The present study investigated microstructural evolution and changes in tensile properties of an Al-Si piston alloy subjected to thermal exposures at 250 and 350 °C for 150, 300, and 500 h. Microstructural and nanoscale precipitates were characterized using a combination of high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images and scanning electron microscopy (SEM). The tensile testing was performed. The results demonstrated that the thermal exposure induced granulation of the δ-Al3CuNi particles, alongside precipitation of the θ-Al2Cu phase particles and AlCu clusters within the matrix. Specifically, an increase in the size and number density of the θ-Al2Cu phase particles was observed with exposure time at 250 °C. Conversely, at 350 °C, the θ-Al2Cu particles exhibited a gradual increase in size with prolonged thermal exposure, coupled with a decrease in their number density. AlCu clusters precipitated solely at a thermal exposure temperature of 350 °C, with precipitation intensifying over time. Moreover, a decrease in the alloy’s tensile strength and an increase in elongation were noted after thermal exposure. Finally, the present study discussed the precipitation mechanisms of θ-Al2Cu particles and AlCu clusters within the grains, suggesting that the AlCu clusters exerted a more effective strengthening effect compared to the θ-Al2Cu particles.

Effect of Squeeze Pressure and Strontium Addition on Tribological and Mechanical Properties of Al–Si Piston Alloy with Varying Amount of Ni

Effect of Squeeze Pressure and Strontium Addition on Tribological and Mechanical Properties of Al–Si Piston Alloy with Varying Amount of Ni

Effect of surface roughness on stress concentration factor of machined surface and cutting parameter optimization in boring of Al-Si piston alloy

Effect of surface roughness on stress concentration factor of machined surface and cutting parameter optimization in boring of Al-Si piston alloy

Dry sliding wear characteristics, corrosion behavior, and hot deformation properties of eutectic Al–Si piston alloy containing Ni-rich intermetallic compounds

Al–Si eutectic alloys could be an alternative for gray and compacted graphite cast iron engine blocks due to their excellent thermal conductivity, low density (resulting in weight reduction and fuel efficiency), and good combination of strength and ductility. However, microstructural softening at service temperature (170–250 °C) due to coarsening/coalescence of metastable precipitates and low wear resistance are a stumbling block limiting their usage in engine components. To meet endless demands for fuel-efficient car engine (i.e. higher temperature and pressure in combustion chamber), optimization of current chemical composition seems to be essential. For this reason and in the present study, different concentration of Ni (from 0.8% to 3.5%) was doped into a eutectic Al–Si alloy to investigate tribological, corrosion, and hot deformation properties. It was pointed out that Al–Si eutectic alloy containing 2–2.6% Ni had the highest wear resistance due to uniform dispersion, skeleton-type morphology of Ni-rich intermetallic compounds, and their better bonding with the matrix. Oxidation and delamination of oxide layer was shown to be the main wear mechanism. The presence of (Ni,Cu)-rich intermetallic compounds increased the corrosion rate by ≈ 60%; however, the repassivation potential was found to be slightly higher in 2.6%Ni sample indicative of easier repassivation of micropits. To better understand the high temperature behavior of Ni-containing Al–Si alloy, hot compression test was carried out within the temperature range of 400–550 °C and strain rates of 0.001 to 1s−1 and constitutive equations were used to develop hot deformation processing maps. The presence of Ni-rich intermetallic compounds enhanced the hot deformation behavior and increased the activation energy for hot deformation. Dynamic recrystallization was found to be the main softening mechanism during hot deformation. At true strain of 0.6, the maximum power dissipation efficiency (≈54%) was obtained at 400 °C and a strain rate of 0.01s−1.

Experimental and numerical evaluation of squeeze cast Al-Si-Cu-Ni-Mg alloy for piston applications

ABSTRACT Al-12% Si is the commercial alloy commonly used to make pistons by using gravity casting method. When this alloy was modified with Ni, Cu, and Mg (modified alloy), it showed an adequate fluidity during casting and achieved higher strength. However, the addition of alloying elements reduces its thermal characteristics. Therefore, an attempt has been made to improve the thermal conductivity through modifying the microstructure. It was done by changing the cast method to squeeze cast, adding 0.04%Sr (Sr-modified alloy) and post-heat treatment (T6 and T7). Results highlighted that the sample of gravity cast-Sr-modified alloy showed enhancement in thermal conductivity. Even though squeeze cast-Sr-modified alloy showed reduction in thermal conductivity of few percentage, it possessed better strength, however the heat treatment (T7 condition) showed a noticeable increment in thermal conductivity. Therefore, it is concluded that T7-heat-treated squeeze cast-Sr-modified alloy showed an excellent combination of strength and thermal conductivity making it an ideal material for piston applications. The piston component was studied by numerical steady state thermal analysis using ANSYS Workbench, obtained a maximum heat flux of 1.80 W/mm2 for Al-Si piston alloy aged for 11 h (T7), which is in agreement with the experimental results.

Influence of Intermetallic Compounds on Age Hardening of Gasoline Pistons in Engine Operations

Intermetallic precipitates hold the tensile strength of alloy at elevated temperature along with enhanced mechanical and thermal properties. In this research work, we have implemented the new material development of Al-Si alloys with Fe-rich intermetallic precipitates for piston application in order to study the influence of intermetallic compounds (IMCs) on piston performance under engine operations in such a way that pistons with three different intermetallic compositions namely type A, type B and type C were developed to investigate the influence of IMC on age hardening. Three IMC pistons were installed in the engine and allowed to run for six consequent engine operations (1-6 hrs). After the designated hours of engine operation, piston was dismantled from the engine and cut sectioned for aging characterization. Hardness, an indicator of age hardening and thermal stability of the material, was measured at various piston zones such as crown, ring land, top land, skirt and gudgeon pin boss. These hardness values reflect the tendency of age hardening of pistons. It was found that higher IMC density piston alloys (type B and type C) showed the enhancement in tensile strength and hardness about 8 and 13%, respectively. Aging behavior of type B and type C piston zones has been predicted as better due to presence of high dense intermetallic compounds in comparison with type A piston. DSC testing showed that type B and type C have longer precipitation than that of type A piston. Type B and type C alloys have exhibited 20-40% higher specific heat capacity than type A piston. It was found that the high dense star-/Chinese script-like IMC has influenced the Al-Si piston alloy to enhance its properties and piston performance as well.

Morphological Evolutions of Ni-Rich Phases in Al-Si Piston Alloys during 250–400 °C Thermal Exposure Processes

The thermal stability of the Al-Si alloys during the thermal exposure process from 250 °C to 400 °C was systematically investigated. The relationships between the morphological evolution and the mechanical changes of the alloys were determined through the Vickers hardness test and materials characterization method. Initially, the alloys exhibited similar thermal degradation behavior. For example, the exposure process of the alloy at 300 °C can be divided into two stages according to the changes of the alloy hardness and the matrix micro-hardness. In detail, the first stage (0–2 h) exhibited a severe reduction of the alloy hardness while the second stage showed a more leveled hardness during the following 98 h. There are three identified morphological characteristics of Ni-rich phases in the alloy. Furthermore, the differences in both composition and the micro-hardness between these Ni-rich phases were confirmed. The underlying relationships between the morphological transformation of the Ni-rich phases and hardness fluctuation in the alloy were correlated and elucidated. The observed alloy hardness increase when the exposure temperature was 400 °C was unexpected. This behavior was explained from the perspectives of both Ni-rich phases evolution and dispersoid formation.

Effect of thermal exposure on microstructure and high-temperature fatigue life of Al-Si piston alloys

A very effective approach is proposed here to increase the high-temperature fatigue life of an Al-Si piston alloy. Compared with traditional T6 heat treatment, the high-temperature fatigue life can be improved by three times of magnitude with thermal exposure treatment, which reduces the negative effects of gradually decreasing high-temperature fatigue life due to over-aging of piston durability. Specimens heat-treated by various processes have been tested in a rotating bending test machine under conditions of 50 MPa, 350 °C, and 80 Hz to observe the effects of thermal exposure on microstructure and high-temperature fatigue life. The thermal exposure treatment provides an effective method to optimize Si and the thermally stable phases of δ-Al3CuNi and γ-Al7Cu4Ni. It is one of the most important methods for improving the high- temperature fatigue life of an Al-Si piston alloy.

The effect of Ni-doping on the microstructure, hot deformation behavior, and processing map of as-cast Al–Si piston alloy

Al–Si alloy is widely utilized in automobile pistons and cylinders due to good castability, high strength-to-weight ratio, excellent thermal conductivity, good resistance to corrosion/oxidation, and acceptable wear resistance. To increase the efficiency of engine motor, it is imperative to modify and engineer the microstructure of Al–Si alloy to enhance the high-temperature behavior. For this reason, in the current study, different concentration of Ni (0.8% and 2%) was doped into Al–Si alloy during casting to generate various Ni-rich intermetallic compounds within the alloy matrix. AlNiCu, AlCu, AlFeSi, and Mg2Si were found to be the main intermetallic compounds precipitated within the dendritic and interdendritic regions, which could enhance the thermal stability and high-temperature performance. High-temperature compression test at high temperatures ranging from 400 to 550 °C and at different strain rates ranging from 10−3 to 1 s−1 was carried out to develop high-temperature processing map and optimize the hot workability parameters of Ni-doped Al–Si alloy. The maximum power dissipation efficiency (η) was observed within the temperature range of 450–500 °C and strain rate of 0.05–0.08 s−1 (lower strain rate) for Al–Si–0.8%Ni alloy and within the temperature range of 450–500 °C and strain rate of 0.3–1 s−1 (medium to high strain rate) for Al–Si–2%Ni alloy. Dynamic recrystallization was found to be the primary softening mechanism at high temperatures (T > 450 °C).

Influence of Ultrasonic Melt Treatment on Ni‐Containing Intermetallic Phases in Al–Si Piston Alloys

The influence of ultrasonic melt treatment (USMT) at different ultrasonic powers (0, 900, 2400, and 3000 W) on the morphology and chemical composition of Ni‐containing intermetallic phases (T‐Al9FeNi and δ‐Al3CuNi phases) in an Al‐11.5Si‐4Cu‐2Ni‐1Mg‐0.45Fe (wt%) piston alloy is investigated. The morphological characterization reveals that the USMT not only decrease the size of the T‐Al9FeNi and δ‐Al3CuNi phases but also change the morphology of T‐Al9FeNi from skeleton‐ to block‐shaped. The block‐shaped T‐Al9FeNi contributes the most to the high‐temperature properties because this morphology is beneficial in the formation of semi‐continuous network structures. The chemical composition of T and δ phases is analyzed. The Cu → Fe in the T phase and Ni ↔ Cu in the δ phase existing in the examined alloys are estimated. With increasing the ultrasonic power from 0 to 3000 W, the ultimate tensile strength, yield strength, and elongation are increased from 98.46 to 124.57 MPa, 82.4 to 101.87 MPa, and 2.95% to 5.12%, respectively, at 350 °C.

Effect of constraint factor on the thermo-mechanical fatigue behavior of an Al-Si eutectic alloy

The thermo-mechanical fatigue (TMF) behaviors and corresponding damage mechanisms of Al-Si piston alloy were investigated in the temperature range of 120~425 °C under different constraint factors (η). The TMF hysteresis loops exhibit asymmetrical, because the yield strength and elastic modulus decrease significantly with the increasing temperature. Taking the phase angles into consideration, the mean tensile stress for out of phase TMF (OP-TMF, η<0) and mean compressive stress for in-phase TMF (IP-TMF, η>0) can be found. The fatigue life of IP-TMF is longer than that of OP-TMF except for higher constraint factor, and the life of TMF decreases with the increasing absolute value of η. There are two different crack initiation mechanisms under the constrain TMF: primary phases broken mainly for OP-TMF and matrix deboned mainly for IP-TMF. The oxidation may have only little influence on the crack behavior and TMF life, considering the obvious damage by deboned or broken primary phases. The tensile stress is key factor for fatigue crack initiation and growth in primary phases (Si and intermetallic) under cyclic deformation. The TMF life under different constrain factors can be predicted by the strain energy model with good precision.

Structure Modification upon Ultrasonic Processing of an AA4032 Piston Alloy: Comparison of Permanent Mold and Direct-Chill Casting

Piston Al-Si alloys have very complex compositions and multi-phase heterogeneous structure, so it is necessary to control the formation of primary and eutectic compounds. In this study, the ultrasonic melt processing (USP) of a eutectic Al-Si piston alloy (AA4032-type) was performed in a permanent mold and during direct-chill (DC) casting to study its effects on the structure refinement and modification. The principal difference between these two ways of casting is that in the permanent mold the solidification front progressively moves towards the ultrasound source, while in the DC casting the position of the solidification front is fixed in space. The results showed that the USP can successfully refine primary Si, Fe-containing intermetallics and aluminum grains. Refinement of primary Si was accompanied by the increase in its amount, which was attributed to both enhanced heterogeneous nucleation and fragmentation. The refinement of Fe-containing intermetallics and Al grains resulted from the fragmentation mechanism and were more pronounced when USP was applied below the liquidus temperature in the permanent mold. However, the eutectic phases coarsened upon USP, and this effect was most pronounced when USP was applied to the semi-solid material. This was related to the strong attenuation of acoustic waves, which effectively heats the semi-solid material and induces corresponding coarsening of the phases. Acoustic streaming induced by an oscillating sonotrode affected the depth of the sump while simultaneously decreasing the macrosegregation, which reflects the dominant role of the melt flow directed against natural convection. The results demonstrated the importance of the solidification stage at which the USP was applied and the specifics of the USP mechanisms acting at the different stages of solidification.

Effect of trace elements on the pinhole fatigue-resistance of gasoline Al-Si piston alloy

Trace elements, i.e., Cu, Ni, Zr and V are added into Al-Si piston alloy to optimize the phase structure. Micro-structures and fatigue performances are discussed. The stress state of pinhole inner-surface is investigated by finite element method (FEM). The comparative tests of original piston alloy and modified piston alloy are conducted on a specialized piston fatigue setup under the same condition as a real engine, and then the fracture analyses are addressed to discuss the relevant mechanisms. Results show the content and size of primary Si particles are restrained after the addition of trace elements. The ultimate tensile strength (UTS) and high cycle (HC) fatigue strength of modified piston alloys under high temperature are increased obviously. The increase of intermetallics and decrease of primary Si are the main reasons.

Property optimization of low-cycle fatigue in Al-Si piston alloy at elevated temperatures by ultrasonic melt treatment

The cyclic deformation and damage behaviors of Al-Si piston alloys are comprehensively studied. It is found that the fatigue cracks mainly initiate from broken primary Si at low temperature and phase/matrix interface debonding at higher temperature. The transgranular crack propagation can be found for all the temperatures and the grain is obviously reduced after low-cycle fatigue at higher temperature. Furthermore, a hysteresis energy-based life prediction model was developed and utilized. Based on the model, the optimum fatigue life was found at intermediate temperature. A strategy for fatigue property optimization was proposed: increasing W0 at low temperature and increasing β at high temperature, to enhance the fatigue life during entire service temperatures with the ultrasonic melt treatment. In this way, the remarkable improvement of the fatigue properties may be achieved.

Evolution and Strengthening Effects of the Heat-Resistant Phases in Al-Si Piston Alloys with Different Fe/Ni Ratios.

The evolution of three major heat-resistant phases (δ-Al3CuNi, γ-Al7Cu4Ni, T-Al9FeNi) and its strengthening effects at high temperature in Al–Si piston alloys with various Fe/Ni ratios were studied using field emission scanning electron microscope (FE-SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). With the increase of Fe/Ni ratios, the heat-resistant phases begin to evolve in category, morphology, and distribution. The results show that a suitable Fe/Ni ratio will cause the T-Al9FeNi phase to appear and form a closed or semi-closed network with δ-Al3CuNi and γ-Al7Cu4Ni phases instead of the originally isolated heat-resistant phases. As a result, the ultimate tensile strength of the optimized alloy reached 106 MPa with a Fe/Ni ratio of 0.23, which was 23.3% higher than that of base alloy at 350 °C, which is attributed to the fact that a closed or semi-closed network microstructure is advantageous to the bearing of mechanical loads. This work may provide useful ideas for the development of high temperature resistant piston alloys.

FEM simulation-based cutting parameters optimization in machining aluminum-silicon piston alloy ZL109 with PCD tool

The Johnson-Cook (JC) constitutive model was utilized to simulate the processing of Al-Si piston alloy ZL109 to obtain the optimal finishing parameters in polycrystalline diamond (PCD) tool, and studied the machining quality of ordinary-turning under the optimized cutting parameters. PCD is brittle, so it is difficult to manufacture orthogonal blades and is easy to break and high cost. We have adopted ordinary-turning quality to verify simulation results. Effect of cutting parameters on residual stress, cutting force and temperature was investigated through simulation, meanwhile, effect of cutting parameters on surface roughness and residual stress was analyzed by experiments. It indicates the verification method is practicable, and residual stress, cutting force and temperature are the smallest in simulation, and the experimental values of surface roughness and residual stress are also the smallest under the optimal cutting parameters, which can guide the cutting parameters selection to obtain higher machining quality.

Effect of Er Addition on the High Temperature Strength of Al-Si-Cu-Ni-Mg-Fe Piston Alloys

In this study, the dependence of Er addition (0-0.49 wt %) on the microstructural evolution and related tensile strength at high temperature (HT) were investigated for the eutectic Al-Si piston alloys. The results showed there were intermetallic compounds of -Al3CuNi, -Al7Cu4Ni, Al4Cu2Mg8Si7, β-AlFeSi and Al3(Er, Cu, Fe) (Ni, Si) 2 identified at T5 states. Herein, the added Er element has led to the formation of the star-like and the irregular-shaped Al3(Er, Cu, Fe) (Ni, Si) 2 phase. The phase was stable which showed little change in both size and composition experienced solid solution at HT during the T6 treatment. For the HT strengths (350 oC), the T5 state alloys have higher UTSs than those at T6 state, both of which were reduced gradually with the increasing Er contents. The variation of HT strengths caused by the Er addition was explained by the microstructral evolution, namely Al3(Er, Cu, Fe) (Ni, Si)2 phase formation

Al-Si piston alloy behavior under combined mechanical and thermal cyclic loading with superimposed high-frequency thermal cycling

Abstract On pistons for diesel direct injection engines, the thermally high loaded bowl rim is exposed to high-cycle mechanical and high-cycle thermal loading during each combustion event in addition to the thermal cycling induced by variable engine loading, e. g. between idle and full load. This combined loading at various overall temperatures during engine operation contributes to material damage and finally to a limited service life. In the past, the high-cycle thermal loading has neither been experimentally investigated nor incorporated into a numerical model for lifetime estimation. The work described in this paper deals with the setup of an experimental test procedure on samples taken from components for which the effects of slow temperature cycling, fast temperature cycling and mechanical cyclic fatigue loading can be combined in different ratios. The aim of the investigation is to characterize the sample lifetime with respect to varied test conditions in order to differentiate alloy- or process-related material variations in support of process and material development.

Improvement in elevated-temperature properties of Al–13% Si piston alloys by dispersoid strengthening via Mn addition

Abstract

Optimized Segmentation of the 3D Microstructure in Cast Al-Si Piston Alloys

Abstract The thermomechanical behavior of cast Al-Si piston alloys is highly dependent on the condition of the highly interconnected hybrid 3D network which is composed of primary and eutectic silicon and intermetallic phases which are found embedded in the α-Al matrix. The very similar X-ray absorption of silicon and the α-Al matrix is problematic for the local identification of these phases and thus for the accurate segmentation and characterization of all rigid phases in the network when laboratory X-ray tomographs are used. A combination of conventional X-ray computed tomography, synchrotron computed tomography, and chemical deep etching performed on the same alloy and at the same position allows for an automatic segmentation and a more accurate characterization of phases across large representative volumes and thus provides the information necessary for a quantification of the whole 3D microstructure of the alloys.