Effects of Extrusion Processing on Microstructure of 7075Al Alloy in the Semi-solid State

A recrystallization and partial melting(RAP) process was introduced to prepare the semi-solid 7075 aluminum alloy used for thixoforming. In order to obtain an ideal semi-solid microstructure, a series of extrusion experiments were conducted to comparatively investigate the optimum extrusion process parameters. Commercial 7075 Al alloy samples were firstly extruded with varying extrusion ratios below the recrystallization temperature followed by homogenization, then these samples were reheated to the semi-solid state and held in the range of 5 to 50 minutes. The experimental results show that varying process cause the difference in the deformation degree and microstructure for as-extruded samples, resulting in various semi-solid microstructure. It is verified that the formation of equiaxed grains in semi-solid microstructure depends on recrystallization behavior of extruded samples during partial melting. Both relative high extrusion temperature and low extrusion ratio lead to high volume fraction of recrystallized area, thus entirely equiaxed solid grains in semi-solid 7075 Al alloy samples can be obtained finally. In addition, Ostwald ripening was determined as the dominate coarsening mechanism of solid grains in semi-solid state for this 7075 Al alloy during the RAP route. The influence of predeformation on recrystallization behavior of this 7075 Al alloy was discussed in detail.

Microstructure and properties of Al-0.70Fe-0.24Cu alloy conductor prepared by horizontal continuous casting and subsequent continuous extrusion forming

A novel process for manufacturing A1-0.70Fe-0.24Cu alloy conductor was proposed, which includes horizontal continuous casting and subsequent continuous extrusion forming （Conform）. The mechanical properties, electrical conductivity and the compressed creep behaviour of the alloy were studied. The results indicate that the Conform process induces obvious grain refinement, strain-induced precipitation of AI7CuzFe phase and the transformation of crystal orientation distribution. The processed alloy has good comprehensive mechanical properties and electrical conductivity. Moreover, a better creep resistance under the conditions of 90 ~C and 76 MPa is shown compared with pure A1 and annealed copper, and the relationship between primary creep strain and time may comply with the logarithmic law. The enhanced properties are attributed to the grain refinement as well as the fine and homogeneously distributed thermally stable A1Fe and A17Cu2Fe precipitation phases.

Deformation and microstructure characterization during semi-solid extrusion of Al-4Cu-Mg alloy

Effects of the process parameters, including deformation temperature, punch velocity and extrusion ratio, on the deformation and microstructure characterization during the semi-solid extrusion of Al-4Cu-Mg alloy, were investigated. The experimental results show that the load decreases with an increase of deformation temperature and/or a decrease of punch velocity. When the displacement is more than 4 mm,the load decreases significantly with an increase of the deformation temperature, which is related to the high liquid fraction. The microstructure varies with the process parameters and deformation regions. It can be found that the dynamic recovery occurs during the semi-solid extrusion of Al-4Cu-Mg alloy at lower deformation temperature. Subsequently, the microstructure elongated gradually polygonizes with an increase of deformation temperature. So, the higher deformation temperature should be chosen during the semi-solid extrusion of Al-4Cu-Mg alloy because the grains polygonized and high liquid fractions are beneficial to deformation.

Heat-treated microstructure and mechanical properties of laser solid forming Ti-6Al-4V alloy

The effects of heat treatment on the microstructure and mechanical properties of laser solid forming （LSF） Ti-6Al-4V alloy were investigated The influences of the temperature and time of solution treatment and aging treatment were analyzed. The results show that the microstructure of LSFed samples consists of Widmanstatten α laths and a little acicular in columnar prior β grains with an average grain width of 300 μm, which grow epitaxiaUy from the substrate along the deposition direction （27）. Solution treatment had an important effect on the width, aspect ratio, and volmne fraction of primary and secondary a laths, and aging treatment mainly affects the aspect ratio and volume fraction of primary α laths and the width and volume fraction of secondary a laths. Globular a phase was first observed in LSFed samples when the samples were heat treated with solution treatment （950℃, 8 h/air cooling （AC）） or with solution treatment （950℃, 1 h/AC） and aging treatment （550℃, above 8 h/AC）, respectively. The coarsening and globularization mechanisms of a phase in LSFed Ti-6Al-4V alloy during heat treatment were presented. To obtain good integrated mechanical properties for LSFed Ti-6Al-4V alloys, an optimized heat treatment regimen was suggested.

Dynamical coarsening processes of microstructures in non dendritic AlSi_7Mg alloy remelted in semi solid state

The microstructural variation in the non dendritic AlSi 7Mg samples remelted in the semi solid state has been investigated. It is proposed that the primary α Al phases are mainly coarsened by connecting the secondary arms or fine primary α Al phases together at the stage of a small quantity of liquid and slowly coarsened through diffusion at the stage of a great quantity of liquid. The dynamical coarsening equation controlled by diffusion is in good agreement with the equation of d 3- d 3 0= kt and the effect of the starting microstructures on the coarsening of primary α Al phases is gradually decreased when the soaking time is long enough.

Effect of Mg and semi solid processing on microstructure and impression creep properties of A356 alloy

The effects of Mg and semi solid processing on the creep properties ofA356 A1 alloy were investigated. The results show that the dislocation climb controlled creep is the dominant creep mechanism and it is not affected by the semi solid processing and further addition of Mg. Mg improves the alloy creep properties probably by forming large Chinese script Mg2Si compounds at the interdendritic regions. The semi solid processed specimens exhibit better creep properties in comparison with the as cast ones. It is attributed to the reduction in the stacking fault energy resulting from the significant dissolution of Mg in the a（A1） phase.

Reheating microstructure of refined AZ91D magnesium alloy in semi-solid state

By means of equal channel angular extrusion (ECAE) test, upsetting test and metalloscope, reheating microstructures of raw casting ingots, materials prepared by SIMA and materials extruded by ECAE in semi-solid state were investigated. The results show that compared with those of raw casting ingots and materials prepared by SIMA, reheating microstructure of materials extruded by ECAE is the best and the final grain size is the finest. With increasing holding time, a growing phenomenon occurs in reheating microstructure of materials extruded by ECAE, which can be described by Ostwald ripening law. The average grain size increases firstly, subsequently decreases and the shape factor of grains approaches to 1 as the reheating temperature increases. With increasing equivalent strain, the average grain size decreases. This demonstrates that reheating material extruded by ECAE technology is a good method to prepare AZ91D magnesium alloy semi-solid billets.

Formation and Evolution of Non-dendritic Microstructures of Semisolid Alloys Prepared by Shearing/Cooling Roll Process

The shearing/cooling roll （SCR） process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe＇s surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures.

Effect of predeformation on semi-solid microstructure of ZK60+RE magnesium alloy

Microscopical techniques were used to provide the semi-solid microstructure evolutions of ZK60+RE alloys formed by compression and equal channel angular extrusion(ECAE), respectively. It is found that after compression and ECAE, as-cast microstructures exhibit an obvious directional characteristic. The predeformation exerts a significant influence on the formation of thixotropic microstructures during partial remelting. Coalescence and Ostwald ripening are operative in the semi-solid mixture for both compression and ECAE formed alloys. Furthermore, the degree of spheroidization of ECAE formed alloy is better than that of compression formed alloy in appearance.

Effects of trace Cr on as-cast microstructure and microstructural evolution of semi-solid isothermal heat treatment ZC61 magnesium alloy

The content and kind of trace elements in magnesium alloys have important effects on their ascast and semi-solid microstructures. In this research work, effects of trace Cr on as-cast and semi-solid microstructures of ZC61 magnesium alloy were investigated by metal mold casting and semi-solid isothermal heat treatment. The results show that the addition of Cr can refine the α-Mg phase without generating a new phase, noticeably change the eutectic phase, and decrease the average size of solid particles at the same isothermal heat treatment conditions. Non-dendritic microstructures of all alloys are constituted of α_1-Mg phases, α_2-Mg phases and eutectic phases after water quenching. With isothermal temperature increased or holding time prolonged, the eutectic microstructure(α-Mg+MgZn_2+CuMgZn) at the grain boundaries in as-cast alloy is melted preferentially and then turned into semi-solid non-dendritic microstructure by processes of initial coarsening, microstructure separation, spheroidizing and final coarsening. Especially when the ZC61-0.1 Cr alloy was treated at 585 ℃ for 30 min, the ideal non-dendritic microstructure can be obtained, and the corresponding solid particle size and shape factor were 37.5 μm and 1.33, respectively. The coarsening process of solid α-Mg phase at higher temperature or longer time, which is affected by both combining growth and Ostwald ripening mechanism, is refrained when Cr is added to the ZC61 alloy.

Effect of original microstructures on microstructural evolution of A2017 semi-solid alloy billets during reheating

The microstructural evolution of the A2017 semi-solid alloy billets provided with rheocasting and extruding/extending forming by shearing-cooling-rolling(SCR) technology during reheating in semi-solid state was investigated. The microstructural differences and their generation causes for both billets were also analyzed. The results show that during reheating, the grains of rheocasting billets grow up and spheroidize gradually with the prolongation of isothermal holding time, the eutectic liquid phase at low melting point forms mainly among the grains. However, the grains of the extruding/extending forming billets grow up abnormally through grain coalescence in the initial stage of the reheating, the entrapment of large amount of liquid within grains occurs, and the grain sizes in the reheating billets are coarse and inhomogeneous. Compared with extruding/extending forming billets, rheocasting billets have smaller and uniform grains in reheating microstructure and can rapidly form liquid phase among grains. Therefore, rheocasting billets are more suitable for the semi-solid forming than the extruding/extending forming billets.

Effect of Hot Extrusion on Microstructure and Properties of (ABOw+SiCp)/6061Al Composites Fabricated by Semi-solid Stirring Technique

6061Al matrix composites reinforced by 5vol.%ABOw and 15vol.%SiCp were fabricated by semi-solid stirring technique successfully at 640 ℃ for 40min with the stirring rate of 300 rpm,and the composites were extruded at a temperature of 500 ℃ using an extrusion ratio of 25:1 subsequently.Tensile tests were performed on as-casted and as-extruded(ABOw+SiCp)/6061Al composites at room temperature,and microstructures were observed by scanning electron microscope(SEM).SEM investigation showed that the as-extruded composite exhibited reduced porosity as well as a more uniform distribution of the reinforcements compared with the as-casted composite.The tensile tests results showed that the ultimate tensile strength and tensile elongation of as-extruded composite are higher than that of as-casted composite.

Effects of processing parameters on microstructure of semi-solid magnesium alloy

In this paper, the effects of pouring temperature of magnesium melt, preheating temperature of the barrel of the screw mixer, and shear rate on the solidified microstructures of semi-solid slurry were investigated by a mechanical stirring semi-solid process. The appropriate processing parameters of slurry preparation were obtained, and the mold filling ability of semi-solid slurry for thin-walled casting was examined. Results indicate that the solid volume fraction of non-dendritic microstructure increases with a decrease in pouring temperature of magnesium melt and the barrel preheating temperature of the screw mixer. Also the grain size of primary α-phase is reduced. Furthermore, the solid volume fraction of semi-solid nondendritic structure decreases with an increase of shear rate. The fine and round granular microstructure with 30～50 μm in size of semi-solid AZ91D magnesium alloy was presented. Finally, a 1.0 mm thin-walled casting with a clear contour and good soundness was successfully made by semi-solid rheo-diecasting.

Reverse design of microstructure and application effectiveness for semi-solid processing of aluminum alloy

The hypoeutectic Al-Si alloy billet with non-dendrite was reheated to meet the needs of the semi-solid thixoforming microstructure by four kinds of reheating power,achieving the same final temperature of 851 K.Subsequently,under the same condition of thixoforming,the microstructure,surface hardness and tensile properties were observed.Afterwards,quantitative analysis was made for the microstructures of the reheated semi-solid of billet and the thixoforming parts.The results show that when the induction reheating power is 90 kW,the average grain size of the semi-solid billet is the minimum,the microstructures of the thixoforming samples also are the finest,and the mechanical properties of the relevant thixoforming samples are the best.Furthermore,after studying on the relationship between the microstructures of the semi-solid billet of aluminum alloy and the mechanical properties of the thixoforming samples,the reverse design of microstructure is primarily achieved.Finally,the effectiveness of the reverse design for semi-solid microstructure is confirmed by an actual automobile part with complex shape.

Interface and microstructure characteristics of SiC_p/2024 aluminium alloy composite

Electron microscope examination of the microstructure, interface and fracture surface of SiC particulate reinforced 2024 aluminium alloy composites produced by powder mixing and semi-solid extrusion process was presented. The microstructure of SiC p/2024 composites fabricated by the present method is characterized by uniformly distributed SiC particulates in well-densified matrix. Conventional transmission electron microscopy(TEM) reveals the interface between the SiC particulates and the aluminium matrix. It is shown that this interface provides very strong bonding which is further evidenced by the fractographic results, and that there is no apparent chemical reaction. Examination of the fracture surface indicates that the bonding strength between the SiC particulates and the aluminium alloy matrix is stronger than that of the matrix. The dimples and tearing edges on the fracture surface of composites are obviously observed.

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angular extrusion in semi-solid isothermal treatment

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angularextrusion(ECAE) in semi-solid isothermal treatment was investigated. The results show that withincreasing semi-solid isothermal treatment temperature, the a phase solid grain size of processedMg-Al-Zn alloy by ECAE increases firstly due to coarsening of a phase solid grains, then decreasesdue to melting of a phase solid grains. With the increase of extrusion passes during ECAE, the aphase solid grain size in the following semi-solid isothermal treatment decreases. The a phase solidgrain size of processed Mg-Al-Zn alloy by ECAE under route B_C is the smallest, while the a phasesolid grain size of processed material by ECAE under route A is the largest. The primary mechanismof spheroid formation depends on the melting of recrystallizing boundaries and diffusion of soluteatoms in the semi-solid state.

Microstructure and Properties of 7075Al Alloy Fabricated by Directly Combined of Spray Forming and Continuous Extrusion Forming under Different Atomization Gas Pressures

A new process consisting of the spray forming and the continuous extrusion forming for manufacturing 7075A1 alloy was proposed. The microstructure evolution, mechanical properties and the resistance to stress corrosion cracking of the alloy were studied. The results indicate that the spray forming process induces obviously grain refinement and greatly lower segregation microstructure. Besides, the Conform process produces finer grains and conduces to more uniform distribution of the precipitates of A1Cu and MgZn2 phases. The fabricated alloy shows good comprehensive mechanical properties and superb performance of stress corrosion resistance. Moreover, a better combination of the mechanical properties and the resistance to stress corrosion cracking could then be obtained under a certain condition of atomization gas pressure of 0.19 MPa. The enhanced properties are attributed to the following factors, which include the grain refinement, the fine and homogeneous distribution of A1Cu and MgZn2 phases, the high density of the extruded products, as well as the discontinuous distribution of the grain boundaries after retrogression and reaging （RRA） heat treatment.

Microstructure and mechanical properties of Al-6Zn-2.5Mg-1.8Cu alloy prepared by squeeze casting and solid hot extrusion

Al-6Zn-2.5Mg-1.8Cu alloy ingots were prepared by squeeze casting under different specific pressures,and the fresh ingot with best mechanical properties was solid hot extruded.With the increase of the specific pressure from 0 to 250 MPa,the dendrites became round and small.Because the applied pressure increased the solid solubility of alloying elements,the number of MgZn2 phases decreased.When the specific pressure increased from 250 MPa to 350 MPa,the grain size increased.After solid hot extrusion,the a（Al） grains were refined obviously and the MgZn2 phases were uniformly dispersed in the microstructure.After solid hot extrusion,the ultimate tensile strength was 605.67 MPa and the elongation was 8.1%,which were improved about 32.22%and15.71%,respectively,compared with those of the metal mold casting alloy.The fracture modes of the billet prepared by the metal mold casting and by squeeze casting were intergranular and quasi-cleavage fractures,respectively,whereas,that of the solid hot extrusion was mainly dimple fracture.The refined crystalline strengthening was the main reason to improve the strength and elongation of alloy.

Microstructure evolution of semi-solid 2024 alloy during two-step reheating process

A two-step reheating process was proposed and applied to perform reheating experiments on the semi-solid 2024 alloy billet. In this process, the semi-solid billet was firstly heated over liquidus temperature and then isothermally held at solid-liquid zone temperature. Microstructure evolution of the semi-solid billet during two-step reheating was studied by optical microscope and compared with that during isothermal reheating. The results show that the remelting rate of the semi-solid billet during two-step reheating is faster than that during isothermal reheating. Under the same reheating time, the grains of the semi-solid billet reheated by two-step reheating process are finer and rounder than those by isothermal reheating process. The present experimental results indicate that accelerating the formation of liquid phase during the two-step reheating process can restrain the coalescence of grains to a certain extent, and thus refine the grain size and promote the grain spheroidization.

Preparation of semi-solid billet of magnesium alloy and its thixoforming

Preparation of semi-solid billet of magnesium alloy and thixoforming was investigated by applying equal channel angularextrusion to magnesium alloy.The results show that mechanical properties of AZ91D alloy at room temperature,such as yieldstrength(YS),ultimate tensile strength(UTS)and elongation,are enhanced greatly by four-pass equal channel angularextrusion(ECAE)at 573 K and microstructure of AZ91D alloy is refined to the average grain size of 20μm.Through using ECAE asstrain induced step in SIMA and completing melt activated step by semi-solid isothermal treatment,semi-solid billet with finespheroidal grains of 25μm can be prepared successfully.Compared with common SIMA,thixoformed satellite angle framecomponents using semi-solid billet prepared by new SIMA have higher mechanical properties at room temperature and hightemperature of 373 K.