Fine Si 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 and mechanical properties of spray -formed hypereutectic Al-Si-Ti alloy

ABSTRACT In the present study, the effect of the addition of Ti on the microstructural and mechanical properties of hypereutectic (Al/15 wt. %Si) spray-formed (SF) alloys has been investigated and compared with as-cast (AC) alloys. Al/15Si and Al/15Si/2Ti alloys were synthesised by spray forming method. The microstructural features of the alloys were explored under an optical microscope, and a scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis were carried out for phase identification. The phase analysis confirmed the presence of α-Al and Si phases in all the alloys, with additional peaks attributed to Al3Ti and AlSi2Ti present in the Al/15Si/2Ti alloys. The microstructure of the SF1 alloy (Al/15Si) consisted of globular, fine Si phases (primary and eutectic) distributed homogeneously in the equiaxed Al matrix. The addition of Ti to the binary SF1 alloy led to the formation of an SF2 alloy (Al/15Si/2Ti) with a fine intermetallic phase of Al3Ti and further spheroidisation of Si particles. The SF1 and SF2 alloys exhibited 30% and 40% greater microhardness, respectively, than did their corresponding AC alloys. With the addition of Ti, the ultimate tensile strength of the SF2 alloy increased by 19% compared to that of the SF1 alloy. Furthermore, a significant increase in yield strength and ductility was observed in the SF alloys compared to the AC alloys. Adding Ti to hypereutectic Al/15Si alloy via the spray-forming method provides a better alternative material for aerospace and automotive applications.

Effect of heat treatment on microstructural characteristics and mechanical properties of hypereutectic B390 aluminium alloys modified by phosphorus and bismuth

This study investigated the effects of solution treatment and aging processes on microstructural characteristics and mechanical properties of hypereutectic B390 Al-Si Alloy modified by phosphorus (P) and bismuth (Bi). The morphology evolution of primary Si, eutectic Si, intermetallic compounds, and secondary dendrite arm spacing (SDAS) of both initial and modified alloys were investigated in as-cast and heat-treated conditions. It was found that the combination of 0.1 wt%P and 0.5 wt%Bi in B390 alloy resulted in a remarkable modification and refinement of primary Si, eutectic Si, and SDAS compared to initial specimens in the as-cast condition. After heat treating, the fine lamellar eutectic Si and intermetallic compounds underwent a higher degree of fragmentation and coarsening, appearing as small round particles, leading to a more pronounced formation of a supersaturated solid solution. Additionally, the peak aging time of the B390-P-Bi alloy was only 7 h, 2 h less than the initial specimen. After T6 treatment and addition of both 0.1 wt%P and 0.5 wt%Bi, the B390-P-Bi alloy exhibited a smaller rounded Mg3Bi2 phase at peak aging with a 62.07% size reduction compared to the B390-Bi alloy. The results show that adding P with Bi enhanced hardness, ultimate tensile strength (UTS) and elongation because of refinement of the SDAS, primary Si and eutectic Si. These refinements played a crucial role in increasing tensile strength by more effective strengthening through precipitation and grain size reduction. The B390-P-Bi alloy is a promising choice for applications requiring a balance between strength and ductility.

Experimental study on particulate matter emissions in a lab-scale pressurized reactor

Pressurized oxy-combustion (POC) is a promising carbon capture and storage (CCS) technology to improve the process efficiency and economics as compared with first-generation oxy-combustion. However, compared with traditional air combustion, pollutant emissions would be significantly changed in pressurized conditions, and the sampling method has always been disputed. A novel particulate matter (PM) sampling system, consisting of a pressurized PM10 cyclone, a three-stage aerosol dilution system, and a depressurization device was developed to investigate PM emissions at elevated pressure. After being depressurized, PM10 size distribution in the flue gas was measured by using an Electrical Low-Pressure Impactor plus (ELPI+). The PM losses in three depressurization devices were evaluated. Results show that, compared to a needle or ball valve as a pressure regulator, the designed nozzle performed better in terms of controlling system pressure and reducing PM loss in the channel. The sampling system was then used to evaluate the effect of pressure on PM formation. At pressure, there is a greater production of PM below 30 nm, while the generation of coarse mode PM is reduced. Sulfur-based compounds in the combusting particles are prone to be evaporated into the gas phase at higher pressure, which contributes to the formation of fine mode PM, while Fe, Si, and Ca are reduced significantly. The coarse mode PM is reduced at higher pressure, which is mainly due to the weak fragmentation; as a result, more PM is captured in the PM10 cyclone. Compared with O2-N2 conditions, the O2-CO2 conditions can promote the formation of the fine mode PM.

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.

Influence of post-process heat treatments on the fatigue crack propagation behaviour of a PBF-LB/M AlSi10Mg alloy

The microstructure has a great influence on short fatigue crack growth in metallic materials. Laser-based Powder Bed Fusion AlSi10Mg alloys exhibit in the as-built condition a fine fibrous Si structure and a supersaturated solid solution of Si in the α-Al matrix, which is significantly modified by heat treatments starting already at temperatures under 260 °C. This study focuses on the influence of post-process heat treatments on the microstructural evolution and the resulting fatigue crack growth resistance. As compared to the as-built condition, two heat treatments at 265 °C/1 h and at 300 °C/2 h are found to be beneficial to the fatigue crack growth resistance of the investigated material.

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.

Effect of casting temperature on halo in the solidification microstructure of direct-chill casting Al-16.5Si-4.5Cu-0.6Mg-0.1Zr alloys

The effect of casting temperature on the halo structure of direct-chill (DC) casting Al-16.5Si-4.5Cu-0.6Mg-0.1Zr alloys was investigated. The results indicated that the microstructure consists of fine primary Si, α-Al dendrite halo, and fine fibrous coupled eutectic structure, Al2Cu, and Al8FeMg3Si6 phase. The formation of halos was systematically analyzed. The temperature gradient at the solid–liquid interface front played an essential role in the development and growth of the halo. As the casting temperature elevated, the temperature gradient of the solid–liquid interface front increased, the solid–liquid interface became stable, the α-Al dendrite halo was less developed, the number of large-sized Si atomic clusters in the melt decreased and the growth of primary Si was inhibited.

Microstructure and mechanical properties of A356.2(Al-7Si-0.3Mg) alloy refined and modified by in-situ Al-SrB6 composites

In this research, in-situ Al-SrB6 composites were prepared and employed to inoculate A356.2(Al-7Si-0.3Mg) alloy. Al-SrB6 ingot and ribbon composites were fabricated by high-frequency melting and melt-spinning, respectively. They both have α-Al and SrB6 phases. In the ribbons, the SrB6 particles have nano-sized dimensions and distribute homogeneously in the Al matrix. With the addition of 1.0% Al-SrB6 ribbons (S4 alloy), A356.2 alloy show the optimal microstructures and properties. The YS, UTS, EL and HV of S4 alloy are 30.0%, 16.0%, 38.8% and 17.7% greater than those of A356.2 alloy (S0), respectively. Furthermore, Al-SrB6 ribbons with nanoparticles demonstrate better inoculant effects than ingots at the same additive content. After adding Al-SrB6 composites, the base alloy exhibits more pronounced ductile fracture characteristics. The clean and stable SrB6/Al interface, formed by Al and SrB6, facilitates heterogeneous nucleation, grain refining and strengthening for A356.2 alloy. It has a low interface crystallographic misfit and a few orientation relationships (ORs), which will be described and calculated. The synergistic effect of free elements Sr and B ultimately increases the formation of twins, which results in fine fibrous and granular Si.

Age hardening of Al-7Si-0.5Mg alloy: Role of Si size and distribution

The age-hardening behavior of the Al-7Si-0.5Mg alloy with different size and distribution of Si phase was investigated. The wedge-shaped mold was used to prepare specimens with different characteristics of Si. The specimen with fine and homogeneous Si showed accelerated age-hardening kinetics and higher peak hardness than the one with coarse and clustered Si due to the higher dislocation density resulted from the mismatch in the thermal contraction of Si and Al. The overlapping of the plastically-affected zone of the adjacent Si phase in the specimen with large and clustered Si led to a reduced volume fraction of the plastically-affected zone, lower dislocation density and lower age-hardening kinetics. The relationship between the radius of Si and the critical inter-particle spacing that minimize the interaction of adjacent Si to maximize the dislocation density and accelerate age-hardening kinetics was established.

Aluminum Cast Alloys Based on the Al–Ce–Si–Mg System: An Influence of Silicon on Crystallization, Phase Composition, and Tensile Properties

An attempt at strengthening the aluminum-cerium-based alloys through additions of silicon was assessed using the experimental Al5Ce3Si0.5Mg (wt. pct) cast hypoeutectic composition, designed based on the commercial A356 (Al–7Si–0.3Mg, wt pct) grade by substituting a portion of Si with Ce. To determine a role of Si, the Al5Ce0.5Mg (wt. pct) reference was cast and tested under identical conditions. An addition of 3 wt. pct Si to the Al5Ce0.5Mg base increased the room temperature yield stress almost three times, from 47 to 135 MPa, but reduced its elongation by an order of magnitude from 8 pct to that below 1 pct. A presence of Si led also to essential changes in the alloy crystallization with the melting range widened substantially from 10 °C to 91 °C mainly due to a reduction in the solidus level. As-cast microstructure of the Al5Ce0.5Mg base consisting the primary αAl along with 44 pct of the αAl + Al11Ce3 eutectic was replaced in the Al5Ce3Si0.5Mg alloy with the αAl + AlCeSi2 coarse lamellae ternary eutectic, bulky compounds having the Ce-rich core of AlCe2Si with external shell of Al2CeSi2 along with the quaternary eutectic of fine αAl, Si, Al2MgSi/Al2Mg2Si, and Al–Si–Mg-Ce phases. The fractographic analysis revealed that additions of Si caused a transition from largely ductile fracture to the predominantly brittle mode with crack paths propagating mainly along the interface between the coarse AlCeSi2 lamellae and Al(Si, Mg) solid solution, which explains the measured drastic reduction in the alloy elongation.Graphical

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.

Evaluation of microstructure and mechanical properties of M-SIMA processed Al–7Si alloy

In this research article, the effect of the strain-induced melt activation (SIMA) process on the microstructure and mechanical properties of the Al–7Si alloy has been investigated. The morphology of α-Al grains is changed from dendritic to fine spherical and flaky type eutectic Si to a fine fibrous eutectic Si after the modified strain-induced melt activation (M-SIMA) process. The intermetallic compounds are refined and distributed uniformly throughout the matrix after the M-SIMA process. The mechanical properties have been improved due to fine spherical grain formation with refined eutectic Si. A high tensile strength (UTS: 204 MPa) with a remarkable improvement in ductility (El: 30%) is achieved after the M-SIMA process at 585°C for 30 min in comparison to the as-cast Al–7Si alloy (UTS: 117 MPa, El: 16%).

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.

Separation mechanism of a Si+TiSi2 eutectic alloy and metal impurity Fe segregation via electromagnetic directional solidification of Ti-80 wt%Si alloy melt

The separation mechanism of Si+TiSi2 eutectic alloy and Fe atom segregation were investigated during the electromagnetic directional solidification of Ti-80 wt%Si alloy melt at pull-down rates from 5 µm/s to 20 µm/s. The experimental results indicated that a fine lamellar Si+TiSi2 eutectic alloy was obtained in the middle of the ingot after electromagnetic directional solidification. Fe impurities were captured by primary SiC particles at the bottom of the ingot, which then segregated at the top of the ingot. When the pull-down rate was 5 µm/s, the solidification front was always close to the induction coil. Electromagnetic stirring produced forced convection that promoted Si+TiSi2 eutectic alloy precipitation by mixing the melt and offsetting the temperature gradient caused by the precipitation of the primary phase, thereby inhibiting segregation. Thus, the optimal pull-down rate for fabricating Si+TiSi2 eutectic alloy was 5 µm/s. At this pull-down rate, the maximum thickness of the Si+TiSi2 eutectic alloy layer was 27 mm, and the removal rate of impurity Fe in this layer reached 98.98%.

Effect of extrusion temperatures on the microstructures, mechanical properties and thermal properties of PM Al-20Si alloy

Al-20Si alloy is fabricated by extruding pre-alloyed powder at different temperatures ranging from 350° to 550°C. Morphologies of the Si phase, mechanical properties and thermal properties of the alloy are influenced by extruding temperature. The alloy extruded at 350 °C mainly contains fine eutectic Si. Although its fine structure imparts high strength to the alloy, it is not stable in high-temperature CTE test and tends to agglomerate into large Si lumps due to Oswald ripening effect, therefore offsetting the restriction effect of Si on the thermal expansion of Al and thus increasing the CTE of the alloy. In contrast, the alloy extruded at 550 °C exhibits primary Si phase, which is large in size and more stable in the CTE test. Moreover, it has in-situ formed amorphous/δ * - Al2O3 at the grain boundaries which reduces the CTE of the alloy further. As a result, the alloy fabricated at 550 °C exhibits the best combination of thermal properties (i.e. TC 184.5 Wm−1K−1 and CTE 20.9 ×10−6 K−1 at 400 ℃).

Preparation of undecanoic acid-terminated Si particles from rice husks

Luminescent fine Si powder was prepared by an Mg reduction process of SiO2 particles extracted from rice husks. The surface of the powder was terminated by the hydrophilic molecules of undecanoic acid via the hydrosilylation process. After the hydrosilylation of the fine Si powder obtained from rice husks, surface termination by undecanoic acid was confirmed to suppress natural surface oxidation. Transmission electron microscopy observations of the rice husk fine Si powder showed that the Si nanoparticles have a crystalline structure. Room temperature photoluminescence in the wavelength range of 500–1000 nm was observed in the samples taken at each step of the process, and the stability of the luminescence properties was improved via undecanoic acid surface termination. The observation of wavelength dependent microsecond-order photoluminescence indicates that the photoluminescence was due to quantum size effects. On the basis of time-transient light transmittance of rice husk Si colloid, the surface termination of undecanoic acid improved the dispersibility of Si particles in distilled water. The production of luminescent rice husk Si colloid is effective for the preparation of luminescent bio labels and will lead to high value-added recycling of rice husks.