Effects of cooling rate on solution heat treatment of as-cast A356 alloy

The effect of cooling rate of the solidification process on the following solution heat treatment of A356 alloy was investigated,where the cooling rates of 96 K/s and 3 K/s were obtained by the step-like metal mold.Then the eutectic silicon morphology evolution and tensile properties of the alloy samples were observed and analyzed after solution heat treatment at 540 °C for different time.The results show that the high cooling rate of the solidification process can not only reduce the solid solution heat treatment time to rapidly modify the eutectic silicon morphology,but also improve the alloy tensile properties.Specially,it is found that the disintegration,the spheroidization and coarsening of eutectic silicon of A356 alloy are completed during solution heat treatment through two stages,i.e.,at first,the disintegration and spheroidization of the eutectic silicon mainly takes place,then the eutectic silicon will coarsen.

Gradual solution heat treatment of AlSiCuMg cast alloys

The microstructure characteristics of AlSiCuMg cast alloys were studied withdifferent Cu content and the gradual solution treatment by DSC, SEM, TEM and mechanical method. Themelting point of alpha (Al) + Si decreases and polynary eutectic phases with low melting point formwith increase of Cu content. Gradual solution treatment includes two steps: solution treating nearthe melting point of polynary eutectic phase to take it dissolve first, and then increasing solutiontemperature to take the remainder copper intermetallics dissolved into alpha (Al). Grain boundariesmelting can be avoided by gradual solution treatment, even the maximum solution temperature isabove final solidification point, and the age hardening response increases correspondingly

Novel high strength Al−10.5Si−3.4Cu−0.2Mg alloy produced through two-stage solution heat treatment

Mechanical properties of aluminum−silicon−copper alloys are enhanced through precipitation hardening.The response of these alloys to age-hardening is very slow.To overcome this problem,0.2,0.4 and 0.7 wt.%magnesium were added to Al−10.5Si−3.4Cu alloy.The new alloys were subjected to two types of precipitation hardening processes different in the solutionizing stage.The results showed that the presence of various amounts of magnesium in the composition of this alloy accelerates the response to ageing treatments,increasing the hardness and strength.Higher mechanical properties can be achieved when the alloys were subjected to a two-stage solution heat treatment.It is found that Al−10.5Si−3.4Cu alloy containing 0.2 wt.%Mg treated through a two-stage solution process,has optimum properties with ultimate tensile strength of 383.9 MPa,yield strength of 289.7 MPa and elongation of 3.97%,and can be used as a substitute for a large number of aluminum castings which need high strength and excellent castability.

Effects of solution heat treatment on microstructure and mechanical properties of AZ61-0.7Si magnesium alloy

The effects of solution heat treatment on the microstructure and mechanical properties of AZ61-0.7Si magnesium alloy were investigated.The results indicate that the solution heat treatment can modify the Chinese script shaped Mg2Si phases in the AZ61-0.7Si magnesium alloy.After being solutionized at 420℃ for 16-48 h,the morphology of the Mg2Si phases in the AZ61-0.7Si alloy changes from the Chinese script shape to the short pole and block shapes.Accordingly,the tensile and creep properties of the AZ61-0.7Si alloy are improved.After being solutionized at 420℃ for 24 h and followed by aging treatment at 200℃ for 12 h,the heat-treated alloy exhibits relatively high tensile and creep properties than those of the as-cast alloy.

High-Superelasticity NiTi Shape Memory Alloy by Directed Energy Deposition-Arc and Solution Heat Treatment

Directed energy deposition-arc(DED-Arc)technology has the advantages of simple equipment,low manufacturing cost and high deposition rate,while the use of DED-Arc has problems of microstructure inhomogeneity,position dependence of macroscopic mechanical properties and anisotropy.Therefore,it is necessary to carry out a subsequent heat treatment to improve its microstructure uniformity,mechanical properties and superelasticity.In this investigation,the DED-Arc 15-layer NiTi alloy thin-walled parts with the solution treatment at different process parameters were studied to analyze the effects of solution heat treatment on microstructure,phase composition,phase transformation,microhardness,tensile and superelasticity.The temperature range of solution treatment is 800-1050℃,and the treatment time range is 1-5.5 h.The results show that after solution treatment at 800℃/1 h,the content of precipitated phase decreases,the grain is refined,the microhardness increases,and the mechanical properties in the 0°direction are improved.The strain recovery rate after 10 tensile cycles has increased from 37.13%(as-built)to 49.25%(solid solution treatment).This research provides an effective post treatment method for high-performance DED-Arc NiTi shape memory alloys.

Mechanical properties,microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu alloy after solution heat treatment

The effect of solution heat treatment （SHT） on mechanical properties, microstructure and surface quality of Al-1.2Mg-0.6Si-0.2Cu-0.6Zn alloy was investigated by tensile test, Erichsen test, surface topography, scanning electron microscope （SEM） and electron back-scattered diffraction （EBSD）. The results indicate that with the increase in SHT temperature, yield strength and cupping test value （IE） of the sheets increase greatly and reach a peak value, then decrease. Meanwhile, intermetallic com- pounds dissolve into matrix gradually. The grains grow up as SHT temperature increases, and abnormal grain growth leads to the surface defects after solution-treated above 560 ~C. Considering mechanical properties, IE value, residual phases, grain size and surface quality of the sheets, SHT temperature for the alloy should not be higher than 550 ℃.

Effect of as-solidified microstructure on subsequent solution-treatment process for A356 Al alloy

For the compromise of mechanical properties and product cost, the end-chilled sand casting technique was applied to studying the microstructure evolution of A356 Al alloy with cooling rate and the effect of different as-cast microstructures on the subsequent solution-treatment process. The experimental results show that the secondary dendrite arm spacing (SDAS) of primaryα(Al), the size of eutectic Si and the volume fraction of Al?Si eutectic are reduced with increasing the cooling rate. Eutectic Si, subjected to solution treatment at 540 °C for 1 h followed by water quenching to room temperature, is completely spheroidized at cooling rate of 2.6 K/s; is partially spheroidized atcooling rate of 0.6 K/s; and is only edge-rounded at cooling rates of 0.22 and 0.12 K /s. Whilst the microhardness is also the maximum at cooling rate of 2.6 K/s. It consequently suggests that subjected to modification by high cooling rate, the eutectic Si is more readily modified, thus shortening the necessary solution time at given solution temperature, i.e., reducing the product cost.

Microstructural evolution during solution treatment of thixoformed AM60B Mg alloy

The microstructural evolution and kinetic characteristics were studied during solution treatment of AM60B Mg alloy prepared by thixoforming. The results indicate that the microstructural evolution includes two stages： the first stage involves rapid dissolution of eutectic β （Mg 17 Al 12 ） phase, homogenization and coarsening, and the second stage is regarded as normal grain growth consisting of primary α-Mg particles （primary particles） and secondary α-Mg grains （secondary grains）. In the first stage, the dissolution completes in a quite short time because the fine β phase can quickly dissolve into the small-sized secondary grains. The homogenization of Al element needs relatively long time. Simultaneously, the microstructure morphology and average grain size obviously change. The first stage sustains approximately 1 h when it is solutionized at 395 ℃ Comparatively, the second stage needs very long time and the microstructure evolves quite slowly as a result of low Al content gradient and thus low diffusivity of Al element after the homogenization of the first stage. The growth model of primary particles obeys power function while that of the secondary grains follows the traditional growth equation in the first stage. In the second stage, both of the primary particles and secondary grains behave a same model controlled by diffusion along grain boundaries and through crystal lattice.

Effects of heat treatments on microstructure and mechanical properties of Mg-15Gd-5Y-0.5Zr alloy

Microstructure and mechanical properties of Mg-15wt.%Gd-5 wt.%Y-0.5wt.% Zr alloy were investigated in a series of conditions. The eutectic was dissolved into the matrix and there was no evident grain growth after solntionized at 525 ℃ for 12 h. The evolution of the phase constituents from as-cast to cast-T4 was as follows： α-Mg solid solution＋Mg5（Gd,Y） entectic compound→α-Mg solid solution＋ spheroidized Mg5（Gd, Y） phase→α-Mg supersaturated solid solution＋cuboid-shaped compound （Mg2Y3Gd2）. And the precipitation sequences of Mg-15Gd-5Y-0.5Zr alloy were observed, according to the hardness response to isothermal ageing at 225-300 ℃ for 0-128 h.

Morphological evolution of γ'precipitates in a nickel-based superalloy during various solution treatments

The morphological evolution of the γ' phase in nickel-based superalloy жc6y during various solution heat treatments was investigated. The significant changes of the γ' precipitates were observed in the solution-treated samples. The coarsening and dissolution of γ' phase simulta-neously occurred at intermediate temperatures. In some areas, the primary precipitates became blunt and the adjacent ones were intercon-nected with each other via a diffuse neck, indicating a coarsening process of the primary γ' population. The coarsening was dominated by the precipitate agglomeration mechanism (PAM) rather than by the well-known Ostwald ripening mechanism. In other areas, the partial dissolu-tion of the γ' precipitates began to occur, spreading gradually from dendrite cores to interdendritic regions. In addition, a flower-like γ' struc-ture was developed during the subsolvus solution treatments. The observable long filaments composed of erraticly shaped precipitates were caused by the heterogeneous nucleation of the cooling precipitates during water quenching.

Influence of heat treatments on incipient melting structures of DD5 nickel-based single crystal superalloy

The evolution of microstructure and formation mechanism of incipient melting microstructure of DD5 single crystal superalloy during solution heat treatment were studied by scanning electron microscopy(SEM),electron probe microanalysis(EPMA),and energy dispersive spectroscopy(EDS).The solidus and liquidus of single crystal alloy were obtained by differential scanning calorimetry(DSC).Results show that the mosaic-like eutectic and fan-like eutectic are dissolved at first,and the coarseγ'phase is dissolved later during the solution heat treatment of 1,390°C/2 h+1,310°C/4 h+1,320°C/10 h+air cooling(AC).The composition segregations of Al,Ta,W and Re are 0.99,0.96,1.04 and 1.16,respectively,which close to 1.The incipient melting is caused by the low local temperature of the alloy,and the micropore region with a lower melting point is the preferred position for incipient melting.

强化固溶对7055铝合金力学性能和断裂行为的影响

残余可溶结晶相颗粒是制约高强度铝合金力学性能的重要因素 .作者通过改变固溶热处理条件并结合金相组织观察和断口分析研究了强化固溶对提高 70 5 5铝合金力学性能的作用 .结果表明 :采取逐步升温固溶处理可使最终固溶温度超过多相共晶温度而不产生过烧组织 ,提高残余可溶结晶相的固溶程度和合金力学性能 .强化固溶的 70 5 5合金的屈服强度和抗拉强度分别达 715MPa和 75 0MPa,且延伸率约为 10 % ;微量元素Zr比Cr更有利于提高 70 5 5合金的力学性能 ,且在强化固溶条件下 ,提高效果更加明显 .通过断口分析显示 ,合金的断裂属晶内韧窝断裂与沿晶断裂的混合断裂 ;强化固溶后 ,残余结晶相引起的晶内韧窝断裂减少 。

A comparative study of Mg-Gd-Y-Zr alloy cast by metal mould and sand mould

The differences of the microstructure and mechanical property between metal mould and sand mould cast Mg-10Gd-3Y-Zr alloy were investigated both under as-cast condition and after solution heat treatment. In the as-cast specimens, the microstructure is similar and composed of a-Mg solid solution and eutectic compound of a-Mg＋ Mg24（Gd,Y）5; whereas the grain size using metal mould and sand mould is 27 pm and 71 IJm, respectively. The eutectic compound of metal mould cast alloy was completely dissolved after solution treated at 500℃ for 8 h, however it needs higher temperature （525 ℃） and longer time （12 h） to achieve the absolute dissolving under sand mould condition. In contrast to metal mould, the peak time of sand mould alloy aged at 225℃ and 250℃ of was advanced by 4 h and 6 h, respectively. The precipitation reaction sequence in sand mould cast Mg-10Gd-3Y-Zr alloy during isothermal ageing at 250℃ follows S.S.S.S.→β（D010）→β′（cboc）→β1（fcc）→β（fcc）, which is similar to that in the alloy cast using metal mould.

Microstructure and mechanical properties of as-cast and heat treated Mg-15Gd-3Y alloy

The microstructure evolution and mechanical properties of Mg-15Gd-3Y alloy were investigated in the as-cast and heat treated conditions. The microstrucmre evolution from as-cast to cast-T4 states involved a-Mg solid solution＋Mg5（Gd,Y） phase→a-Mg supersaturated solid solution＋rare earths compound Mg3（Gdl.26,Y0.74）→a-Mg supersaturated solid solution＋rare earths compound Mg3（Gd0.Tas,Y1.255）. It showed that 480 ℃/4 h was the optimal solution treatment parameter. If the solution temperature was high or the holding time was long, such as 520 ℃/16 h, an overheating phenomenon would be induced, which had a detrimental effect on the mechanical properties. When ageing at 225 and 200℃, the alloy would exhibit a significant age-hardening response and great long-time-age-hardening potential, respectively. The best mechanical properties were obtained at the parameters of 480 ℃/4 h＋225 ℃/16 h, with the UTS of 257.0 MPa and elongation of 3.8%.

Effect of Cooling Rate on Microstructure and Mechanical Properties of K465 Superalloy

K465 superalloy, as a material for production of turbine nozzle, shows high mechanical properties as well as microstructure stability under critical and severe service conditions. The appropriate microstructure and strength of this alloy can be obtained by solid solution strengthening mechanism. Heat treatment parameters such as time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from solid solution temperature affect the microstructure of the alloy. Among these parameters, cooling rate from solid solution temperature is the most effective. Therefore, the effect of cooling rate on microstructure and mechanical properties （tensile and stress properties） was investigated. For this purpose, three different cooling rates were applied to the cast K465 specimens subjected to solution treatment at 1 483 K for 4 h. Microstructures of the specimens then were studied using optical and electron microscopy. Also, tensile tests were performed at room temperature and stress rupture tests were performed under the condition of 1 248 K and 230 MPa. It was found that with increasing the cooling rate, the size of the γ＇ precipitates decreases and the mechanical properties of specimens increases. Also, it was found that the shape and volume fraction of primary γ＇ particles are largely influenced by the cooling rate after solution treatment at 1 483 K for 4 h.