Fine Si Particles Research Articles

Study on microstructure and mechanical properties of 3003 aluminum alloy joints brazed with Al–Si–Cu–Ni paste brazing materials

Paste-type brazing materials have advantages such as adjusting the complexity of the parts to be soldered, easy storage and production in certain quantities. They can be used for brazing heat exchangers, liquid tanks and corrosion resistant parts. In this work, the microstructures and thermal behaviors of Al–Si–Cu–Ni brazing materials with different contents were investigated, and the effect of brazing process on the microstructural evolution and mechanical properties of brazed joints produced under nitrogen-filled environment was examined. It was found that the melting temperature of brazing material Al–5Si–20.5Cu–2Ni were ranged from 512.86 to 549.37 °C. The microstructure of Al–5Si–20.5Cu–2Ni consisted of α-Al solid solution, CuAl2 intermetallic compounds, the Al–Si–Cu phase, and some fine irregular Si particles in a homogenous manner. The microstructure of the brazed joints was uniformly formed during the brazing condition of 580 °C for 20 min, and the shear strength of the joints reached 41.76 MPa.

Microstructural features of AlFeSi spherical particles in hypoeutectic Al-Si alloy

The Al-Si alloy ingots used for the study were produced by diluting an Al-Si master alloy containing impurity Fe with pure Al. A type of micron-sized AlFeSi spherical microstructure was observed in the diluted alloy ingot. These spherical particles have an overall diameter range of 2.8–6.1 µm, with polyhedral Al9Fe2Si2 particles inside and clusters of fine Si particles with sizes concentrated at 200–250 nm outside. During solidification, the AlFeSi atomic clusters in the alloy melt firstly form the polyhedral Al9Fe2Si2 and then a shell of fine Si particles. The internal polyhedral Al9Fe2Si2 particles and the external fine Si particles have different formation mechanisms, with the polyhedral Al9Fe2Si2 particles being dominated by faceted growth and the fine Si particles being dominated by finite growth after nucleation.

Improving Mechanical Property of Hyper-Eutectic Al-Si Alloys via Regulating the Microstructure by Rheo-Die-Casting

The microstructure plays a key role in the mechanical properties of hyper-eutectic Al-Si alloys. In this study, we investigate the microstructural evolution of rheo-die-casting (RDC) on the Al-15Si-4Cu-0.5Mg alloy using a mechanical rotational barrel system. Our findings demonstrate that higher rotational speed and pouring temperature reduce the size and roundness of primary Si particles in the semisolid slurry. Additionally, during RDC, the dendritic aluminum matrix and skeletal iron-containing inter metallics are sheared off, leading to a more uniform and dispersed Al2Cu phase. Ultimately, our rheo-diecasting results indicate the formation of a near globular aluminum matrix, fine primary Si particles, and a homogeneous Al2Cu phase, thus highlighting the efficacy of this processing method for improving the microstructure and properties of the Al-15Si-4Cu-0.5Mg alloy. We suggest that these results hold promise for enhancing the quality of aluminum-based alloys in various industrial applications.

Enhancing mechanical performances by unconventional Si precipitates obtained in pressure-induced Al-Si-Cu-(Mg) solid solutions

The influence of 6 GPa-level high pressure on the microstructures and mechanical properties of Al-7Si-3Cu and Al-7Si-3Cu-0.4 Mg alloys is investigated. It is found that both alloys break the solubility limit of Si and become near-complete solid solutions of Al under 6 GPa. Upon direct aging treatment, the precipitates of Al-7Si-3Cu alloy consist of θ′ phases, coarse irregular Si particles, and Al2Cu particles, while unconventional fine rod-like Si particles lying along the <100>Al directions precipitate in Al-7Si-3Cu-0.4 Mg alloy, which leads to the superior mechanical properties with a compressive stress of 1161.9 MPa and a microhardness of 259.6 HV. The APT analysis further shows that these unconventional Si phases are enriched with Mg and segregated by Cu. The mechanism behind the precipitation and the corresponding mechanical performance are further explored.

Dry sliding behavior of AlSi10Mg alloy produced by Laser-based Powder Bed Fusion: influence of heat treatment and microstructure

The L-PBF AlSi10Mg alloy is widely used in the production of structural parts in the transportation sector. However, the high stresses caused by the severe operating conditions require an optimal combination of mechanical and tribological properties. This paper reports on the effect of optimized T5 heat treatment (direct artificial aging: 4 h at 160 °C) and novel T6 heat treatment (rapid solution: 10 min at 510 °C, followed by artificial aging: 6 h at 160 °C) on the tribological behavior of the L-PBF AlSi10Mg alloy. Dry sliding tests (ball-on-disk) were carried out using AlSi10Mg samples as rotating disks against an Al2O3 stationary ball. The optimized T5 led to the formation of a stable protective oxide layer well adherent on the worn surface, increasing the wear resistance of the alloy. In addition, the novel T6 improved wear resistance compared to conventional T6 due to microstructural refinement induced by shorter solutionizing. The sub-surface analysis of the wear tracks highlighted the higher cohesion between the more homogeneous and finer Si particles and the α-Al matrix, as well as the improved load-bearing support compared to the coarser microstructure induced by conventional T6. Therefore, the new T6 could be the optimal solution for high-performance components.

Effect of artificial aging on the strength, hardness, and residual stress of SLM AlSi10Mg parts prepared from the recycled powder

This study investigated the effect of artificial aging on the hardness, strength, and residual stress of SLM (Selective Laser Melting) AlSi10Mg parts prepared from recycled powder used for 30 SLM printing cycles. First, an analysis was conducted for a comparison of recycled and virgin powders; it revealed that the powders have the same shape and phase and chemical composition. The only significant differences were the slightly increased average particle size and higher oxygen content in the recycled powder, especially on the powder surface. Typical molten pools were found in the microstructure, and a fine cellular type of structure was observed. The peripheral areas of the molten pools contained coarser cells with an average size of 0.6 μm2, and the middle part of the molten pools contained finer cells with an average size of 0.17 μm2. Fine Si precipitates with an average size of 25 nm were observed inside the cells. In terms of microstructure, no significant differences were observed when compared to the published data relating to SLM using virgin powders. This suggests that recycled powder after 30 printing cycles can be utilized for SLM component preparation to achieve cost-effectiveness. However, comparison of products with the use of virgin and recycled powders showed that the use of recycled powder affects the submicron characteristics (precipitate size increase), density and the resulting mechanical properties. Artificial aging of samples from the recycled powder at 170 °C caused an increase in hardness, Young's modulus and strength, while ductility remained essentially the same after aging. Peak values of hardness and strength were determined after 6 h of artificial aging. The increase in strength of the prepared samples can be explained by the precipitation of fine Si particles and their coarsening inside the cells. However, after longer aging times, the precipitates continued to coarsen, coalesced and displayed a plate-like shape, which led to the reduction in strength. The residual stress measured by the bridge curvature method (BCM) showed a significant decrease with increasing artificial aging time. The lowest residual stress was determined for an aging time of 100 h; however, the difference between residual stress for 6–7 h and 100 h was not significant. In order to obtain higher strengths and remove internal stresses after SLM during one heat treatment, the most suitable time for artificial aging at 170 °C seems to be 6 h.

The effect of pre-drilling on the characteristics of friction drilled A356 cast aluminum alloy

The friction drilling process utilizes frictional heat to increase the thickness of sheet metal parts by making holes with bushes using a non-traditional drilling tool. Unfortunately, cracks are found in the formed bushings during friction drilling of brittle materials. The purpose of the present study is to improve the quality of friction-induced bushes in brittle A356 aluminum sheets via a performance of a pre-drilling with different diameters (2, 2.5, and 3 mm). The pre-drilling is aimed to reduce cracks and petal formation as compared with the solid sheets. The friction drilling processes were carried out by a K100 conical tool, with 10 mm diameter and 58° cone angle. The friction drilling parameters involve different rotational speeds (2000, 3000, and 4000 rpm) as well as different feed rates (40, 60, and 80 mm/min). The characteristics of the thermally induced bushes (shape, dimensions, and surface roughness) were investigated in solid and pre-drilled sheets. Furthermore, we studied the microstructure evolution and hardness distribution in the thermo-mechanical affected zone and the heat-affected zone surrounding the bush. It was found that, the formed bushings in the pre-drilled sheets did not have cracks or petal formation in comparison with the other formed bushes in the solid sheets. The pre-drilled specimens showed an average bushing height increase from 5 to 6 ± 0.8 mm with the increase of the tool rotational speed from 2000 to 4000 rpm. At rotational speeds of 2000 and 4000 rpm, the pre-drilled specimens with a diameter of 2.5 mm had a bushing height which is 0.5 ± 0.2 mm greater than the other pre-drilled specimens. In addition, the surface roughness of the pre-drilled sheets decreased with the increase of the rotational speed and/or reduction of the feed rate. Moreover, the microstructure near the drilling zone exhibited a fine Si particles distribution. However, the hardness values near the drilled surfaces were 15 ± 8 HV lower than those of the base metal in both solid and pre-drilled sheets.

Influence of Modification on Flow Stress Behavior and Corrosive Properties of a Hypoeutectic Al-Si Alloy

The flow stress behavior and corrosive properties of Al-7Si alloy were studied by thermal compression test, electrochemical test, and electron probe microanalysis. The influences of temperatures, strain rates, strains, and morphology of Si particles on stress-strain curves of Al-7Si alloy were analyzed. The peak stress of unmodified-Al-7Si alloy is higher than that of Sr-modified-Al-7Si alloy at the same deformation conditions, and the phenomenon is obviously relevant to the distribution and morphology of Si particles. The morphology of Si particles of unmodified-undeformed-Al-7Si alloy is a typical thick layer, and that of unmodified-deformed-Al-7Si alloy is broken into relatively small particles and the distribution is relatively even in homogeneous deformation zone II. The distribution of Si particles in Sr-modified-deformed-Al-7Si alloy is obviously more even than undeformed alloy. The effect of a small deformation of 20% on the distribution and morphology of Si particles is obviously smaller than that of a large deformation of 50%. The electrochemical self-corrosion potential of Sr-modified-Al-7Si alloy is higher than that of unmodified-Al-7Si alloy, and it proves that the distribution and morphology of Si particles have a certain influence on the corrosive properties of Al-7Si alloy. That is, the even fine Si particles are more conducive to improving the corrosive properties of Al-7Si alloy.

In situ alloying of AlSi10Mg-5 wt% Ni through laser powder bed fusion and subsequent heat treatment

In the current study, the effect of in-situ alloying of AlSi10Mg with 5 wt% Ni (pseudoeutectic composition) through the LPBF additive manufacturing technique was investigated. Fabricated samples underwent supplementary annealing treatment at 300 °C for 15 and 120 min, eventually. During the thermal treatment, the fish-scale grains created after printing inside the melt pools after printing vanished gradually. In the as-built sample, molten Ni particles settled as big chunks of Al3Ni in the center and a thin strip of tiny Ni-rich masses in the borders of the melt pools. FIB/SEM and AFM images revealed that after 15 min annealing, silicon cellular dendrites fragment into fine Si particles, and during annealing for 120 min, the aluminum matrix expels out the supersaturated solute silicon atoms. Consequently, Si atoms leave the substrate and diffuse to pre-existing Si particles in cell walls and the triple junctions. Such a collective diffusion of Si atoms leads to the formation of coarse Si particles widespread in the Al matrix. Based on the XRD outputs, annealing for 15 min had not any major effect on Ni-rich phases. however after 120 min, a more brittle intermetallic shell of a Ni-rich phase formed on pre-exist coarse phases. The comparison of the mechanical properties of the Ni-reinforced AlSi10Mg alloy with those reinforced via the addition of other elements/compounds revealed that in a size range close to AlSi10Mg particles, Ni could not be an appropriate candidate for in-situ alloying through the LPBF method. Furthermore, spheroidization of the silicon particles and formation of fragile Ni-rich intermetallic shells having weak interfacial bonding to the Al matrix during the long-term annealing results in a significant reduction in mechanical strength of the specimens.

Dry Sliding Wear Behavior of Wrought Al-12.7Si-0.7Mg Alloy

In present paper, the dry sliding wear behaviors of wrought Al-12.7Si-0.7Mg alloy have been investigated using a pin-on-disc machine. The effect of various parameters, such as alloy conditions, sliding speed, and normal pressure, has been investigated. Wear surface was characterized by SEM/EDX microanalysis. The microstructure, hardness and tensile properties of the extruded profile were investigated with the aim of understanding the wear and friction mechanisms. The present results reveal an improvement in the mechanical and tribological properties, obtained due to microstructure characterized by fine Si particles uniformly distributed in the Al matrix of fine equiaxed grains, as promoted by hot extrusion. It was also revealed that higher hardness and the lower aspect ratio of eutectic Si particles contributed to the better wear resistance.

High strength and ductility of Al–Si–Mg alloys fabricated by deformation and heat treatment

Al–Si–Mg alloys with 7% and 12% (mass ratio) Si contents were subjected to friction stir processing (FSP), warm rolling (WR), and heat treatment. The effects of process routes on the microstructure and mechanical properties of the Al–Si–Mg alloys were studied. FSP could break the eutectic structure and lead to the uniform distribution of the generated fine Si particles in the two Al–Si–Mg alloys. WR could effectively decrease the porosity of the Al–Si–Mg alloys. T6 could lead to the precipitation of the β'′ phases in these alloys. The structure characteristics, including fine grain and second phases, uniformly distributed Si particles, high density of β'′ phases, and low porosity, resulted in the satisfactory mechanical properties of the Al–Si–Mg alloys after FSP, WR, and T6 treatments.

A comparative study of tensile properties of eutectic Al-Si / ZrO2 composites fabricated by spray forming and stir casting methods

ABSTRACT The study aims to investigate the microstructural and mechanical properties of Al-12.5%Si matrix composites reinforced with ZrO2 were fabricated by spray forming method and stir casting (SC) technique by varying reinforcement of 2%, 6% and 10 wt.% ZrO2 powder (size 25 μm).The spray-formed (SD) composites were subjected to hot (400°C) isostatic pressing to reduce the porosity. The microstructure of spray-formed and SC composites were examined using optical and SEM/ EDS. The microstructure of the spray-formed composites showed uniform distribution of fine Si, eutectic phase and ZrO2 particles in the equiaxed Al-matrix. In contrast, SC composites consist of a coarse Si, needle like eutectic phase and clustering of reinforcement particles at grain junctions as a continuous network along the grain boundaries. The hardness, ultimate tensile strength of both spray-formed and SC composites improved as the ZrO2 content increased, while yield strength decreased. Spray-formed composites showed better mechanical properties. This was attributed to uniform distribution fine of Si particles, reinforced particles, equiaxed matrix induced during spray deposition process. Tensile fracture surface of spray-formed composites possess ductile and cleavage fractures. SC composites showed intragranular cracks in coarse Si particles and increase in micro-cracks with increasing reinforcement.

Fatigue Behavior of an Al-12.7Si-0.7Mg Alloy Processed by Extrusion and Heat Treatment

The fatigue life of a hot extruded Al-12.7Si-0.7Mg alloy under T1, T4, and T6 conditions was studied. The microstructure and tensile properties of the alloy were investigated in order to analyze the fatigue behavior. The results of the fatigue test showed that an extruded Al-12.7Si-0.7Mg alloy provided greater fatigue life compared to a cast Al-Si alloy, which was explained by the refined microstructure characterized by fine Si particles uniformly distributed in the Al matrix of fine equiaxed grains promoted by hot extrusion. The fatigue property of the alloy in T6 treatment was higher than that in the T4 and T1 conditions due to strengthening precipitation.

Influence of heat treatment on the high-cycle fatigue properties and fatigue damage mechanism of selective laser melted AlSi10Mg alloy

The high-cycle fatigue properties and fatigue damage mechanism of AlSi10Mg alloy manufactured by selective laser melting (SLM) were investigated. The influence of the post-heat treatment on the microstructure and high-cycle fatigue properties was also examined. The SLM-manufactured AlSi10Mg alloy was heat-treated under two different conditions, namely T6 and direct aging (DA). The results of the high-cycle fatigue tests, which were carried out at room temperature, imply that, under all three conditions (as-built, T6, DA), the fatigue strength decreases as the number of cycles increases along with the estimated endurance limit. Predominantly, the fatigue properties were influenced by the morphology and size of the Si particles, both controlled by adjusting the heat treatment. The alloy that was processed using DA heat treatment had superior fatigue properties due to its cellular structural morphology characterized by a significant amount of fine Si precipitates. Compared with the as-built and T6 specimens, the superior fatigue limit of the DA alloy is attributable to its outstanding strength characteristics. The reliability of the fatigue life of the DA alloy is relatively high. Fatigue fracture samples revealed that the DA alloy had evenly distributed dislocations along the cellular boundaries. The fine Si particles were found to play an important role in preventing fatigue crack propagation, thereby enhancing fatigue properties. The effect of the heat treatment on the microstructure and mechanical properties of the SLM AlSi10Mg specimens was investigated with a specific focus on fatigue strength. The results highlighted that an appropriate DA process could significantly enhance the fatigue performance of the AlSi10Mg alloy under investigation. The resulting characteristics are highly advantageous and are superior to those obtained with other conventional heat treatment processes.

Microstructural evolution and mechanical performance of in-situ TiB2/AlSi10Mg composite manufactured by selective laser melting

In-situ TiB2/AlSi10Mg composites were prepared by casting, gas atomization, and selective laser melting (SLM), respectively. Microstructural evolution of the composites in as-cast, atomized, and SLMed were investigated. Compared with the as-cast composite, the SLMed sample has no texture and is composed of extremely fine grains and crystal cells. The in-situ TiB2 particles uniformly distribute over the grain boundaries, the acicular Si phase is replaced by a network of fine Si particles and there are nano-sized Si precipitations in the crystal cell. The formation of this microstructure is due to the continuous solidification of non-equilibrium eutectic Al–Si during the rapid cooling process of SLM. The SLMed TiB2/AlSi10Mg composite obtains excellent strength (tensile strength ∼ 451 MPa, bending strength ∼ 674 MPa), hardness (∼151 HV0.1) and good friction coefficient (∼0.4), which are higher than that of the as-cast TiB2/AlSi10Mg composite and the SLMed AlSi10Mg alloy. The formation mechanism of this particular structure and its strengthening effect were discussed.

Solidification and microstructural formation of a ternary eutectic Al-Cu-Si cast alloy

A novel ternary eutectic Al-27%Cu-5%Si cast alloy was investigated to understand its solidification behavior and microstructural evolution. The studied alloy was cast, remelted and subjected to a rapid cooling in ice-water mixture, and also cooled slowly in air. The solidification sequence of the alloy was studied using Thermo-CalC software with a subsequent DSC measurement. Micro- and nano-hardness tests were conducted at room temperature to assess the change of hardness associated with the microstructural changes due to the different cooling paths the alloy was subjected to. Thermodynamic analysis showed that the equilibrium solidification path of the eutectic alloy consists of three stages: precipitation of Al2Cu primary phase at 527.6 °C, evolution of binary eutectic (α-Al + Al2Cu) at 525.9 °C and the formation of ternary eutectic (α-Al + Al2Cu + Si) at 521.7 °C. DSC analysis of the alloy suggests only one endothermic peak at 536.4 ± 0.2 °C due to the melting of the alloy, which also corresponds to the ternary eutectic transition. The quenched alloy has a well-refined and uniform distribution of bimodal eutectic microstructure when compared with the slowly cooled alloy. The hardness of the quenched alloy was observed to be 16% higher than the slowly cooled alloy. The increased microhardness of the quenched alloy is attributed to the solid solution strengthening of the alloying elements and homogeneous dispersion of fine Si particles, and partly due to the higher volume fractions of both θ-Al2Cu and Si phases.

Developing a high-strength Al–11Si alloy with improved ductility by combining ECAP and cryorolling

In this study, a high-strength Al–11Si alloy with an improved ductility was developed via a two-step processing route - equal-channel angular pressing (ECAP) and post-cryorolling. Specifically, the cast alloy was ECAP processed at 623 K for 16 passes, followed by multi-pass cryorolling with 50% thickness reduction. Herein, its mechanical properties was evaluated, and the underlying mechanisms for the improvement of both strength and ductility was investigated. Through systematic comparisons with other samples, including the cast, the cast-rolled, the ECAP-processed and the ECAP-annealed samples, we found the respective origin for the high strength and the improved ductility in the processed alloy. On the one hand, the high strength resulted from both grain boundary strengthening and dislocation strengthening caused by the thermo-mechanical processing. On the other hand, the improved ductility was primarily dictated by the uniform distribution of the very fine Si particles due to the attendant fragmentation of the Si phase during ECAP process. In addition, the results showed that the superior combination of strength and ductility could not be achieved by either processing step alone. This two-step processing route is also promising to benefit other Al–Si alloys for structural applications by optimizing their mechanical properties.

Friction stir processing of squeeze cast A356 with surface compacted graphene nanoplatelets (GNPs) for the synthesis of metal matrix composites

Friction stir processing (FSP) was applied to graphene nanoplatelets (GNPs) physically compacted on the surface of squeeze cast A356 alloy to incorporate GNPs within the matrix and to improve its mechanical properties. Squeeze casting resulted in finer size silicon and intermetallic compounds in cast microstructure, and subsequently FSP further refined the microstructure of squeeze cast A356 alloy, and GNP reinforced A356 alloy. The finer Si particles, intermetallics and graphene dispersed in the matrix increased the yield and ultimate tensile strength of FSP squeeze cast A356 alloy compared to the results reported in prior literature for FSP A356 alloy. Eutectic Si needles have been converted to fine spherical particles during FSP and were uniformly distributed within the nugget zone. The crystallite size of GNPs which were physically adhered to the surface of squeeze cast alloy prior to FSP decreased after FSP as a result of deformation. Thus, a combination of squeeze casting, and friction stir processing and incorporation of GNPs reinforcement in the A356 matrix is a promising route to further improve its mechanical properties.

Abnormal corrosion behavior of selective laser melted AlSi10Mg alloy induced by heat treatment at 300 °C

Heat treatment is often employed to control the mechanical properties of selective laser melted (SLMed) AlSi10Mg, which also alters its microstructure and corrosion resistance. Therefore, a SLMed AlSi10Mg alloy was heat treated at 300 °C and 400 °C for 2 h to investigate its microstructural evolution and corrosion behavior. Abnormal corrosion behavior is observed in the sample heat treated at 300 °C. Heat treatment destroys the cellular Si network in the microstructure of the SLMed AlSi10Mg sample and promotes the formation of extremely fine Si particles; in contrast, such a microstructure cannot form a protective film on the surface of the sample, which leads to rapid corrosion along the cracked Si network inward to the inner Al matrix without the microgalvanic effect.

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height.In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth.The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult.