Effect of quenching cooling rate on residual stress and microstructure evolution of 6061 aluminum alloy

In this study,the cooling rate was manipulated by quenching with water of different temperatures(30,60 and 100℃).Surface and internal residual stresses in the quenched 6061 aluminum alloy samples were measured using hole-drilling and crack compliance methods,respectively.Then,the processability of the quenched samples was evaluated at cryogenic temperatures.The mechanical properties of the as-aged samples were assessed,and microstructure evolution was analyzed.The surface residual stresses of samples W30℃,W60℃and W100℃is−178.7,−161.7 and−117.2 MPa,respectively along x-direction,respectively;and−191.2,−172.1 and−126.2 MPa,respectively along y-direction.The sample quenched in boiling water displaying the lowest residual stress(~34%and~60%reduction in the surface and core).The generation and distribution of quenching residual stress could be attributed to the lattice distortion gradient.Desirable plasticity was also exhibited in the samples with relatively low quenching cooling rates at cryogenic temperatures.The strengthes of the as-aged samples are 291.2 to 270.1 MPa as the quenching water temperature increase from 30℃to 100℃.Fine and homogeneous β"phases were observed in the as-aged sample quenched with boiling water due to the clusters and Guinier-Preston zones(GP zones)premature precipitated during quenching process.

Exploring the effect of cooling rate on non-isothermal crystallization of copolymer polypropylene by fast scanning calorimetry

Polypropylene is commonly used as a binder for ceramic injection molding,and rapid cooling is often encountered during processing.However,the crystallization behavior of polypropylene shows a strong dependence on cooling rate due to its semi-crystalline characteristics.Therefore,the influence of cooling rate on the quality of final product cannot be ignored.In this study,the fast differential scanning calorimetry(FSC)test was performed to study the influence of cooling rate on the non-isothermal crystallization behavior and non-isothermal crystallization kinetics of a copolymer polypropylene(PP BC03B).The results show that the crystallization temperatures and crystallinity decrease as the cooling rate increases.In addition,two exothermic peaks occur when cooling rate ranges from 30 to 300 K·s^(-1),indicating the formation of another crystal phase.Avrami,Ozawa and Mo equations were used to explore the non-isothermal crystallization kinetics,and it can be concluded that the Mo method is suitable for this study.

Effect of solidification cooling rate on microstructure and tribology characteristics of Zn-4Si alloy

The main objective of this work was to modify the microstructure and enhance the tribological properties of a new Zn-4Si al-loy through a high solidification cooling rate(SCR).According to the results,by increasing the SCR from 2.0 to 59.5℃/s the average size of primary Si particles and that of the grains reduced from 76.1 and 3780μm to less than about 14.6 and 460μm,respectively.Augment-ing the SCR also enhanced the microstructural homogeneity,decreased the porosity content(by 50%),and increased the matrix hardness(by 36%).These microstructural changes enhanced the tribological behavior.For instance,under the applied pressure of 0.5 MPa,an in-crease in the SCR from 2.0 to 59.5℃/s decreased the wear rate and the average friction coefficient of the alloy by 57%and 23%,respect-ively.The wear mechanism was also changed from the severe delamination,adhesion,and abrasion in the slowly-cooled alloy to the mild tribolayer delamination/abrasion in the high-cooling-rate-solidified sample.

The effects of deformation parameters and cooling rates on the aging behavior of AZ80+0.4%Ce

The extruded AZ80+0.4%Ce magnesium alloy was twisted in the temperature range of 300-380℃by using a Gleeble 3500 thermal simulation test machine with a torsion unit.The deformed cylindrical specimens were cooled at a cooling rate of 10℃/s or 0.1℃/s,respectively,and aged at 170℃.The microstructure analysis results showed that the grain size decreased with increasing specimen radial position from center(SRPC),and that the strong initial basal texture of the extruded magnesium alloy was weakened.Both continuous and discontinuous dynamic recrystallization mechanisms were involved in contributing to the grain refinement for all specimens investigated.And a novel extension twinning induced dynamic recrystallization mechanism was proposed for specimen deformed at 300℃.For the specimens deformed at 300℃and 340℃followed by a slow cooling rate(0.1℃/s),precipitates of various shapes(β-Mg_(17)Al_(12)),with the dominant precipitates being on the grains boundaries,appeared on the surface section.For specimen deformed at 380℃,lamellar precipitates(LPS)in the interiors of the grains were predominant.After aging,the LPS still dominated for specimens twisted at 380℃;however,the LPS gradually decreased with decreasing deformation temperatures from 380℃to 300℃.Dynamically precipitatedβ,especially those decorating the grain boundaries,changed the competition pictures for the LPS and precipitates of other shapes after aging.Interestingly,LPS dominated the areas for the center section of the specimens after aging regardless of deformation temperatures.Low temperature deformation with high SRPC followed by rapid cooling rate increased the micro hardness of the alloy after aging due to refined grain,reduced precipitates size,decreased lamellar spacing as well as strain hardening.

Impact of cooling rate on exfoliation corrosion resistance of a Li-containing 7xxx aluminum alloy

The impact of cooling rate after solution heat treatment on exfoliation corrosion resistance of a Li-containing 7xxx aluminum alloy was investigated by accelerated immersion and electrochemical impedance spectroscopy test,optical microscope,electron backscatter diffraction and scanning transmission electron microscope.With the decrease of cooling rate from 1700℃/s to 4℃/s,exfoliation corrosion resistance of the aged specimens decreases with rating changing from EA to EC and the maximum corrosion depth increasing from about 169.4μm to 632.1μm.Exfoliation corrosion tends to develop along grain boundaries in the specimens with cooling rates higher than about 31℃/s and along both grain boundaries and sub-grain boundaries in the specimens with lower cooling rates.The reason has been discussed based on the changes of the microstructure and microchemistry at grain boundaries and sub-grain boundaries due to slow cooling.

Impact of cooling rate on mechanical properties and failure mechanism of sandstone under thermal-mechanical coupling effect

High geo-temperature is one of the inevitable geological disasters in deep engineering such as resource extraction,space development,and energy utilization.One of the key issues is to understand the mechanical properties and failure mechanism of high-temperature rock disturbed by low-temperature airflow after excavation.Therefore,.the experimental and numerical investigation were carried out to study the impact of cooling rate on mechanical properties and failure mechanism of high temperature sandstone.First,uniaxial compression experiments of high temperature sandstone at different real-time cooling rates were carried out to study the mechanical properties and failure modes.The experimental results indicate that the cooling rate has a significant effect on the mechanical properties and failure modes of sandstone.The peak strain,peak stress,and elastic modulus decrease with an increase in cooling rate,and the fragmentation degree after failure increases gradually.Moreover,the equivalent numerical model of heterogeneous sandstone was established using particle flow code(PFC)to reveal the failure mechanism.The results indicate that the sandstone is dominated by intragrain failure in the cooling stage,the number of microcracks is exponentially related to the cooling rate,and the higher the cooling rate,the more cracks are concentrated in the exterior region.Under axial loading,the tensile stress is mostly distributed along the radial direction,and the damage in the cooling stage is mostly due to the fracture of the radial bond.In addition,axial loading,temperature gradient and thermal stress mismatch between adjacent minerals are the main reasons for the damage of sandstone in the cooling stage.Moreover,the excessive temperature gradient in the exterior region of the sandstone is the main reason for the damage concentration in this region.

Effect of Cooling Rates on Solidification Microstructures and Tensile Property of a Novel Wrought Superalloy

The effects of cooling rates on solidification behaviors,segregation characteristics and tensile property of GH4151 alloy were investigated using microstructure characterization and tensile test.Firstly,a relationship between the secondary dendrite arm spacing and cooling rate was determined and it was confirmed to be valid.Secondly,it can be found from microstructure observations that the morphology of(Nb,Ti)C carbides transits from blocky and script type to fine script type and spotty type,and the refinedγ'phase was observed due to decrease of segregation with increasing cooling rates.Thirdly,the solidification microstructures of the industrial-scale samples were analyzed.The morphology ofηphase changes from indistinguishable shape,fine needle-like shape to large block-like shape with increasing ingot diameter.As a result,the mechanical properties of alloy decrease due to increase of brittle precipitations.The experimental results show that the precipitation behavior of GH4151 is affected by segregation degree of elements,and the segregation degree is determined by solute distribution process and solid back-diffusion process.

Impact of Cooling Rate on the Results of Vibration Treatment of the Aluminum Casting

The effect of vibration(50 Hz)on the formation of aluminum castings of 99.5%purity at various cooling rates was studied.It was found that the presence of vibration leads to an increase in the cooling rate of the castings.It was found that the higher the speed without vibration,the stronger the effect of increasing the speed when vibration was applied.Apparently,this effect is associated with additional mixing of the melt by free-floating crystals.

Effects of cooling rate on microstructure,mechanical and corrosion properties of Mg-Zn-Ca alloy

Mg69Zn27Ca4 alloys with diameters of 1.5, 2 and 3 mm were fabricated using copper mold injection casting method. Microstructural analysis reveals that the alloy with a diameter of 1.5 mm is almost completely composed of amorphous phase. However, with the cooling rate decline, a little α-Mg and MgZn dendrites can be found in the amorphous matrix. Based on the microstructural and tensile results, the ductile dendrites are conceived to be highly responsible for the enhanced compressive strain from 1.3% to 3.1% by increasing the sample diameter from 1.5 mm to 3 mm. In addition, the Mg69Zn27Ca4 alloy with 1.5 mm diameter has the best corrosion properties. The current Mg-based alloys show much better corrosion resistance than the traditionally commercial wrought magnesium alloy ZK60 in simulated sea-water.

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.

Effect of cooling rate on solidification parameters and microstructure of A1-7Si-0.3Mg-0.15Fe alloy

The effects of cooling rate on the solidification parameters and microstructure of Al-7Si-0.3Mg-0.15 Fe alloy during solidification process were studied.To obtain different cooling rates,the step casting with five different thicknesses was used and the cooling rates and solidification parameters were determined by computer-aided thermal analysis method.The results show that at higher cooling rates,the primary α（Al） dendrite nucleation temperature,eutectic reaction temperature and solidus temperature shift to lower temperatures.Besides,with increasing cooling rate from 0.19 ℃/s up to 6.25 ℃/s,the secondary dendritic arm spacing decreases from 68 μm to 20 μm,and the primary dendritic volume fraction declines by approximately 5%.In addition,it reduces the length of Fe-bearing phase from 28 μm to 18 μm with a better uniform distribution.It is also found that high cooling rates make for modifying eutectic silicon into fibrous branched morphology,and decreasing block or lamella shape eutectic silicon.

Influence of cooling rate on phase transformation and microstructure of Ti-50.9%Ni shape memory alloy

Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature （Ms） and martensitic finish transformation temperature （Mf） decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis （Af-Mf） increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.

Influence of cooling rate on solidification behavior of sand-cast Mg-10Gd-3Y-0.4Zr alloy

The effect of the cooling rate ranging from 1.4 °C/s to 3.5 °C/s on the solidification behavior of the sand-cast Mg?10Gd?3Y?0.4Zr alloy was studied by computer aided cooling curve analysis (CA-CCA). With the increase in cooling rate, the nucleation temperature (Tα,N) increases from 634.8 °C to 636.3 °C, the minimum temperature (Tα,Min) decreases from 631.9 °C to 630.7 °C, the nucleation undercooling (ΔTN) increases from 2.9 °C to 5.6 °C, the beginning temperature of the eutectic reaction (Teut,N) increases, the time of the eutectic reaction shortens, solidus temperature decreases from 546.0 °C to 541.4 °C, and solidification temperature range (ΔTS) increases by 6.1 °C. The increased nucleation rate (N&) is supposed to be the main reason for the increased?TN. Increased value (Teut,N?Teut,G) and shortened time of the eutectic reaction cause the change in the volume fraction and morphology of the second phase.

Effect of cooling rate on morphology of primary particles in Al-Sc-Zr master alloy

Al-1.0%Sc-1.0%Zr （mass fraction） master alloy was prepared at different cooling rates. The morphology and thermodynamics data of the primary particles of the master alloy were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and differential scanning calorimetry （DSC）. It shows that the primary particles are dendrite-shaped particles comprised of several attached small cubic, cusped-cubic or crucifer shape particles at slow cooling rate. However, the primary particles are separated with crucifer shape at intermediate cooling rate, and they are cubic with cusped-cubic shape at high cooling rate. Meanwhile, the separated and attached particles present AlaSc/AlaZr1-xScx core-shell structure. The formation mechanism of the structure was systematically investigated by a mathematical model.

Effect of cooling rates on clustering towards icosahedra in rapidly solidified Cu_(56)Zr_(44) alloy

The rapid solidification processes of liquid Cu56Zr44 alloys at different cooling rates （γ） were simulated by a molecular dynamics （MD） method. In order to assess the influence of cooling rate on the clustering tendency and degree towards icosahedrons, a ten-indices＇ cluster-type index method was suggested to characterize the local atomic structures in the super-cooled liquid and the rapidly solidified solid. And their clustering and ordering degrees as well as the packing density of ieosahedral clusters were also evaluated by an icosahedral clustering degree （fI）, the chemical order parameter （ηαβ） and densification coefficients （D0, DI and DIS）, respectively. Results show that the main local atomic configurations in Cu56Zr44 alloy system are Z12 clusters centered by Cu, and most of which are （12 0 12 0 0 0 0 0 0 0） standard icosahedra and （12 0 8 0 0 0 2 2 0 0） as well as （12 2 8 2 0 0 0 0 0 0） defective icosahedra. Below glass transition temperature （Tg）, these icosahedral clusters will be coalesced to various icosahedral medium-range orders （IMROs） by IS linkages, namely, icosahedral bond, and their number N, size n, order parameter ηαβ as well as spatial distributions vary with y. As the cooling rate exceeds the critical value （γc） at which a glassy transition can take place, a lower cooling rate, e.g., γ1=10^1K/ns, is demonstrated to be favorable to uplift the number of icosahedra and enlarge the size of IMROs compared with the higher cooling rates, e.g., γ5=10^5 K/ns, and their packing density and clustering degree towards icosahedra in the rapidly solidified solid can also benefit from the slow cooling process.

Effects of cooling rate on thermal expansion of Cu_(49)Hf_(42)Al_9 metallic glass

Effects of cooling rate on thermal expansion of Cu49Hf42Al9 metallic glass were studied. Five types of amorphous samples with different sizes were prepared in order to get a broad range of cooling rates （from 102 to 107 K/s）. The average thermal expansion coefficients （αaver） of as-quenched samples range from 6.14×10-6 to 9.20×10-6 K-1. When the temperature is below the glass transformation temperature （Tg）, αaver of as-quenched samples has a negative correlation with cooling rate; the values of αaver of annealed and crystallized samples are closed to each other. The results indicate that the amount and motion of free volume play important roles in thermal expansion of metallic glasses.

Effects of cooling rate on bio-corrosion resistance and mechanical properties of Mg-1Zn-0.5Ca casting alloy

Mg?1Zn?0.5Ca alloys were prepared by traditional steel mould casting and water-cooled copper mould injection casting at higher cooling rate. Microstructure, mechanical properties and bio-corrosion resistance of two alloys were contrastively investigated. Grain size reduces remarkably and microstructure becomes homogenous when raising cooling rate. The bio-corrosion behaviour in 3.5% sodium chloride solution (3.5% NaCl) and Hank’s solution at 37°C was investigated using electrochemical polarization measurement and the results indicate that the alloy prepared at higher cooling rates has better corrosion resistance in both types of solution. Further mass loss immersion test in Hank’s solution reveals the same result. The reason of corrosion resistance improvement is that raising cooling rate brings about homogeneous microstructure, which leads to micro-galvanic corrosion alleviation. The tensile test results show that yield strength, ultimate tensile strength and elongation are improved by raising cooling rate and the improvement is mainly due to grain refinement.

Effect of cooling rates on dendrite spacings of directionally solidified DZ125 alloy under high thermal gradient

The dendrite morphologies and spacings of directionally solidified DZ125 superalloy were investigated under high thermal gradient about 500 K/cm. The results reveal that, with increasing cooling rate, both the spacings of primary and secondary dendrite arms decrease, and the dendrite morphologies transit from coarse to superfme dendrite. The secondary dendrite arms trend to be refined and be well developed, and the tertiary dendrite will occur. The predictions of the Kurz/Fisher model and the Hunt/Lu model accord basically with the experimental data for primary dendrite arm spacing. The regression equation of the primary dendrite arm spacings 21 and the cooling rate Vc is λ1 = 0.013 Vc^-0.32. The regression equation of the secondary dendrite arm spacing λ2 and the cooling rate Vc is λ2 = 0.00258 Vc^-0.31, which gives good agreement with the Feurer/Wunderlin model.

Effect of cooling rate on solidification microstructure and mechanical properties of TiB2-containing TiAl alloy

Effects of cooling rate and 0.25 at.%TiB2 addition on solidification microstructure and mechanical properties of Ti−48Al−2Cr−2Nb alloys fabricated by the investment casting with different thicknesses were studied.The results show that with the cooling rate increasing from 37 to 2×102 K/s,the solidification path of the studied alloys is unchanged.The grain size of the matrix alloy is refined from 650 to 300μm,while the grain size of Ti−48Al−2Cr−2Nb−TiB2 is reduced from 550 to 80μm.The lamellar spacing of matrix alloy is reduced from 360 to 30 nm with increasing the cooling rate from 37 to 2×102 K/s,while TiB2 addition shows little refinement effect on the lamellar spacing.Ti−48Al−2Cr−2Nb−TiB2 sample under medium cooling rate(69 K/s)exhibits superior microhardness(HV 550)and ultimate tensile strength(570 MPa)among the studied alloys.The refined grain size,lamellar spacing and fine TiB2 particles could account for the favorable mechanical properties of the studied TiB2-containing alloy.The microstructure evolution was discussed in light of cooling rate,constitutional supercooling and borides addition.

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution,morphology and volume fraction ofγ′phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods.Oil quenching and air cooling were adopted with cooling rate of 183°C/s and 4?15°C/s,respectively.The experimental results show that the average size of the secondaryγ′after oil quenching is 24.5 nm compared with 49.8 nm under air cooling,and corresponding volume fractions ofγ′are 29%and 34%,respectively.Meanwhile,the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens.The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen,but the difference approaches each other as the temperature increases to 650°C.The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature,in contrast to the intergranular fracture at 650°C or even higher temperature.These two mechanical responses indicate the strengthening effects ofγ′precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.