High-cycle fatigue and fracture behaviours of SLM AlSi10Mg alloy

The high-cycle fatigue and fracture behaviours of the selective laser melting(SLM)AlSi10Mg alloy were investigated.Flat specimens were designed directly in the shape required for the fatigue tests under pulsating loading in tension(R=0,R is the dynamic factor).The fatigue−life(S−N)curves were modelled with a conditional Weibull’s probability density function,where the real-valued genetic algorithm(GA)and the differential ant-stigmergy algorithm(DASA)were applied to estimating the needed Weibull’s parameters.The fractography of the fatigue specimens showed that the fatigue cracks initiated around the surface defects produced by SLM and then propagated in an unstable manner.However,the presence of large SLM defects mainly influenced the crack initiation period and did not have a strong influence on the crack propagation.The obtained experimental results present a basis for further investigation of the fatigue behaviour of advanced materials and structures(e.g.cellular metamaterials)fabricated by additive manufacturing(AM).Especially,in the case of two-dimensional cellular structures,the cross-section of cellular struts is usually rectangular which corresponds to the specimen shape considered in this work.

Microstructure evolution of modified die-cast AlSi10MnMg alloy during solution treatment and its effect on mechanical properties

To optimize the solution treatment process of a modified high-pressure die-cast AlSi10MnMg alloy, the influence of the solution treatment on the microstructure, mechanical properties and fracture mechanisms was studied using OM, SEM, EBSD and tensile test. The experimental results suggest that the solution treatment could be completed in a shorter time at a temperature much lower than the conventional practice. Surface blistering could be avoided and substantial strengthening effect could be achieved in the following aging process. Prolonging solution treatment time and elevating solution temperature would be meaningless or even harmful. The rapid evolution of eutectic silicon during solution treatment, especially at the early stage, affected the way of interaction among α-Al grains during plastic deformation, and changed the ultimate mechanical properties and fracture mode.

Microstructures of an AlSi_7Mg alloy by near-liquidus casting

Semi-solid ingots of an A1SiTMg alloy were obtained using the method of near liquidus casting. Their microstructures exhibit the characteristics of free, equiaxed, and non-dendrite, which are required for semi-solid forming. The influences of casting temperature, heat preservation time, and cooling rate on the microstructure were also investigated. The results show that in the temperature region near liquidus the grain size becomes small with a decrease in casting temperature. Prolonging the heat preservation time makes grain crassitude at the same temperature. And increasing the cooling rate makes grain fine. The microstructure of the alloy cast with iron mould is freer than that with graphite mould.

Preparation of semisolid AlSi7Mg alloy slurry through weak traveling-wave electromagnetic stirring

The semisolid AlSi7Mg alloy slurry with large capacity was prepared by low superheat pouring and week traveling-wave electromagnetic stirring. The effects of electromagnetic stirring power and frequency on the shape and distribution of primary α-A1 grains in the AlSi7Mg alloy slurry were discussed. The experimental results show that the AlSi7Mg alloy slurry with fine and spherical primary α-A1 grains distributed homogeneously can be obtained. Under the condition of low superheat pouring and week traveling-wave electromagnetic stirring, when the pouring temperature is 630℃, raising the stirring power or frequency appropriately can gain a better shape of primary α-Al grains; but if the stirring power or frequency is increased to a certain value （1.72 kW or10 Hz）, the shape of primary α-A1 grains cannot be obviously improved and spherical primary α-Al grains distributed homogeneously can be still obtained.

Effect of Pouring Process on the Microstructures of Semi-Solid AlSi_7Mg Alloy

The effect of different pouring temperatures and different pouring heights, the distance between the mouth of the pouring ladle and the top of the mold, on the microstructure of AlSi7Mg alloy have been researched in the paper. When the pouring temperature is close to the liquidus temperature, the primary alpha -Al in 'the billets of AlSi7Mg alloy solidified into spherical and nodular fine grains distributed homogeneously. The optimum pouring temperature for semi-solid AlSi7Mg billet with spherical or nodular primary alpha -Al is 615 degreesC. At the same pouring temperature, the higher the pouring ladle, the more easily the spherical and nodular primary alpha -Al obtained in the semi-solid AlSi7Mg billet. When the pouring temperature is close to the liquidus temperature and the pouring ladle is relatively high, it is the great cooling rate, the flow of the molten allay caused by pouring and the large simultaneous solidification region induced by the near liquidus temperature, that promote the formation of spherical or nodular primary cr-Al.

New method for the preparation of semi-solid AlSi7Mg alloy slurry

The effects of pouring temperature and short electromagnetic stirring with low strength on the microstructures of AlSi7Mg alloy were investigated. The results show that if liquid AlSi7Mg alloy is poured at 630 or 650℃, many primary α-Al grains in the solidified melt are rosette-like, and only a small number of them are spherical. However, if liquid AlSi7Mg alloy is poured at a lower superheat and meanwhile is stirred by an electromagnetic field at a low power for a short time, then most primary α-Al grains in the solidified melt become spherical, and only a few are rosette-like. The theoretical analysis indicates that the strengthened melt flow motion induced by the short electromagnetic stirring makes the temperature field more homogeneous in the melt, which is poured at a lower superheat, and makes the primary α-Al grains deposit in a larger region at the same time, so this new solidification kinetic condition leads to the microstructure of spherical primary α-Al grains. The experiments also demonstrate that pouring at an appropriate superheat and stirring by an electromagnetic field at a low power for a short time is a good new method for preparing the semi-solid slurry or billet of AlSi7Mg alloy.

Effect of pouring height on the solidified microstructure of AlSi7Mg alloy

The effect of different pouring heights and evenly soaking process in the liquidus and solidus range on the solidified microstructure of AlSi7Mg alloy has been studied. The results show that if the pouring temperature is 630 or 650℃ and the pouring height is 40 mm, the microstructure of the solidified melt is not homogeneous and there are many rosette-like primary α-Al grains. But if the pouring height is increased to 400 mm, the solidified microstructure becomes more homogeneous and favorable to obtain spherical primary α-Al grains in the solidified melt. With further being evenly soaked in the liquidus and solidus range for some time, the temperature difference between the melt center and the melt periphery can be controlled within ±2℃ and the primary α-Al grains will evolve into more spherical grains. The theoretical analysis indicates that the higher pouring height promotes the melt flow motion and makes the temperature field in the melt more homogeneous and restrains the large rosette primary α-Al grains. This flow motion can also promote the ripening effect and the primary α-Al grains in the melt are gradually changed into spherical grains. It can be concluded from the experiments that pouring at an appropriate superheat and from a proper height is a good new method for preparing the semisolid slurry of AlSi7Mg alloy, its process control is easy and the preparation cost is lower.

Microstructure and properties of AlSi7Mg alloy fabricated by selective laser melting

The AlSi7Mg alloy was fabricated by selective laser melting(SLM),and its microstructure and properties at different building directions after heat treatment were analyzed.Results show that the microstructure of SLM AlSi7Mg samples containes three zones:fine grain zone,coarse grain zone,and heat affected zone.The fine-grain regions locate inside the molten pool,and the grains are equiaxed.The coarse-grain regions locate in the overlap of molten pools.After T6 treatment,the microstructure at the molten pool boundary is still the network eutectic Si,but the network structure becomes discrete,and is composed of intermittent,chain-like eutectic Si particles.The yield strength at three directions(xy,45°,z direction)of the AlSi7Mg alloy samples fabricated by SLM is improved after T6 heat treatment.The fracture mechanism of the samples is a mixed ductile and brittle fracture before heat treatment and ductile fracture after heat treatment.

High-strength friction stir welding of selective laser melted AlSi10Mg alloys

Selective laser melted AlSi10Mg alloys with eutectic network structures were successfully joined by friction stir welding.Sound butt-lap joints have been achieved.A novel enlarged pin was designed to overcome kissing bonding defects and increase interfacial bonding area.The hook defect in a conventional butt-lap joint was avoided due to the limitation of the upward flow of interfacial materials,and the interfacial joining width was 2.5 times of the plate thickness.The eutectic Si network structure was broken into the re-dispersed rich-Si phases,improving joint performance.The ultimate tensile strength reached 83.1%of the matrix,higher than those of conventional melting techniques.

Preparation for Semi-Solid Aluminum Alloy Slurry under Weak Electromagnetic Stirring Conditions

The effects of pouring temperature, short electromagnetic stirring with low strength and then soaking treatment on the microstructure of AISi7Mg alloy were investigated. The results show that if AlSi7Mg alloy is poured at 630 or 650℃ and meanwhile stirred by an electromagnetic field at a low power for a short time, the pouring process can be easily controlled and most solidified primary α-Al grains become spherical and only a few of them are rosette-like. Weak electromagnetic stirring makes the temperature field more homogeneous and makes the primary α-Al grains disperse in a larger region, which leads to the spherical microstructures of primary α-Al grains. When the AISi7Mg alloy is soaked or reheated at the semisolid state, the primary α-Al grains ripen further and they become more spherical, which is favorable to the semi-solid forming of AlSi7Mg alloy.

Influence of process parameters and aging treatment on the microstructure and mechanical properties of Al Si8Mg3 alloy fabricated by selective laser melting

Many studies have investigated the selective laser melting(SLM)of AlSi10Mg and AlSi7Mg alloys,but there are still lack of researches focused on Al-Si-Mg alloys specifically tailored for SLM.In this work,a novel high Mg-content AlSi8Mg3 alloy was specifically designed for SLM.The results showed that this new alloy exhibited excellent SLM processability with a lowest porosity of 0.07%.Massive lattice distortion led to a high Vickers hardness in samples fabricated at a high laser power due to the precipitation of Mg_(2)Si nanoparticles from theα-Al matrix induced by high-intensity intrinsic heat treatment during SLM.The maximum microhardness and compressive yield strength of the alloy reached HV(211±4)and(526±12)MPa,respectively.After aging treatment at 150℃,the maximum microhardness and compressive yield strength of the samples were further improved to HV(221±4)and(577±5)MPa,respectively.These values are higher than those of most known aluminum alloys fabricated by SLM.This paper provides a new idea for optimizing the mechanical properties of Al-Si-Mg alloys fabricated using SLM.

Effect of withdrawal rate on microstructure of a hypoeutectic Al-Si alloy by low superheat DC casting

To obtain the billet with homogeneous and spheroidized primary grains is the key step in the semi- solid forming process. Among the semi-solid billet preparation methods, the process of low-superheat direct chill （DC） casting is simpler and more effective. In this study, the billets of AISi7Mg alloy were prepared by low- superheat DC casting at various withdrawal rates. The effect of withdrawal rate on the surface quality of the billets was investigated, and the evolution mechanism of the microstructural morphology in the billets was analyzed. The results show that the periodic corrugations and a large quantity of fine shrinkage pits appear in the billet surface when the withdrawal rate is 100 mm.min-1, and the above defects in the billet surface can be eliminated completely when the withdrawal rate is above 150 mm.min-1. But when the withdrawal rate is too fast （250 mm.min-1）, the primary a grains, except the ones in the billet center, have not enough time for ripening due to the high silidification rate, and will solidify as the dendrite structure. When the withdrawal rate is between 150 - 200 mm.min-1, the dendritic growth of the primary a grains is effectively inhibited, and a billet of AISi7Mg alloy with a smooth surface and homogeneous, fine, non-dendritic grains can be obtained.

Grain Refinement by Electromagnetic Stirring and the Impact on the Mechanical Properties of AlSi-Alloys

This present study examines the directional solidification of A1Si7 alloys from a water cooled copper chill.A rotating magnetic field was used for melt agitation.Different magnetic field configurations were considered to demonstrate the impact of diverse flow conditions on the resulting microstructure and the mechanical properties.The solidified structure was evaluated in comparison to an unaffected solidified ingot.Measurements of the phase distribution, the grain size,the hardness and the tensile strength were performed.Our results reveal the potential of magnetic fields to control the grain size,the formation of segregation freckles and the mechanical properties.In particular,time-modulated rotating fields demonstrated their capability to homogenize both the grain size distribution and to improve the mechanical properties.

Flow Control During Solidification of AlSi-Alloys by Means of Tailored AC Magnetic Fields

This paper presents an experimental study which in a first stage is focused on obtaining quantitative information about the isothermal flow field exposed to various magnetic field configurations.Melt stirring has been realized by utilizing time-modulated AC magnetic fields in different variants.We consider time-modulated fields or combinations of traveling magnetic fields(TMF)and rotating magnetic fields(RMF).In a second step solidification experiments are carried out to verify the effect of a certain flow field on the solidification process.Our results demonstrate that the melt agitation using modulated magnetic fields offers a considerable potential for a well-aimed modification of casting properties by an effective control of the flow field.

Effects of direct aging treatment on microstructure,mechanical properties and residual stress of selective laser melted AlSi10Mg alloy

Direct aging treatment is an important post-processing procedure,yet little research has been done on how it balances the mechanical properties and the stress removal for selective laser melted(SLMed)AlSi10Mg alloys.Here,we proposed a typical direct aging treatment on SLMed AlSi10Mg alloys,and studied the effects on their microstructure,properties and residual stress evolution.The results indi-cate that the as-built microstructure is mainly composed of fine cellularα-Al and reticulated Si phases,and some pre-existing precipitates and dislocations are found in these cells.The direct aging treatment promotes the precipitation of nano-scaled Si phases and preserves a network-like Si structure.Therefore,the strength of the peak-aged alloy increases while the ductility decreases.As the aging temperature in-creases from 160 to 200℃,aging hardening behavior was accelerated significantly.Aging at 160℃ for 4-9 h removes 32.0%-43.0%of the residual stress,which is attributed to the decomposition of the su-persaturatedα-Al matrix,the precipitation of the nano-Si phase and the exposure of low-angle grain boundaries(LAGBs).Considering the overal alloy performance obtained,over-aging at 160℃ for 4 h is the optimized heat treatment regime.Under this condition,the yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)of the alloy in the transverse and longitudinal direction are 309.5 MPa,464.4 MPa and 8.3%and 286.4 MPa,464.9 MPa and 5.1%,respectively.

Microstructure and Mechanical Properties of AlSi10Mg Alloy Manufactured by Laser Powder Bed Fusion Under Nitrogen and Argon Atmosphere

In order to study the effect of gas atmosphere on forming performance of laser powder bed fusion(LPBF),AlSi10 Mg alloy was prepared by direct forming and in situ laser remelting under the shielding gas of argon and nitrogen in this study,and its micro structure and properties were characterized and tested,respectively.The results show that the forming performance of AlSi10 Mg under nitrogen atmosphere is better than that of argon.Moreover,in situ laser remelting method can effectively enhance the relative density and mechanical properties of AlSi10 Mg,in which the densification is increased to 99.5%.In terms of mechanical properties,after in situ remelting,ultimate tensile strength under argon protection increased from444.85±8.73 to 489.45±3.20 MPa,and that under nitrogen protection increased from 459.21±13.77 to 500.14±5.15 MPa.In addition,the elongation is nearly doubled and the micro-Vickers hardness is increased by 20%.The research results provide a new regulation control method for the customization of AlSi10 Mg properties fabricated by LPBF.

Evolution of globular microstructures during processing of aluminium slurries

Slurry processing experiments were performed with AlSi7Mg0.6 to identify the globularization mechanisms.The melt sample water quenched slightly above the liquidus point is predominantly dendritic while that cooled into the semi-solid temperature range internally via stirring the melt with a rotating cylindrical block of the alloy itself becomes fully globular.The globules are much smaller when internal cooling and stirring are employed longer to achieve higher solid fractions before casting.Coarse dendrite fragments of various sizes are revealed,in the case of stirring after an initial fraction of solid is first formed without the benefit of additional internal cooling.

Achieving Superior Strength and Ductility of AlSi10Mg Alloy Fabricated by Selective Laser Melting with Large Laser Power and High Scanning Speed

AlSi10Mg alloy was prepared by selected laser melting(SLM)in a high laser power range 300–400 W.The effects of energy density on the relative density,microstructure and mechanical properties of the SLMed AlSi10Mg alloy were studied.The results showed that the SLMed AlSi10Mg alloy fabricated at a laser power of 400 W and a scanning speed of 1800 mm/s had a relative density of 99.4%,a hardness of 147.8 HV,a tensile strength of 471.3 MPa,a yield strength of 307.1 MPa,and an elongation of 9.6%,exhibiting excellent comprehensive mechanical properties.The unique combination of the melt pool structure and microstructure caused by the large laser power and fast scanning was responsible for the excellent performance.The wide and shallow melt pool structure with few defects and proper overlapping between the continuous melt pools were obtained.The growth of columnar crystals was inhibited by a large proportion of equiaxed grains formed at the border of melt pools,and numerous sub-structures were observed within theα-Al grains.This study provided a more efficient process parameters for the preparation of the SLMed AlSi10Mg alloy.The enhanced mechanical property will help to broaden the application of the AlSi10Mg alloy in industry.

Achieving superior mechanical properties of selective laser melted AlSi10Mg via direct aging treatment

For additive manufactured aluminum alloys,the inferior mechanical properties along the building direction have been a serious weakness.In this study,an optimized heat treatment was developed as a simple and effective solution.The effects of direct aging on microstructure and mechanical properties along the building direction of AlSi10Mg samples produced via selective laser melting(SLM)were investigated.The results showed that,compared with the conventional heat treatment at elevated temperatures,direct aging at temperatures of 130-190℃ could retain the fine grain microstructure of SLM samples and promote further precipitation of Si phase,however,the growth of pores occurred during direct aging.With increasing aging temperature,while finer cell structures were obtained,more and larger pores were developed,resulting in decreased density of the samples.Two types of pore formation mechanisms were identified.Considering the balance between the refinement of cell structure and the growth of pores,aging at 130℃ was determined as the optimized heat treatment for SLM AlSi10Mg samples.The tensile strength along the building direction of the 130℃ aged sample was increased from 403 MPa to 451 MPa,with relatively high elongation of 6.5%.

Formation of spherical alloy microparticles in a porous salt medium

This study describes the development of a one-pot strategy to produce spherical alloy microparticles for advanced near-net-shape manufacturing processes, including additive manufacturing and powder injection molding. The Al Si12 eutectic alloy(ca. 12 wt% Si) system was chosen as the model with which the main experiments were carried out. The proposed process synergistically integrates a few common,low-cost processing techniques including the mixing of Al micrometer size particles with silicon and sodium chloride, heat-treating the mixture at temperatures of 650–810°C, and the dissolution of salt in water to produce spherical Al Si12 alloy particles without the need to rely on costly melting and atomizing techniques. This new process can use laow-cost source Al and Si powders as the raw materials to produce10–200 um-sized spherical particles of Al Si12. The Ansys-CFX computational fluid dynamics software was used to analyze the flow behavior of Al Si12 liquid droplets and particle size refinement in the narrow voids of the sample.