Research progress on microstructure evolution of semi-solid aluminum alloys in ultrasonic field and their rheocasting

The effects of ultrasonic vibration(UV)treatment on microstructure of semi-solid aluminum alloys and the application of UV in rheocasting process are reviewed.Good semi-solid slurry can be produced by high-intensity UV process for aluminum alloys.The microstructures of Al-Si,Al-Mg and Al-Cu alloys produced by rheocasting assisted with UV are compact and with fine grains.The mechanical properties of the UV treated alloys are increased by about 20%-30%.Grain refinement of the alloys is generally considered because of cavitation and acoustic streaming caused by UV.Apart from these mechanisms,a hypothesis of the fuse of dendrite root caused by capillary infiltration in the ultrasonic field,as well as a mechanism of crystallites falling off from the mould-wall and crystal multiplication by mechanical vibration effect in indirect ultrasonic vibration are proposed to explain the microstructure evolution of the alloys.

Microstructure and property of rheocasting AZ91 slurry produced via ultrasonic vibration process

The microstructure and mechanical properties of rheocasting AZ91 magnesium alloy were investigated. The semisolid slurry of this alloy was prepared by ultrasonic vibration （USV） process and then shaped by high pressure diecasting （HPDC）. The results show that fine and spherical a-Mg particles were obtained by USV at the nucleation stage, which was mainly attributed to the cavitation and acoustic streaming induced by the USV. Extending USV treatment time increased the solid volume fraction and average particle size, the shape factors were nearly the same, about 0.7. Excellent semisolid slurry of AZ91 magnesium alloy could be obtained within 6 rain by USV near its liquidus temperature. The rheo-HPDC samples treated by USV for 6 min had the maximum ultimate tensile strength and elongation, which were 248 MPa and 7.4%, respectively. It was also found that the ductile fracture mode prevailed in the rheocasting AZ91 magnesium alloy.

Microstructure and properties of rheo-diecast Al-20Si-2Cu-1Ni-0.4Mg alloy with direct ultrasonic vibration process

The microstructure and properties of Al-20Si-2Cu-1Ni-0.4Mg alloy fabricated with semi-solid rheo-diecasting process were studied.A newly developed direct ultrasonic vibration process(DUV process) was used in the preparation of the semi-solid slurry of this alloy.The results show that the primary Si particles in this alloy is about 20 μm in size under DUV for 90 s in the semi-solid temperature range,compared to about 30 μm in the alloy without DUV.It is discovered that the primary Si particles distribute more homogeneously and have regular shape,but have lower volume fraction after DUV.The tensile strength at room temperature is about 310 MPa,and the tensile strength and elongation of the semi-solid die castings are increased by 34% and 45%,respectively,compared with the traditional liquid die castings.The high-temperature tensile strength at 300 ℃ of this high Si aluminum alloy reaches 167 MPa,and the coefficient of thermal expansion is 17.37×10-6/℃ between 25 and 300 ℃.This indicates that this high Si content Al-Si alloy produced with the DUV process is suitable to be used in the manufacture of pistons or other heat-resistant parts.

Effects of ultrasonic vibration on performance and microstructure of AZ31 magnesium alloy under tensile deformation

Ultrasonic vibration can reduce the forming force, decrease the friction in the metal forming process and improve the surface quality of the workpiece effectively. Tensile tests of AZ31 magnesium alloy were carried out. The stress–strain relationship, fracture modes of tensile specimens, microstructure and microhardness under different vibration conditions were analyzed, in order to study the effects of the ultrasonic vibration on microstructure and performance of AZ31 magnesium alloy under tensile deformation. The results showed that the different reductions of the true stress appeared under various ultrasonic vibration conditions, and the maximum decreasing range was 4.76%. The maximum microhardness difference among the 3 nodes selected along the specimen was HV 10.9. The fracture modes, plasticity and microstructure of AZ31 magnesium alloy also were affected by amplitude and action time of the ultrasonic vibration. The softening effect and the hardening effect occurred simultaneously when the ultrasonic vibration was applied. When the ultrasonic amplitude was 4.6 μm with short action time, the plastic deformation was dominated by twins and the softening effect was dominant. However, the twinning could be inhibited and the hardening effect became dominant in the case of high ultrasonic energy.

Microstructure Evolution of AS41 Magnesium Alloy Fabricated by Ultrasonic Vibration

The effects of ultrasonic vibration on the grain size and morphology of Mg2Si in Mg-4 wt% Al-1 wt%Si（AS41） alloys designed were evaluated. The results show that the major constituents of the alloy include β-Mg17Al12 and Mg2Si phase, and no difference in the type of constituents between without ultrasonic vibration and with ultrasonic vibration. Without any ultrasonic vibration, the grain size and Mg2Si phase in AS41 alloy are coare structure. However, the microstructure with fine uniform grains and Mg2Si particles are achieved with ultrasonic vibration. The crystal grains and Mg2Si particles refine with increase in the ultrasonic vibration intensity. When the ultrasonic vibration intensity was too low or too high, coarse structures could be obtained. The analysis of refinement mechanism indicates that the acoustic cavitation and flows induced by ultrasonic vibration lead to the fine uniform microstructure.

Influence of non-uniform ultrasonic vibration on casting fluidity of liquid aluminum alloy

The application of ultrasonic vibration to the casting process can be realized through mould(die)vibration.However,the resonant vibration of the mould is always accompanied by a non-uniform vibration distribution at different parts,which may induce a complex liquid flow and affect the casting fluidity during the mould filling process.The influence of non-uniform ultrasonic vibration on the fluidity of liquid AlSi9Cu3 alloy was studied by mould vibration with different vibration gradients.It is found that ultrasonic mould vibration can generate two opposite effects on the casting fluidity:the first,ultrasonic cavitation in melt induced by mould vibration promotes the casting fluidity;the second,the non-uniform mould vibration can induce a melt flow toward the weak vibration areas and turbulence there,consequently decreasing the casting fluidity.When the melt flow and turbulence are violent enough to offset the promoting effect of cavitation on fluidity,the ultrasonic vibration will finally induce a resultant decrease of casting fluidity.The decreasing effect is proportional to the vibration gradient.

Microstructures and mechanical properties of AZ80 alloy treated by pulsed ultrasonic vibration

Pulsed ultrasonic field was employed in the melt of the AZ80 magnesium alloy.The effects of pulsed ultrasonic field on microstructure and mechanical properties of AZ80 magnesium alloy were investigated.The results show that the as-cast microstructure of the AZ80 alloy with pulsed ultrasonic treatment is significantly changed.Pulsed ultrasonic field significantly decreases the grain size,changes the morphologies of theβ-Mg_(17)Al_(12) phases and reduces their area fraction.It is found that pulsc width of ultrasonic plays an important role on the microstructure formation of AZ80 alloy.With increasing pulse width,grains become finer and more uniform.In the range of experimental parameters,the optimum pulse width for melt treatment process is found to be 210μs.The mechanical tests show that the mechanical properties of the as-cast AZ80 magnesium alloy with pulsed ultrasonic treatment are much higher than those of AZ80 alloy without ultrasonic field.

Effect of cooling condition on microstructure of semi-solid AZ91 slurry produced via ultrasonic vibration process

The effects of cooling conditions on the microstructure of semi-solid AZ91 slurry produced via ultrasonic vibration process were investigated. AZ91 melts were subjected to ultrasonic vibration in different temperature ranges under different cooling rates. The results show that fine and spherical α-Mg particles are obtained under ultrasonic vibration at the nucleation stage, which is mainly attributed to the cavitation and acoustic streaming induced by the ultrasonic vibration. The reduction of lower limit of ultrasonic vibration temperature between the liquidus and solidus increases the solid volume fraction and average particle size. Increasing cooling rate increases the solid volume fraction and reduces the average shape factor of particles. The appropriate temperature range for ultrasonic vibration is from 605 °C to 595 °C or 590 °C, and the suitable cooling rate is 2-3 °C/min.

Evolution of microstructure in semi-solid slurries of rheocast aluminum alloy

Semi-solid metal processing is being developed in die casting applications to give several cost benefits. To efficiently apply this emerging technology, it is important to understand the evolution of microstructure in semi-solid slurries for the control of the theological behavior in semi-solid state. An experimental apparatus was developed which can capture the grain structure at different times at early stages to understand how the semi-solid structure evolves. In this technique, semi-solid slurry was produced by injecting fine gas bubbles into the melt through a graphite diffuser during solidification. Then, a copper quenching mold was used to draw some semi-solid slurry into a thin channel. The semi-solid slurry was then rapidly frozen in the channel giving the microstructure of the slurry at the desired time. Samples of semi-solid 356 aluminum alloy were taken at different gas injection times of 1, 5, 10, 15, 20, 30, 35, 40, and 45 s. Analysis of the microstructure suggests that the fragmentation by remelting mechanism should be responsible for the formation of globular structure in this rheocasting process.

Degassing effect of ultrasonic vibration in molten melt and semi-solid slurry of Al-Si alloys

In the process of semi-solid slurry preparation with direct ultrasonic vibration (UV) by dipping the horn into the melt, one of the questions is whether the gas content in the melt would be increased or not by the cavitation effect of ultrasonic vibration. By application of quantitative gas content measurement technique, this paper investigated the effect of the ultrasonic vibration on the gas content of both the melt and the semi-solid slurry of Al-Si alloys, and the variations of the gas contents in two kinds of aluminum alloys, i.e., A356 alloy and Al-20Si-2Cu-1Ni-0.6RE alloy (Al-20Si for short). The results show that ultrasonic vibration has an obvious degassing effect on the molten melt, especially on the semi-solid slurry of Al-Si alloy which is below the liquidus temperature by less than 20 ℃. The ultrasonic degassing efficiency of the A356 alloy decreases with the reduction of the initial gas content in the melt, and it is nearly unchanged for the Al-20Si alloy. The gas content of both alloys decreases when the ultrasonic vibration time is increased. The best vibration time for Al-20Si alloy at the liquid temperature of 710 ℃ and semi-solid temperature of 680 ℃ is 60 s and 90 s, respectively; and the degassing efficiency is 48% and 35%, respectively. The mechanism of ultrasonic degassing effect is discussed.

Microstructure and properties of high silicon aluminum alloy with 2% Fe prepared by rheo-casting

The morphology changes of both Fe-containing intermetallic compounds and the primary Si phase of Al-20Si-2Fe- 2Cu-0.4Mg-1.0Ni-0.5Mn （mass fraction, %） alloy produced by semi-solid rheo-diecasting were studied. The semi-solid slurry of high silicon aluminum alloy was prepared by direct ultrasonic vibration （DUV） which was imposed on the alloy near the liquidus temperature for about 2 rain. Then, standard test samples of 6.4 mm in diameter were formed by semi-solid rheo-diecasting. The results show that the DUV treatment suppresses the formation of needle-like ,β-Al5（Fe,Mn）Si phase, and the Fe-containing intermetallic compounds exist in the form of fine Al4（Fe, Mn）Si2 particles. Additionally, the primary Si grows up as fine and round particles with uniform distribution in α（Al） matrix of this alloy under DUV treatment. The tensile strengths of the samples at the room temperature and 573 K are 230 MPa and 145 MPa, respectively. The coefficient of thermal expansion （CTE） between 25 ℃ and 300 ℃ is 16.052 8×10^-6 ℃^-1, and the wear rate is 1.55%. The hardness of this alloy with 2% Fe reaches HB146.3. It is discovered that modified morphology and uniform distribution of the Fe-containing intermetallic compounds and the primary Si phase are the main reasons for reducing the CTE and increasing the wear resistance of this alloy.

Effect of semi-solid processing on microstructure and mechanical properties of 5052 aluminum alloy

The microstructure and mechanical properties of rheocasted 5052 aluminum alloy were investigated.The semi-solid slurry of this alloy was prepared by ultrasonic vibration(USV) process and then shaped by gravity casting(GC) and high press diecasting(HPDC).The experimental results indicate that fine and globular primary α(Al) particles are distributed uniformly in the rheocasting samples.The tensile strength and elongation of the rheo-GC sample are 191 MPa and 7.5%,respectively.Compared with the conventional GC samples,they increase by 22.4% and 82.9% respectively.The tensile strength and elongation of the rheo-HPDC samples reach 225 MPa and 8.6%,respectively,and they are 14.8% and 75.5% higher than those of the conventional HPDC samples,respectively.It is also found that the ductile fracture mode prevails in the rheocasting samples.

Formation of hypereutectic silicon particles in hypoeutectic Al-Si alloys under the influence of high-intensity ultrasonic vibration

The modification of eutectic silicon is of general interest since fine eutectic silicon along with fine primary aluminum grains improves mechanical properties and ductilities. In this study, high intensity ultrasonic vibration was used to modify the complex microstructure of aluminum hypoeutectic alloys. The ultrasonic vibrator was placed at the bottom of a copper mold with molten aluminum. Hypoeutectic AI-Si alloy specimens with a unique in-depth profile of microstructure distribution were obtained. Polyhedral silicon particles, which should form in a hypereutectic alloy, were obtained in a hypoeutectic AI-Si alloy near the ultrasonic radiator where the silicon concentration was higher than the eutectic composition. The formation of hypereutectic silicon near the radiator surface indicates that high-intensity ultrasonic vibration can be used to influence the phase transformation process of metals and alloys. The size and morphology of both the silicon phase and the aluminum phase varies with increasing distance from the ultrasonic probe/radiator. Silicon morphology develops into three zones. Polyhedral primary silicon particles present in zone I, within 15 mm from the ultrasonic probe/radiator. Transition from hypereutectic silicon to eutectic silicon occurs in zone II about 15 to 20 pm from the ultrasonic probe/radiator. The bulk of the ingot is in zone III and is hypoeutectic AI-Si alloy containing fine lamellar and fibrous eutectic silicon. The grain size is about 15 to 25 IJm in zone I, 25 to 35 IJm in zone II, and 25 to 55 pm in zone II1. The morphology of the primary a-AI phase is also changed from dendritic （in untreated samples） to globular. Phase evolution during the solidification process of the alloy subjected to ultrasonic vibration is described.

Effect of electromagnetic and ultrasonic cast rolling on microstructure and properties of 1050 aluminum substrate for presensitized plate

The 1050 aluminum alloy strip was prepared by means of electromagnetic and ultrasonic cast rolling on the modified asymmetric twin roll caster, and then the aluminum substrate for presensitized plate was prepared through cold rolling and annealing.The effects of electromagnetic and ultrasonic cast rolling on microstructure, mechanical properties, surface roughness and electrolytic corrosion properties of 1050 aluminum substrate were studied. The results show that electromagnetic and ultrasonic cast rolling can decrease the average crystallite size of aluminum substrate by 5 μm, increase the crystal boundaries with uniform distribution, and make the second-phase particles with smaller size distributed dispersively in the substrate, meanwhile, it can increase the tensile strength, elongation and micro-hardness by 4.58%, 9.85% and HV 2, respectively, reduce the surface roughness, make the surface appearance more even, electrolytic corrosion polarization curve of aluminum substrate more smooth and the surface corrosion pits with regular shape more dispersive.

Effect of Compound Energy-Field with Temperature and Ultrasonic Vibration on Mechanical Properties of TC2 Titanium Alloy

To solve the problem of the poor plasticity and to meet the requirements of high temperature for forming titanium alloy,mechanical properties of TC2 titanium alloy under the compound energy-field(CEF)with temperature and ultrasonic vibration were studied.The effects of CEF on tensile force,elongation,microstructure and fractography of the TC2 titanium alloy were compared and analyzed.The results show that,under the same thermal conditions,the deformation resistance of TC2 titanium alloy decreases with the increase of ultrasonic vibration energy.The formability is also improved correspondingly due to the input of ultrasonic vibration energy and its influence on the microstructure of the material.However,when the ultrasonic vibration energy is larger,the fatigue fracture will also appear,which reduces its formability.

Preparation of semi-solid aluminum alloy slurry poured through a water-cooled serpentine channel

A water-cooled serpentine channel pouring process was invented to produce semi-solid A356 aluminum alloy slurry for rheocast ing, and the effects of pouring temperature and circulating cooling water flux on the microstmcture of the slurry were investigated. The results show that at the pouring temperature of 640-680~C and the circulating cooling water flux of 0.9 m3/h, the semi-solid A356 aluminum alloy slurry with spherical primary a（A1） grains can be obtained, whose shape factors are between 0.78 and 0.86 and the grain diameter can reach 48-68 ~am. When the pouring temperatures are at 660-680~C, only a very thin solidified shell remains inside the serpentine channel and can be removed easily. When the serpentine channel is cooled with circulating water, the microstructure of the semi-solid slurry can be improved, and the serpentine channel is quickly cooled to room temperature after the completion of one pouring. In terms of the productivity of the special equipment, the water-cooled serpentine channel is economical and efficient.

Microstructural characteristics and mechanical properties of Al-2024 alloy processed via a rheocasting route

This article reports the effects of stirring speed and T6 heat treatment on the microstructure and mechanical properties of Al-2024 alloy synthesized by a rheocasting process. There was a decrease in grain size ofα-Al particles corresponding to an increase in stirring speed. By increasing the stirring speed, however, the globularity of matrix particles first increased and then declined. It was also found that the hardness, compressive strength, and compressive strain increased with the increase of stirring speed. Microstructural studies revealed the presence of nonsoluble Al15（CuFeMn）3Si2 phase in the vicinity of CuAl2 in the rheocast samples. The required time for the solution treatment stage was also influenced by stirring speed;the solution treatment time decreased with the increase in stirring speed. Furthermore, the rheo-cast samples required a longer homogenization period compared to conventionally wrought alloys. Improvements in hardness and compres-sive properties were observed after T6 heat treatment.

Microstructure and mechanical properties of Mg-to-Al dissimilar welded joints with an Ag interlayer using ultrasonic spot welding

Lightweight ZEK100-0 Mg alloy and A16022-T43 Al alloy with an Ag interlayer were joined via ultrasonic spot welding(USW),focusing on the microstructural change and tensile lap shear strength of the welded joints in relation to welding energy.Mg/Al interface was superseded by Mg/Ag and Al/Ag interfaces,and unfavorable Mg门A-intermetallic compound was eliminated.Ag foil was observed to be intact in the nugget center,while it was broken or dissolved at the nugget edge at high welding energy levels.The diffusion layer at the Mg/Ag interface consisted of two distinctive sub-layers:Mg3Ag intermetallic compound adjoining Ag foil,and Mg3Ag-l-Mg eutectic structure adjacent to Mg.Only a thin diffusion layer consisting mainly of Ag3Al occurred al lhe Al/Ag interface.The tensile lap shear strength first increased,reached its peak value,and then decreased with increasing welding energy.The shear strength achieved in the present study was〜31%higher than that of the joint without interlayer.Interfacial failure occurred at all energy levels,with Ag foil particles or fragments being stuck on both Mg and Al sides due to its intense interaction with Mg and Al via accelerated diffusion during USW.The results obtained pave the way for the challenging dissimilar welding between Mg and Al alloys.

Influence of mechanical vibration on the solidification of a lost foam cast 356 alloy

Mechanical vibration was applied to the solidification of a lost foam cast(LFC) 356 aluminum alloy.Effects of mechanical vibration,with different peak acceleration,on the size and morphology of α-Al phase,and also on the mechanical properties of the castings were studied.Results indicated that α-Al dendrites gradually grow into equiaxed grains as the peak acceleration of vibration is increased.When the peak acceleration is between about 1 to 4 g,α-Al phase distribution is uniform and is refined obviously.α-Al dendrites are reduced and the mechanical properties of the castings are improved significantly when compared to those of the castings that are produced without vibration.However,when the peak acceleration is higher than 4 g,strong vibration will lead to defects formation,such as sand adhesion,while the amount and size of pores will be increased.And due to the turbulent flow that caused by strong vibration,the chance of forming large pores in the matrix has been increased significantly.The increase in defects will result in the deterioration of mechanical properties.

Interfacial microstructure evolution and mechanical properties of Al/Sn joints by ultrasonic-assisted soldering

Soldering aluminum alloys at low temperature have great potential to avoid softening of base metals.Pure Al was solderedwith pure tin assisted by ultrasound.The influence of primaryα(Al)on the microstructure of Al/Sn interface and its bonding strengthwas studied.It is found that the primaryα(Al)in liquid tin tends to be octahedron enclosed by Al{111}facet with the lowest surfacefree energy and growth rate.The ultrasonic action could increase the nucleation rate and refine the particles of primaryα(Al).For thelonger ultrasonic and holding time,a large amount of the octahedral primaryα(Al)particles crystallize at the Al/Sn interface.Thebonding interface exhibits the profile of rough dentation,resulting in an increment of bonding interface area and the effect ofmechanical occlusion.The bonding strength at interface could reach63MPa with ultrasonic time of40s and holding time of10min.