Quench Sensitivity and Phase Transformation Kinetics of AlSi7MnMg High Pressure Vacuum Die Casting

The quench sensitivity of an AlSi7MnMg alloy in high-pressure vacuum die (HPVD) casting was investigated by time-temperature-transformation and time-temperature-property diagrams with an interrupted quench technique. The quench sensitive temperature range of the alloy is from 260 to 430 °C and its nose temperature is 350 °C. The mechanical strength versus cooling rates of the HPVD casting was predicted using quench factor analysis method and verified by experimental results. The critical cooling rate is 6 °C/s to remain 95% of the maximal mechanical strength. The coefficients k2 - k5, related to the nucleation and precipitation kinetics of TTP curves, and phase transformation diagrams were determined. The precipitation of Mg2Si phase in the castings was observed during isothermal treatment using transmission electron microscope. Moreover, the quench sensitivity and kinetics of the phase transformation of AlSi7MnMg alloy and AlSi10MnMg alloys were compared. It reveals that the quench sensitivity and phase transformation rate of the former are lower than that of the latter.

Investigation of the Quench Sensitivity of an AlSi10Mg Alloy in Permanent Mold and High-Pressure Vacuum Die Castings

The quench sensitivities of an AlSi10Mg alloy in permanent mold (PM) and high-pressure vacuum die (HPVD) castings were investigated with time–temperature–transformation and time–temperature–property diagrams using an interrupted quench technique. The quench-sensitive temperature range of the HPVD casting sample is 275–450 °C, and its nose temperature is 375 °C. The quench-sensitive range of the PM casting sample is 255–430 °C, and the nose temperature is 350 °C. The mechanical strength versus the cooling rate in both casting samples were predicted via a quench factor analysis and verified experimentally. The critical cooling rate of the HPVD casting sample is 20 °C/s whereas it is 17 °C/s for the PM casting sample. With a shorter critical time, higher nose temperature, and higher critical cooling rate, the HPVD casting sample exhibits a higher quench sensitivity than the PM casting sample. The differences in the quench sensitivities of the AlSi10Mg alloy due to the different casting processes is explained via the different precipitation behavior. At the nose temperature, coarse β-Mg2Si precipitates mainly precipitate along the grain boundaries in the HPVD casting sample, whereas rod-like β-Mg2Si precipitates distribute in the aluminum matrix in the PM casting.

Austenite to Ferrite Transformation Kinetics in Fe-9 %Cr Alloys - I. General Kinetic Analysis

The paper considers theoretical aspects of the kinetics of austenite → ferrite transformation in an Fe–9 %Cr alloy, a common model of diffusionless transformation. In previous studies it was shown that this transformation under isothermal conditions shows a behaviour typical for nucleation site saturation, including the change of the Avrami exponentn(determined as the slow of transformation curve on double logarithmic scale) from 4 to 1. Activation energies determined in two ways: by the ‘nose’ temperature of the normal C-curve and by the slope of the C-curve re-drawn on a reverse temperature scale are unexpectedly similar (272–315 kJ/mole) and not temperature-dependent. But the complete TTT diagrams calculated using these values determined directly from experimental data and the precise formula of Cahn’s solution of grain face nucleated transformation problem do not provide good agreement with experiment in the whole temperature range. This may mean that the theory of site saturation needs some correction.

Constitutive Behavior for Quenching of Al–Cu–Mg Alloy With Consideration of Precipitation

The deformation behavior of as-quenched 2024 Al–Cu–Mg alloy has been experimentally studied. The experiments are designed to cool specimens to the desired temperature with a constant cooling rate, i.e., 5 K/s. Isothermal tensile tests are performed over a range of 573–723 K temperature and (0.01, 0.1, and 1 s−1) strain rates to find out the flow stresses and microstructures after deformation. Due to the nonuniform deformation mechanisms (solid solution versus solid solution and precipitation), two types of Arrhenius model are established for the temperature range of 573–673 K and 673–723 K, respectively. For temperature between 573 and 673 K, the activation energy is dependent on temperature and strain rate, and the value of activation energy decreases with the increases of temperature and strain rate. Compared with the ideal variation trend with no consideration of precipitation, the largest difference of activation energy is found at the temperature of 623 K which is the nose temperature of 2024 alloy.

Measurement of TTP Curves of 7050 Aluminum Alloy by Conductivity

The conductivity of 7050 alloy after isothermal quenched and isothermal quenched then T6 aged was measured by a simple and efficient method of measuring conductivity, its TTP curves fitted by conductivity data and also the isothermal treated microstructure of this alloy studied by the transmission electron microscopy (TEM). At the same time, the TTP curves fitted respectively by hardness and conductivity data of isothermal quenched then T6 aged samples were compared. The results show that it is feasible to fit TTP curves of 7xxx series high strength aluminum alloy by the method of conductivity measurement which can obtain its TTP curves by measuring transient properties of alloy. The nose temperature of the TTP curves of 7050 alloy fitted by conductivity method is about 330°C and at this temperature the incubation period for decomposition and precipitation of supersaturated solid solution is 1.25 s. By isothermal treating at 330°C for 2 s the precipitates in 7050 alloy form mainly by heterogeneous nucleation. It is indicated that for current TTP curves of aluminum alloy fitted by the method of measuring properties, the precipitates corresponding with the TTP curve which represents properties of alloy variations about 0.5% of the target properties are heterogeneous nucleation precipitation.

The Effect of Heat Treatment on the Long-Term Corrosion Resistance in Er Containing 5083 Cold Rolled Sheets

The effect of heat treatment on the long-term corrosion resistance of Er containing 5083 aluminum alloy was studied using the micro-hardness test and mass loss test. The microstructure was analyzed by TEM. To maintain the strength, the annealing temperature was selected to be 100-230°C below the recrystallization onset temperature determined by the micro-hardness test. The plot of the annealing temperature versus the Intergranular Corrosion (IGC) initial time, which is determined by the Nitric Acid Mass Loss Test, showed a C-curve. The shortest IGC initial time (~1h) happened at 175°C, the nose temperature of the C-curve. When annealed at 125-200°C, the samples were IGC sensitive with the initial time less than 3h. The entirely IGC resistant (stabilized) samples were obtained when annealed at 220-230°C. The 175°C sensitized treatment was performed on the 220°C-stabilized samples, which showed that the 3-12h stabilization could significantly improve the resistance for long-term corrosion. TEM results showed that, for the IGC sensitive samples, β phases (Al3Mg2) grew along the grain boundary continuously, while for the stabilized samples, they were isolated on triangle grain boundary and phase boundary.

Effect of Austenitization Temperature on Formation of Low Temperature Bainite

The influence of austenitization temperature on the incubation period and bainitic transformation behaviours of the high-carbon silicon steel has been investigated. It was found that the nose temperature of bainite transformation and incubation period decreases with increasing austenitization temperature. The microstructure characteristics of the bainitic transformation products have been also observed. After isothermal heat treatment at 230°C for 20 mins, all samples austenitized at different temperatures produced a bainitic structure, which consists of packets of parallel ferrite laths. The major difference lies in the edge boundary morphology. Bainitic laths formed in low-temperature austenitization conditions has sharp saw-tooth edge boundaries, whereas bainite transformed from high-temperature austenitization conditions, have smooth wedge boundaries. Key Words: austenitization temperature; low-temperature bainite; incubation period;edge boundary