Waldo E. Stumpf

Natural and artificial aging response of semisolid metal processed Al–Si–Mg alloy A356

Abstract International standards for aluminium alloys often permit significant fluctuations in the content of alloying elements. This allows metal suppliers more freedom in preparing these alloys. It is shown that the magnesium content of semisolid metal processed Al–Si–Mg alloy A356 has a significant influence on the natural and artificial aging behaviour of the alloy. Furthermore, natural aging before artificial aging causes the time to peak hardness (T6) to be longer compared to the time when only artificial aging is used. The optimum quality index in this study was obtained using a short solution heat treatment of 1 h at 540°C, no natural aging and artificial aging at 180°C for 1 h. An increase in the magnesium content of the alloy resulted in an increase in the quality index for all the T6 heat treatment cycles studied.

Comparison of heat treatment response of semisolid metal processed alloys A356 and F357

Abstract The heat treatment response of semisolid metal high pressure die cast Al–7Si–Mg alloys A356 and F357 was studied and compared. It was found that the heat treatment behaviour of alloy F357 is influenced markedly by the stability of the Mg containing π phase. This phase, which dissolves in alloy A356 during solution treatment, persists in F357 and decreases the amount of magnesium in solid solution. This is the likely origin of the decrease in the aging response of the F357 alloy. The tensile properties (yield strength and ultimate tensile strength) of the alloys correlate much better with the Mg concentration in solid solution than with the bulk Mg content of the alloys. The recently developed shortened T6 heat treatment cycles for rheocast A356 were tested on alloy F357. The optimum artificial aging treatment was determined to be 180°C for 4 h, regardless of the prior natural aging period.

The Natural and Artificial Aging Response of Semi-solid Metal Processed Alloy A356

The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for dendritic casting alloys. These heat treatments are not necessarily the optimum treatments for non-dendritic microstructures. For rheocast alloy A356, it is shown that natural aging prior to artificial aging causes the time-to-peak-hardness to be longer compared to the time when only artificial aging is used. Furthermore, a hardness plateau is maintained during artificial aging at 180oC between 1 and 5 hours without any prior natural aging. A natural aging period as short as 1 hour results in a hardness peak (rather than a plateau) to be reached during artificial aging after 4 hours at 180oC.

Application of shortened heat treatment cycles on A356 automotive brake calipers with respective globular and dendritic microstructures

Since the automotive industry has many possible applications for semi-solid metal (SSM)−high-pressure die casting (HPDC) parts, the newly developed heat treatment cycles, as well as the traditional heat treatment cycles, were applied to A356 brake calipers cast using a LK DCC630 HPDC machine. Vickers hardness measurements at a cross section of the brake calipers were performed, indicating that similar values can be obtained when using the significantly shorter heat treatment cycles. Finally, the typical tensile properties that can be obtained for SSM-HPDC A356 brake calipers are compared with those manufactured by gravity die casting. Results indicate that the differences in microstructures (globular or dendritic) do not have a noteworthy effect on the heat treatment response. This implies that the short heat treatment cycles originally developed for globular SSM-HPDC A356 castings can

Influence of temper condition on microstructure and mechanical properties of semisolid metal processed Al-Si-Mg alloy A356

Abstract The microstructures and mechanical properties of strontium modified semisolid metal high pressure die cast A356 alloy are presented. The alloy A356-F (as cast) has a globular primary grain structure containing a fine eutectic. Solution treatment results in spheroidisation of the eutectic silicon particles under the T4 and T6 temper conditions. The A356-T5 maintains the fibrous silicon morphology after artificial aging. A356-T4 has better ductility and impact strength than A356-T5 due to its spheroidised silicon morphology. The impact properties of semisolid metal high pressure die cast A356 are controlled mainly by the silicon morphology and alloy strength (hardness), whereas tensile strength is determined by the degree of solid solution coupled with precipitate formation during aging.

Influence of elevated Fe, Ni and Cr levels on tensile properties of SSM-HPDC Al-Si-Mg alloy F357

Abstract The microstructures and tensile properties of semi-solid metal high pressure die cast (SSM-HPDC) F357 alloys with low and high levels of Fe, Ni and Cr were compared in different temper conditions. ThermoCalc software was used to predict the different intermetallics that can be expected in the alloys, and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used to investigate the actual intermetallics that formed. The influence of these intermetallics on tensile properties was quantified. The results show that lower strength is obtained in the alloy with high Fe, Ni and Cr levels. This is attributed mainly to the formation of more ?-Al 8 FeMg 3 Si 6 phase, which removes strengthening Mg atoms from solid solution. Also, the ductility of the high Fe, Ni and Cr levels alloy is decreased significantly due to microcracking of the higher volume fraction ?-Al 8 FeMg 3 Si 6 and Al 9 FeNi phases. The combination of lower strength and ductility results in a decrease of the quality index of this alloy compared with the alloy with low levels of Fe, Ni and Cr.

The T5 Heat Treatment of Semi-Solid Metal Processed Aluminium Alloy F357

The T5 heat treatment of semi-solid metal (SSM) processed alloy F357 was investigated by considering the effects of cooling rate and natural aging after casting, as well as artificial aging parameters on tensile properties. In addition, the tensile properties of SSM-HPDC F357 in different temper conditions (F, T4, T5 and T6) are compared. The Quality Index (QI) is used to compare the influence of different T5 heat treatment parameters and different temper conditions.

Comparison of the heat treatment response of SSM-HPDC 6082 and 6004 wrought alloys with A356 and F357 casting alloys

Semi-solid metal high pressure die casting was used to produce plates from traditional wrought Al-Mg-Si alloys 6082 and 6004, as well as from traditional casting Al-Si-Mg alloys A356 and F357. The high Si-content of the casting alloys offer several advantages, including a faster artificial aging response, higher strength for comparable Mg contents and less sensitivity to prior natural aging on peak strength. However, over-aging occurs earlier in the casting alloys than in the wrought alloys.

The Influence of Prior Natural Aging on the Subsequent Artificial Aging Response of Aluminium Alloy A356 with Respective Globular and Dendritic Microstructures

Alloy A356 is one of the most popular alloys used for semisolid metal forming. The heat treatment cycles that are currently applied to semisolid processed components are mostly those that are in use for dendritic casting alloys. The assumption has been made that these heat treatments are not necessarily the optimum treatments, as the difference in solidification history and microstructure of SSM processed components should be considered. The objective of this study is to determine whether dendritic A356 behaves in a similar way to globular A356 in terms of its response to artificial aging with or without prior natural aging. The results indicate that the differences in microstructures (globular or dendritic) do not have a noteworthy effect on the heat treatment response. It is also shown that strong linear correlations are found between T4 and T6 hardness and wt% Mg of A356, regardless of the casting technique used.

Dynamic Globularization of α-Phase in Ti6Al4V Alloy during Hot Compression

Ti6Al4V samples were isothermally compressed using a Gleeble(TM) 1500D thermo-mechanical simulator. Differential scanning calorimetry (DSC), microstructural analyses, and thermodynamic calculations were used to investigate the sequence of transformation of β into α or vice-versa and the presence of different phases in the compressed Ti6Al4V sample. Globular alpha phase was revealed in the isothermally compressed sample in addition to martensitic and lamellar α/β structures. The transition temperature range of β into α-phase was determined using the DSC thermograms and thermodynamic calculated diagrams. The fraction of α-phase globulized increased as the strain rate decreased from 0.01s-1 to 10-3s-1, and the spheroidization of the α-phase is only possible in a specific range of deformation temperatures.