Aluminum Casting Heat Treating: Looking for Faster Cycles
Pacifica (Melbourne, Australia) designs and manufactures automotive components and friction materials for world markets. Pacifica's technology arm, Pacifica Group Technologies (PGT), has developed innovative casting technology which has been pivotal in the emergence of PBR aluminum brake calipers in North America. Pacifica manufactures aluminum brake calipers in Melbourne, and has casting plants in Tennessee and South Carolina, USA.
The challenge for today's casting manufacturers is to balance high productivity without compromising safety or product quality. The aluminum T6 solution heat treatment process discussed in this paper is appropriate for aluminum magnesium-silicon alloys. Pacifica has used gas fired and electric convection furnaces for the T6 solution treatment of A357 aluminum brake calipers for over 25 years.
Conventional batch type aluminum heat treatment systems
For many years, standard equipment for production of heat treated aluminum castings has been the pit furnace (Fig. 1). Most often electrically heated, hot air is circulated vertically through the load by fans placed either at the bottom of the pit or in the furnace lid. Because of the importance of even airflow, cylindrical furnaces are usually used, thus minimizing zones of differential airflow within the chamber. This type of equipment is relatively inexpensive, simple and because it is usually installed below floor level, can be more energy efficient than an above-ground furnace.
To prevent furnace inefficiencies, the lid must provide an airtight seal and is commonly clamped in position pneumatically. The furnace is then raised to and held at the set temperature for a predetermined time to achieve solutionizing of the cast structure. It is particularly difficult to maintain an even temperature in a vertical direction when heat treating castings in large pit furnaces. A degree of temperature stratification in the furnace is inevitable and, in cases where the load mass and shape is not identical from one batch to another, there can be problems with localized overheating or under-heating. Typically, ramping the casting up to temperature in the furnace takes up to one third of the total processing time (Fig. 2).
To minimize the risk of localized melting (eutectic or insipient melting) of the casting structure due to temperature stratification, pit furnaces are usually operated well below the eutectic temperature of the alloy being heat treated, with the load held at temperature for a correspondingly longer time. Where solutionizing temperatures as low as 530°C (985°F) are used, a time at temperature of between 9 and 12 hours is not uncommon.
Following solutionizing, the load must be removed from the furnace and quenched in water immediately. It is vital that solutionized structure is maintained, and the importance of the quenching operation cannot be overstressed. A correlation between a decrease in tensile strength and quench delay has been reported  and ideally the load should be quenched within 30 seconds of the furnace being opened. Accordingly, the process of removing the furnace lid, positioning the crane, lifting the load out of the furnace, moving it to the quench bath and lowering it into the water should be streamlined as far as possible. The inherent equipment constraints of crane winch and drive speed need to be optimized to achieve best practice time to quench in a production environment.
The most effective method of shortening the time to quench is to place the furnace above ground and the quench tank in the pit immediately below it. These types of heat treatment systems are known as drop-bottom furnaces. As they can achieve very fast quench rates, and thus optimum solutionizing, these systems are frequently used for structural components, particularly castings for aerospace applications.
Following quenching, the load is placed back into the furnace pit and subjected to a time at temperature precipitation aging treatment. Heating at a temperature either over or under the optimum aging temperature will result in less than optimum results. It follows that poor temperature control in the furnace will result in difficulties in achieving the correct exposure time.
Continuous aluminum heat treatment systems
Equipment that can continuously process castings through a temperature-controlled oven from an inlet point to an outlet point and then immediately quench them is likely to overcome many of the negative aspects of batch heat-treating. While a continuously fed oven is simple in principle, there are many technical challenges in the design and manufacture of this type of heat treatment furnace.
The most basic type of continuous system uses a continuous chain or conveyor mesh to carry parts through the oven where they are subjected to the treatment temperature for the time taken to travel the length of the oven. Another method is to suspend the component from an overhead conveyor as it is carried through the furnace.
The challenge with continuous systems thus becomes how to efficiently transport a batch of as many castings as possible through the furnace while achieving optimum heat treatment and thus consistent mechanical properties throughout the batch. Continuous systems that were built in the 1970s and 1980s often required each new basket entering the furnace to literally push the previous basket along so that a continuous "train" was required if processed baskets were to exit the furnace. Consequently, the furnace was often partially filled with empty baskets. Due to improved roller and basket design, improvements in roller installation and roller alignment techniques, today's continuous systems are able to process individually loaded baskets, which move through the furnace independently.
The latest continuous systems are designed using computer simulation to optimize the direction of hot air through the load. PBR's heat treatment system has an internal profile and system of fixed baffles that directs the hot airflow through each basket. Closed loop temperature monitoring above the moving basket gages the air temperature on the down wind or cool side of the basket and directs the furnace control system to react accordingly.
This method allows full, partially full and empty baskets to be subjected to a preset amount of heating for a predetermined length of time. In calibration tests using the latest data-acquisition technology, temperature variation within the basket of parts has been shown to be less than +/-3°C (+/-5°F). PBR's heat treatment system supplied by Seco/Warwick Corp. (Meadville, Pa.; www.secowarwick.com) is shown in Fig. 3. The continuous system uses a unique basket tracking system developed by PBR and Seco/Warwick to ensure that any part in any basket can be matched to the heat treatment regime it was subjected to.
At the end of the continuous solution treatment furnace, each basket is indexed at high speed and automatically quenched. The system is able to fully immerse a 2 metric ton load in the quench tank within 25 seconds of the exit door opening.
Even with the minimum of quench delay times, the resultant material properties may still vary within a given load with certain types of castings and quench-induced casting distortion can still occur, so correct racking of castings and sound casting design is essential.
The final step in the T6 process is to artificially age the aluminum structure. This involves subjecting the castings to a time at temperature heating regime of which there are many combinations in use. Due to the relatively low oven temperature required, if there is a subsequent painting process applied to the casting, it may be possible to dispense with this step entirely and artificially age during the paint curing process.
Tomorrow's solution heat treatment process
Heat treating aluminum castings presents several technical challenges, which can be overcome by continuously minimizing limitations found in the equipment. Current industry practice for T6 solution and age of aluminum castings has been defined by the equipment capabilities in terms of process control, production rates and processing efficiencies.
Furthermore, there is a disparity between the theoretical time temperature regime for a T6 solution and age process and current academic thinking and R&D emphasis . Work at Pacifica's technical arm, Pacifica Group Technologies (PGT), indicates that very short solution and age cycle times are technically possible. The improvements in efficiency and productivity promised by rapid heat treatment process for aluminum castings should be of interest to all manufacturers of high performance aluminum castings.
Modification of the aluminum microstructure is carried out by heating and quenching the casting. Initial heating brings the magnesium and silicon into solution within the cast structure (solution treatment). An intermediate quenching operation ensures that the magnesium and silicon remains in solution and this is followed by a further heating process to bring about precipitation and achieve hardening (aging). Typical minimum properties for T6 solution treated A357 are 290 MPa (42 ksi) tensile strength, 250 MPa (36.2 ksi) 0.2% yield strength, 3% elongation and 95 HBN hardness.
The aim of the solution treatment process is to dissolve the constituents of the interdendritic secondary phase existing in the alloy. The rate at which the constituents of the secondary phase are dissolved can be described in terms of a time-temperature relationship called the index of residual segregation . For a binary alloy, please refer to equation 1, where f/f° is the ratio of residual segregation for time at temperature, t and t°; D is the coefficient of diffusion for Mg; t is the time at the solutionizing temperature in seconds and l° is one half of the measure (µ) of secondary dendrite arm spacing. The equation becomes (equation 2) where a = (-0.625 x 10-3)t. The degree of solutionizing as a function of time at a given temperature can be determined as shown in Fig. 4.
A further heating process (precipitation hardening) is required following the quenching operation (which maintains the solution condition) to fully realize the potential properties of the alloy. Precipitation hardening involves complex time-temperature dependent mechanisms involving precipitation of magnesium silicide. The outcome of a time and temperature dependant chemical reaction can be quantitatively determined using the Arrhenius equation (equation 3) where K is the rate coefficient (sec), A is a constant (sec), E is the activation energy for the reaction (Q mol), R is the gas constant (8.314 x 10-3 kJ mol K) and T is temperature (K). By ignoring the intercept, the equation becomes (equation 4) so that a graph of ln t against 1/T has a slope of -Q/R. PGT conducted practical trials and Design of Experiments to generate data points for 100 Brinell tests and derived a slope of 1,500 used to generate the Arrhenius plot shown in Fig. 5.
Although the age temperature-time relationship is nonlinear, a straight line was plotted using limited data, and a matrix was generated as shown below.
Time, s = Temp., °C
900 = 220
1,800 = 200
2,700 = 189
3,600 = 183
4,500 = 178
The work showed that using specialized equipment designed for rapid heat treating, the process time and temperature relationships likely to achieve satisfactory results for A357 were: Solution treat at 547°C +/-1.5°C for 45 min and age at 185°C +/-1.5°C for 60 min.
This study was supported by Design of Experiments work using test bars and production castings to confirm that the required mechanical properties can be consistently achieved. Typical mechanical properties were 316 MPa UTS, 253 MPa 0.2% YS, 4.24% elongation and 103 HBN hardness.
The reference literature often is more conservative in the suggested process parameters. For example, the ASM Metals Handbook gives a T6 solution and age treatment of: ST 540C 8h/Q/A 175C 6h . Thus, there is a disparity between theory, the needs of the end user and the current state of the art. As the demand for high strength aluminum castings continues to grow, it is likely that casting producers will increasingly challenge equipment manufacturers to develop viable rapid heat treatment processes together with next-generation heat treatment equipment.
As casting manufacturers pursue increased manufacturing performance from all their processes including the heat treat process, suppliers of heat treating equipment will be challenged to respond. IH