Cast Al Si Research Articles

Pulsed Current Effect on the Hard Anodizing of an AlSi10Mg Aluminum Alloy Obtained via Additive Manufacturing

The hard anodizing treatments of cast Al-Si alloys are notoriously difficult. Indeed, their microstructural features hinder the growth of a uniform, compact, and defect-free anodic oxide. In this paper, AlSi10Mg samples, produced via Gravity Casting (GC) and Additive Manufacturing, i.e., Laser Powder Bed Fusion (L-PBF), were hard anodized in a sulfuric acid bath, in order to verify how the particular microstructure obtained via L-PBF affects the thickness, hardness, compactness, and defectiveness of the anodic oxide. Moreover, for the first time, Pulsed Direct Current (PDC) procedures were used to perform the hard anodizing treatments on additively manufactured AlSi10Mg alloy. Several combinations of temperature and electrical parameters, i.e., current density, frequency, and Duty Cycle, were tested. The anodized samples were characterized through optical microscopy analysis, Scanning Electron Microscopy (SEM) analysis, and accelerated corrosion tests, i.e., Potentiodynamic Polarization (POL) and Electrochemical Impedance Spectroscopy (EIS) measurements. The PDC procedures allowed improvement of the compromise between evenness, compactness, and defectiveness. Among the attempted PDC procedures, a specific combination of electrical parameters and temperature allowed the best results to be obtained, i.e., the highest hardness and the lowest volumetric expansion values without compromising the oxide quality rating and the corrosion resistance behavior. However, none of the attempted PCD strategies allowed the hardness values obtained on samples produced via GC to be reached.

Combined effect of Sr-addition and pressure induced solidification on eutectic-Si morphology and mechanical properties of squeeze cast Al-Si binary alloy

Combined effect of Sr-addition and pressure induced solidification on eutectic-Si morphology and mechanical properties of squeeze cast Al-Si binary alloy

Effect of Zn Addition on the Microstructural Heterogeneity and Mechanical Properties of Vacuum‐Assisted High‐Pressure Die Casting Al–Si–Mg–Cu alloys

Herein, Al–8Si–0.5Mn–0.5 Mg–0.2Cu–xZn alloys (x = 0, 1, 2, and 3 wt%) are produced by a vacuum‐assisted high‐pressure die casting process. The effects of Zn addition on the surface layer thickness and segregation band, the number density of externally solidified crystals (ESCs), and mechanical properties of the alloys are systemically investigated. As the Zn content increases, the size and number density of the ESCs increase. The morphology and size of the eutectic Si phase could be modified by the addition of Zn. In addition, the microstructural heterogeneity indicates that the thickness of the surface layer and segregation band zone increase with increasing Zn content. Proper Zn addition can effectively promote the improvement of strength and elongation in the as‐cast state. When the Zn content is 2 wt%, the yield strength, ultimate tensile strength, and elongation of the alloy are 167, 311 MPa, and 10.8%, respectively.

Nugget zone characterization of friction stir welded hypereutectic Al-Si alloy

Despite excellent properties of aluminium silicon (Al-Si) alloys in several aircraft and automotive applications, its usage is limited because near net shape casting of Al-Si alloys may cause several defects like surface pinholes and grooves. Fabrication of components and then welding them via fusion-based welding processes imposes issues like porosity and slag inclusions which may severely degrades its properties. Thus in present study, friction stir welding (FSW) was employed for welding of hypereutectic Al-Si cast alloy. The nugget zone (NZ) of welded alloy and its as-cast counterparts were characterized for their performance in terms of mechanical and tribological behaviour. The results depicted fragmentation of α-Al dendrites, refinement and redistribution of acicular Si particles as a result of stirring with high strain rate, and improvement in mechanical properties such as ultimate tensile strength, yield strength, ductility, hardness and wear resistance as compared to as-cast alloy. It is also shown that disparate wear mechanisms occured in friction stir welded (FSWed) and as-cast alloys. A sizeable amount of adhesion with plastic deformation and delamination pits were observed for as-cast alloy, while the dominant mechanism in FSWed alloy was peeling with little adhesion.

Microstructural evaluation in dissimilar liquid phase bonding of cast Al–Si alloy and stainless steel using pure zinc filler metal

The metallurgical joining between cast A359 aluminium alloy and AISI321 stainless steel using pure Zn as the interlayer was performed slightly above Al–Zn eutectic temperature. The liquid phase was formed by a eutectic reaction between Al and Zn and subsequently isothermally solidified by diffusion of Zn into the A359 base metal (i.e. reactive transient liquid phase bonding). The formation of a Zn-rich diffusion affected zone in A359, incorporation of Si particles into the bond zone, formation of a thin Al–Fe–Si intermetallic at the steel interface, and isothermal solidification of the liquid phase and subsequent Al–Zn eutectoid transformation govern the microstructure of the joint. The resultant bond microstructure does not deleteriously affect the heat sink function of Al base metal.

Revealing the correlation of microstructure configuration and mechanical properties of Al–Si–Mg alloy reinforced by C-doped TiB2 and SiC

A new Al–Si–Mg alloy was prepared and refined by C-doped TiB2 (C-TiB2) and strengthened by SiC, and the effects of microstructure configuration on the mechanical properties of the alloys were investigated in this study. The results show that the eutectic Si and reinforcing particles distribute in a network configuration in the cast Al–Si–Mg alloy with globular grains. In addition, the distribution of the eutectic Si and reinforcing particles is streamlined along the extrusion direction after hot extrusion. The streamlined distribution of eutectic Si and reinforcing particles is beneficial to the ductility of the alloy, the elongation of which is almost twice that of the Al–Si–Mg alloy with a network configuration that possesses higher strength. The reinforcing phase with a network configuration can better withstand the stress and prevent the deformation of the matrix, thus improving the strength of the alloy. By contrast, the reinforcing phase with a streamlined configuration provides conditions for the plastic deformation of the matrix and hinders the contact of microcracks in the matrix, which increases the elongation of the alloy.

On the origin of deformation mechanisms in a heterostructured aluminum alloy via slip trace and lattice rotation analyses

In this study the micro-deformation mechanisms in an Al–Si cast alloy under stepwise compressive loading up to a cumulated plastic strain of 9.7% are examined via detailed EBSD and slip trace analyses. The presence of heterogeneous microstructural features in the alloy like eutectic structure and second-phase particles significantly affects slip trace characteristics. Four types of slip traces (namely, bowed, ‘C’-type curved, ‘S’-type curved, and flat slip traces) present in primary α-Al grains are identified, which unveil the nature of dislocation motion in the alloy and reflect the extent of deformation difficulty associated with α-Al grain orientations and eutectic Si particles. Crystallographic factors like lattice rotations, misorientation angles, active slip systems, and Schmid factors are jointly analyzed for the first time to understand the deformation behavior of the heterogeneous alloy.

Recycling of titanium as reinforcement in cast aluminium alloy A356 for improving the microstructural refinement and mechanical performance towards a sustainable material development

A356 is one of the most commonly used Al-Si cast alloy containing copper, magnesium and titanium, as other predominant alloying elements. Due to its wide application in domains where dominant loading is compressive in nature, various research work is being carried out to improve the compressive performance of A356 alloy by adding particulate reinforcement in it, converting it to metal matrix composite. However, ceramic particles added as reinforcement have always been posing challenges due to the poor wettability and weak interface bonding. In the present research work, an attempt is made to recycle titanium in a greener way of adding it to Aluminium alloy as a strengthening reinforcement which shall address the interface boding issues due to better compatibility of Titanium and Aluminum – both being metals. 5% by weight of micro and nano-sized Titanium particles is added to A356 through stir casting method. Cast material was then subjected to hot extrusion. The as-cast and extruded composites are studied for the micro structural evolution and mechanical performance against the base alloy. Results indicate that the performance of the composite is found to be significantly enhanced as compared with the base alloy in terms of strength, hardness and ductility. Increased mechanical performance is attributed to refinement of dendrite size, presence of uniformly distributed Ti particle, and partial diffusion of Ti into the Al matrix.

Effects of Cu/Er on Tensile Properties of Cast Al-Si Alloy at Low Temperature.

The current protocol presents the effects of the addition of Cu, rare earth Er, and Cu-Er composite elements on the microstructure of the Al-10Si-0.3Mg alloy. The variations in their low-temperature tensile properties were also investigated. The addition of rare earth Er elements, Cu elements, and Cu-Er composite elements increased the strength of all three groups of alloys when stretched at low temperatures (-60 °C). Further, the elongation of the alloy increased with the addition of Er, while the elongation of the other two groups decreased. The low-temperature (-60 °C) tensile strength of the alloy with the same composition was higher than that at room temperature (20 °C), but the elongation decreased. Notably, by adding rare earth Er to the Al-10Si-0.3Mg alloy, the three-dimensional morphology was changed from coarse dendritic to fine fibrous, the secondary dendritic arm spacing (SDAS) of the alloy was reduced, and the grains were refined. The Al2Cu phase, Al-Si-Cu-Mg quaternary phase, and Cu-rich phase appeared in the alloy with the addition of Cu elements, but the Si phase morphology and α-Al dendrites were not significantly improved. Interestingly, the Si phase morphology of the alloy was improved by adding Cu-Er composite elements, and SDAS was reduced. Still, the Al2Cu phase, Al-Si-Cu-Mg quaternary phase, and Cu-rich phase were not much improved.

Thermo-Mechanical Fatigue Behavior and Resultant Microstructure Evolution in Al-Si 319 and 356 Cast Alloys.

The out-of-phase thermo-mechanical fatigue (TMF) behavior of the two Al-Si cast alloys most widely used for engine applications (319 and 356) were investigated under temperature cycling (60-300 °C) and various strain amplitudes (0.1-0.6%). The relationship between the microstructural evolution and TMF behavior was closely studied. Both alloys exhibited asymmetric hysteresis loops with a higher portion in the tensile mode during TMF cycling. The two alloys showed cyclic softening behavior with regard to the maximum stress, but an earlier inflection of cyclic stress was found in the 356 alloy. The TMF lifetime of the 319 alloy was generally longer than that of the 356 alloy, especially at higher strain amplitudes. All the precipitates (β'-MgSi in 356 and θ'-Al2Cu in 319) coarsened during the TMF tests; however, the coarsening rate per cycle in the 356 alloy was significantly higher than that in the 319 alloy. An energy-based model was applied to predict the fatigue lifetime, which corresponded well with the experimental data. However, the parameters in the model varied with the alloys, and the 356 alloy exhibited a lower fatigue damage capacity and a higher fatigue damage exponent.

Mechanism of Corrosion of Cast Aluminum-Silicon Alloys in Seawater. Part 1: Characterization and Field Testing of Bare Alloys in the Adriatic Sea

Aluminum-silicon cast alloys are increasingly used in various applications, including marine. Two Al-Si cast alloys were investigated as materials for marine exposure: Al-Si9-Cu3 and Al-Si7-Mg0.3. Microstructure, chemical composition, and electrochemical properties of nonimmersed alloys were studied, revealing their heterogeneous nature. Eight months of field testing by immersion in the Adriatic Sea was performed to test long-term corrosion and biofouling resistance. Morphological and chemical changes at the surface induced by immersion were investigated using surface analytical techniques. Top and cross-section surfaces were investigated. Electrochemical measurements of immersed samples in artificial seawater revealed that alloys were re-passivated by micrometer-thick oxide layers. Al-Si9-Cu3 alloy forms a layer rich in magnesium, while that formed on Al-Si7-Mg0.3 is rich in silicon. Electrochemical results indicate that both alloys are suitable for use in marine environments, where Al-Si7-Mg0.3 is more resistant than Al-Si9-Cu3. Biofouling formed up to one month of immersion can be removed by sonication. At the end of the immersion period, macro biofouling was firmly attached to the alloys’ surface, as evidenced by scanning electron microscopy.

Uniform fatigue damage tolerance assessment for additively manufactured and cast Al-Si alloys: An elastic-plastic fracture mechanical approach

• Influence of additively manufacturing and casting on microstructure and porosity characteristics as well as quasi-static and cyclic stress-strain behavior of Al-Si alloys. • Evaluation of Murakami-Noguchi approach and Shiozawa approach for fatigue limit and lifetime estimation, however, both approaches are not sufficient for AM Al-Si alloys. • Linking J integral (elastic-plastic fracture mechanics) including cyclic stress-strain curve by means of morrow parameter (K’, n’), defect size and position, stress amplitude and stress ratio. • New approach enables a uniform fatigue damage tolerance assessment of fatigue lifetime and limit for additively manufactured alsi10mg alloy as well as die-cast and sand-cast alsi7mg alloy. Lightweight Al-Si alloys are used in the automotive and railway industry due to their excellent strength-to-weight ratio and near-net-shape manufacturing. In additive manufacturing and casting, the near-net-shape manufacturing results in a non-homogenous solidification and cooling of the parts, leading to significant local gradient in microstructural and defect features as well as deformation behavior. In this paper, a uniform damage tolerance assessment was developed based on fracture mechanical approaches of Murakami (√area) and Shiozawa for a reliable defect-based mechanical design of fatigue-loaded structures. The linear-elastic fracture mechanical (LEFM) approaches of Murakami and Shiozawa were used to calculate defect-based lifetime curves, where the cyclic stress intensity factor (ΔK) at the failure-initiating defect (√area) was used to describe the local stress concentration conditions, called K-N curves, instead of nominal stress-based S-N curves. The LEFM-based K-N curves did not allow to describe the fatigue behavior in terms of a unified fatigue design of AM and casting material. Therefore, the cyclic stress-strain (CSS) behavior was used for a plasticity-modification of the LEFM approach by calculating the effective cyclic J integral (ΔJ eff ) to plot J-based K-N curves, called Kj-N curves. This elastic-plastic fracture mechanical (EPFM) approach allowed a uniform fatigue damage tolerance (FDT) assessment of AM and cast Al-Si alloys for the HCF regime by FDT law and fatigue limit by FDT limit. Exemplarily, the FDT limit could be used to predict synthetic Haigh diagrams for specific AM and cast batches and derive process- and service-relevant knowledge on the effect of tensile or compressive mean stress.

Uniform fatigue damage tolerance assessment for additively manufactured and cast Al-Si alloys: size and mean stress effects

• Influence of additively manufacturing and casting on microstructure and porosity characteristics as well as quasi-static and cyclic stress-strain behavior of Al-Si alloys • Linear-elastic (LEFM: Murakami, Murakami-Schijve) and elastic-plastic fracture mechanics (EPFM: Fischer) were used for determination of stress intensity factor at failure-initiating defect and plotted in Shiozawa diagrams • LEFM and EPFM enables to consider the size effect within fatigue damage tolerance assessment • Benchmark of Woehler, Murakami, Murakami-Schijve and Fischer approaches for fatigue damage tolerance assessment regarding size effect and mean stress effect, whereby only Fischer approach enables a uniform fatigue damage tolerance assessment The near-net-shape manufacturing in additive manufactured and cast of Al-Si alloys results in a heterogeneous solidification and cooling of the parts, leading to significant gradients in microstructural and defect features as well as deformation behavior. In this paper, an elastic-plastic fracture mechanical model for uniform fatigue damage tolerance assessment of Al-Si alloys was further qualified and extended for a uniform view of different testing volumes as well as various stress ratios between R = -2…0.5 based on the fracture mechanical approaches of Murakami (√area) and Shiozawa for a reliable main crack defect-based mechanical design of fatigue-loaded structures. The linear-elastic fracture mechanical (LEFM) approaches of Murakami, Murakami-Schijve and Shiozawa were used to calculate defect-based lifetime curves, where the cyclic stress intensity factor (ΔK) at the failure-initiating defect (√area) was used to describe the local stress concentration conditions (so-called K-N curves) instead of nominal stress-based S-N curves. The LEFM-based K-N curves did not allow a unified assessment of fatigue behavior. Therefore, the cyclic stress-strain (CSS) behavior (K’, n’) was used for a plasticity-modification of the LEFM approach based on the elastic-plastic fracture mechanical (EPFM) approach of Fischer by calculating the effective cyclic J integral (ΔJ eff ) to plot J-based K-N curves, called Kj-N curves. This EPFM approach could be qualified for a uniform and reliable fatigue damage tolerance assessment of AM and sand cast Al-Si alloys for the HCF regime.

INFLUENCE OF DIE CASTING SPEED ON PROPERTIES OF CASTING BASED ON AL-SI ALLOYS

The paper focuses on the research of the influence of chosen technological parameters of die casting on the selected mechanical properties of castings based on Al-Si alloys. The experiments conducted within the frame of the practical part examined the plunger pressing speed in the mould cavity influencing the mechanical properties of a casting which were represented by ultimate tensile strength and percentage share of porosity of Al-Si castings. Plunger pressing speed is closely related to mould cavity filling mode which affected the final porosity of casting. In the case of increased percentage porosity share, the ultimate tensile strength decrease was detected due to the reduction of the cross-section area of the casting. The increase in percentage porosity share in the casting was caused by the increase in plunger pressing speed. To achieve the desired quality of castings, higher production efficiency, and reduced occurrence of faulty pieces during production, an optimal set of technological parameters of die casting is inevitable.

Solidification microstructure manipulation mechanism of hypoeutectic and hypereutectic Al–Si alloys controlled by trace in-situ nano-Fe2B and nano-Fe3Si

In aluminum alloys, the most often utilized cast Al–Si alloy system, the microstructure often needs to be improved by modification and inoculation treatment. This study comprehensively examines the impact of nanocrystals produced by Fe–B–Si amorphous alloys on the solidification behavior and microstructure of hypoeutectic Al–Si7 alloy and hypereutectic Al–Si17 alloy. The nucleation temperature of α-Al is increased and the nucleation of α-Al is easier as well as crystallization platform becomes shorter. For primary Si, the nucleation temperature is decreased and the nucleation is inhibited. The eutectic transformation is promoted and the reaction time is shortened. For α-Al, in situ nano-Fe2B can act as its heterogeneous nucleation sites and promote its nucleation. Meanwhile, the growth of α-Al dendrites is constrained by nearby grains and nanocrystals that do not participate in nucleation, therefore refining the dendrites. Furthermore, nano-Fe2B promotes the eutectic transformation and refines the eutectic microstructure. At the same time, the microstructure of the eutectic groups is refined and the distribution is more dispersed due to the confinement of finer adjacent grains and the hindrance of nanocrystals that do not participate in nucleation.

Optimization of Thermo-Mechanical Fatigue Life for Eutectic Al–Si Alloy by the Ultrasonic Melt Treatment

The eutectic cast Al–Si alloys with excellent high-temperature and casting performance are widely used in engine pistons. During frequent starts and stops, the thermo-mechanical fatigue (TMF) is the most important failure cause. Ultrasonic melt treatment (UT) was chosen to compare and investigate the influence of micro-structures on fatigue life and damage mechanisms of as-cast (AC) eutectic Al–Si alloys under TMF loading. After UT, the grain size, primary Si, and intermetallic particles are reduced significantly in the alloy; fatigue life increases obviously. As a result of pilling-up of dislocations, the competitive effects of the critical strain/stress for fatigue crack nucleation can be found. There are two different crack initiation mechanisms under TMF: one is primary Si fracture for AC alloys with limited critical strain/stress for fatigue crack nucleation at fractured Si particles, and the other is primary Si debonding for UT alloys with increasing critical fracture strain/stress. After the crack initiation, the fractured or debonded primary phases provide the advantages for the further development of main cracks for both alloys. The UT alloy (805 ± 253 cycles) has about twice the TMF life of the AC alloy (403 ± 98 cycles). The refinement of micro-structures is instrumental in improving the fatigue resistance and life of TMF for the UT alloy.

Equiaxed grain formation by intrinsic heterogeneous nucleation via rapid heating and cooling in additive manufacturing of aluminum-silicon hypoeutectic alloy

The high strength of Al-Si hypoeutectic alloys additively manufactured by powder-bed fusion is of great scientific interest. To date, the mechanism of grain refinement near the fusion line, which contradicts conventional Hunt’s columnar–equiaxed transition criteria, remains to be elucidated. Here we present the first report on the mechanism of grain refinement. When a laser was irradiated on cast Al-Si alloy consisting of coarse α-Al grain and α-Al/Si eutectic regions, grain refinement occurred only near the eutectic regions. This strongly suggests that the Si phase is crucial for grain refinement. Multi-phase-field simulation revealed that rapid heating due to the laser irradiation results in unmelted Si particles even above the liquidus temperature and that the particles act as heterogeneous nucleation sites during the subsequent re-solidification. These results suggest the feasibility of a novel inoculant-free grain refinement that is applicable to eutectic alloys comprising phases with a significant melting point difference. • The grain refinement near fusion line in Al-Si alloy irradiated with laser was investigated by experiment and multi-phase-field simulation. • The heating is so rapid that Si particles remain unmelted and act as heterogeneous nucleation sites in the subsequent cooling. • Inoculant-free grain refinement is suggested to be possible and applicable to eutectic alloys comprising phases with a large gap of melting point.

Microstructure- and plasticity-based fatigue and defect tolerance assessment of age-hardenable Al-Si cast alloys in LCF and HCF regime

Due to their excellent strength-to-weight ratio and near-net-shape manufacturing possibility, precipitation hardening Al-Si cast alloys are promising candidates for cost-efficient lightweight components in the automotive and railway industry. In this work, the microstructure and the fatigue properties of cast AlSi7Mg0.3-T6 (A356-T6) were analyzed, whereby the influence of a variation in solution annealing, hot isostatic pressing (HIP) densification, solidification rates as well as casting processes, i.e., gravity die and sand casting on the fatigue behavior and fatigue damage tolerance was investigated. Sand castings exhibit a decelerated fatigue crack initiation and enhanced LCF damage tolerance compared to die castings.

Corrosion and tribocorrosion protection of novel PEO coatings on a secondary cast Al-Si alloy: Influence of polishing and sol-gel sealing

PEO coatings have been developed on a secondary cast Al-Si alloy, using a novel aluminate-based electrolyte, containing Na4P2O7, Na2WO4 and K2TiF6. The protective performance of the PEO coatings has been improved throughout the application of two post-treatments: polishing of the outermost coating layer and sealing with a sol-gel layer. While the sol-gel has notably improved the corrosion resistance and dielectric properties, due to the sealing of surface pores, the decrease in surface roughness achieved by polishing have enhanced the tribocorrosion performance. The post-treatments studied would enable the future application of PEO coatings on components with high requirements in corrosive environments.

Effect of Zn content on microstructure and properties of hypoeutectic and near-eutectic Al-Si alloys

In this research, the microstructure and mechanical properties of permanent mould cast Al-Si (7 wt.% and 12 wt.%) alloys without and with different amounts of Zn (1 wt.%, 2 wt.% and 4 wt.%) were analysed. The Al-xSi-yZn alloys have been prepared through a stir casting process using an Al-15 wt.% Si master alloy at 800 °C. Optical microscopic analyses have revealed variation in the α-Al matrix and Si needles in the alloy with the variation in Zn content. The mechanical properties analysis has shown that the hardness, tensile strength and wear resistance of the alloys increase with the addition of Si but decrease with the addition of Zn, within the limit of the alloying additions of the present research.