Short-Cycle Solution Heat Treatment In a Modern Fluidized Bed
Advanced heat-treated alloys make possible many of today's technological achieve-ments. These alloys are used wherever one looks-from automobiles to airplanes. In precipitation-hardenable aluminum alloys, heat treatment, quenching, and aging are used to develop the desired properties following the casting process. As essential as this process is, it often is a manufacturing bottleneck since the process currently can take up to 30 hours. Technomics believes these long process times are not essential, and is investigating faster cycle times using fluidized bed technology. An automated process that greatly reduces the time and energy requirements of the heat treatment cycle has been developed using advanced fluidized bed technology optimized for the heat-treating industry. As part of this effort, the company has been working in concert with Worcester Polytechnic Institute, or WPI (Worcester, Mass.) and other selected partners in industry. One of these critical partners is ANSYS Inc. (Pittsburgh, Pa.), a manufacturer of finite element analysis (FEA) software. In addition, Technomics has received a federal NIST grant to study and validate the advantages of quenching aluminum castings in a fluidized bed.
Traditional solution heat treatment
Heat treatment is used to change mechanical properties, microstructure, and residual stress state. For example, a T6 heat treatment would be used to increase the strength and hardness of a part by precipitation hardening. The process starts by heating the part to near its solidus temperature of approximately 1000 F (540 C), which varies between specific alloys. The part is allowed to soak at this temperature for a time to dissolve the soluble phases. Subsequent quenching locks these phases within the matrix, resulting in a supersaturated solution. An aging process follows where the part is held at a comparatively low temperature to form finely dispersed strengthening precipitates . The quality of this process is affected by many factors, but four significant issues are: how close the part can be brought to its solidus temperature; the repeatability of the heat treatment cycle from part to part and batch to batch; the uniformity of the heating and cooling processes; and the rate of heating and cooling. The effect of this process is to improve the properties of the material for the end user, such as optimizing strength, hardness, elongation or machinability.
Currently, heating in furnaces in air is the most common method of solution heat-treating parts in industry. However, these furnaces are inherently inefficient at transferring energy to the parts, which leads long cycle times. To overcome this problem, they typically are loaded with a large basket of similar parts before starting a heat-treating cycle. Castings coming from shakeout must be held until an entire basket is loaded before heat treating . This creates a production bottleneck that is a major problem with the implementation of Just-In-Time (JIT) manufacturing. It both slows the process and results in large amounts of work in progress (WIP); that is, product that is taking up precious space on the factory floor.
Problems caused by using the batch method of heat treatment are not limited to the slowing of production throughput. The quality of the finished product also is negatively affected. Loading the furnace in this way slows the rate of heat transfer to the parts and causes nonuniform heating. When the parts are packed together, each piece is exposed to different heating profiles. Parts loaded in the convection flow will heat much more rapidly than those that are blocked from the air flow by other parts in the basket. Even within one part, significant differences in heating rate can be found. Analysis shows this can lead to temperature gradients across a part of 100 F (55 C) or more (Fig. 1).
Another quality-related drawback of air furnaces is the precision of their temperature control. Both the process control temperature and the uniformity of the temperature profile throughout the process chamber are critical to part quality. When loaded, air furnaces often can be controlled to +/-10 F (\+/-5 C). This leads to two problems: lack of consistency and repeatability of the process; and lower quality of the heat treatment. The temperature uncertainty in the chamber, along with the significant variation in heating rates for parts within a basket, result in large variations in the heat treating cycles between parts in the batch. Heating times between individual parts in one basket may vary by as much as one hour or more.
Finally, since the actual temperature of the oven is only known within a 20 F (10 C) window, the operating set point of the furnace must be kept a safe margin below the solidus temperature of the alloy. The temperature any given part reaches may be even lower. This means optimum material properties cannot be achieved because the heat treating furnace is not capable of operating near the solidus point without running the risk of overheating and damaging parts.
The next phase of the heat treatment process is quenching. Water or a mixture of water and glycol is a typical quenchant, but it has several key drawbacks. One is the relatively small window between freezing and boiling in which the water temperature can be varied. Another problem with water quenching is the occurrence of random, localized boiling on the surfaces of 1000 F (540 C) parts. Where boiling occurs, a pocket of water vapor is created which locally affects the rate of heat transfer. This area then experiences a different rate of thermal contraction, causing distortion and induced stresses. Often this is dealt with by adding additional material (and cost) to the casting that is machined off later (at additional cost) to achieve the desired finished geometry.
Newly developed fluidized bed
To address these drawbacks, Technomics is involved in an extensive study of a promising technology. This technology is solution heat treating, quenching and aging in a fluidized bed. A fluidized bed is a bed of granular media that behaves like a fluid when a gas is passed through it. The medium can be a wide range of materials, though sand is often used. Fluidized beds provide excellent heat transfer from the bed to the parts being processed (Fig. 2); in some cases, ten times the heat-transfer rate of a convection system .
In the past, fluidized beds had their own drawbacks. First, the beds were heated via the fluidizing air; that is, the air was heated externally and then the heat was transferred from the fluidizing air to the media. This was both inefficient and produced an inconsistent process chamber. The second problem was slow temperature recovery in the beds after a part was processed, limiting production rates. Both of these issues were significant reasons fluidized beds have not seen wider use in industry. Believing there was potential in the rapid process times the fluidized bed could generate, the company developed a fluidized bed system to address these issues.
To overcome the inefficient transfer of heat to the fluidizing media, the bed is no longer heated via the fluidizing air. Instead, radiant heaters are immersed in the media just down stream from the fluidizing air entrance, allowing the media to be heated directly via conduction, radiation, and convection. In addition to being efficient at transferring thermal energy to the process chamber (99% efficiency when using electric heating elements), uncoupling the heating from the fluidization air allows for temperature control of +/-3 F (+/-1.5 C) throughout the process chamber.
The issue of temperature recovery after a part is processed is addressed both by the direct heating of the media as well as the zoning of the fluidized bed into heating and holding zones. The use of immersed heaters allows very fast temperature recovery because of the efficient heat transfer to the media. Zoning makes it possible to add energy to the bed in the heating zones individually, where it is most needed. This minimizes recovery time for the process chamber, as well as isolating the rest of the process chamber from a drop in temperature. These innovations allow for a solution heat treatment process that is fast, efficient, offers exceptional temperature control, and can be used inline with the manufacturing process.
The use of newly developed fluidized-bed technology offers several improvements over current heat treating methods. Heat transfer rates can be an order of magnitude faster than those seen in air ovens. As mentioned previously, this allows for in-line heat treatment, which makes possible two important energy saving techniques. Parts can be put in heat treatment before they have fully cooled from casting. Since the parts have fully solidified but are still hot, less energy is needed to heat the parts to solution temperature. The other possibility this rapid heat transfer offers is short cycle heat treating. In independent testing using the fluidized-bed system, A356 aluminum alloy parts were solution heat treated to 1030 F (555 C), and peak tensile and elongation properties were achieved in as little as a 30-minute heat-treating cycle (including heating and soak time). By comparison, over four hours of soak time alone is prescribed in a conventional air furnace . The microstructures from these tests showed much more rapid coarsening kinetics and spheroidization of eutectic silicon particles in the samples heated in the fluidized bed .
Improved product quality because of tightened temperature control is one of the most significant benefits to this fluidized-bed heat-treating process. The entire process chamber can be controlled to within +/-3 F at 1000 F. This causes the fluidized bed to heat parts more uniformly than conventional systems. Every exposed surface of each part (internal and external) is in direct contact with the fluidizing media, and all of the media is at the same temperature. This uniformity results in true repeatability between components in the process: every part sees the exact same heating profile.
The fluidized bed can also be used in the quenching process, and it offers several advantages. In contrast to a water quench, there is no random, localized boiling. As a result, the stresses and distortions associated with this phenomenon are reduced. In addition, the quench rate can be very precisely fine tuned because of the excellent temperature control in the bed.
The fluidized bed also is used in the aging process. The same qualities that make a fluidized bed ideal for solutionizing (rapid heat transfer, process uniformity, and the ability to perform the process in line) also apply to the aging process. Process times can be shortened, and the quality of the end results can be improved compared with conventional aging. Again, the parts all see identical thermal cycles and so repeatability, and therefore quality, is enhanced.
Using the fluidized bed in all three phases of solution heat treatment allows the processing time to be shortened by hours (total reduction in process time depends on the process in question) and the process made completely automated. After the parts are loaded in the system, they are conveyed through the solution bed and then automatically transferred to the quench bed. When quenching is complete this transfer is repeated as the parts move into the aging bed; after aging, the parts are placed in an unloading station. All of these transfers occur automatically.
Analysis of the process using FEA
Finite element analysis software is used to predict the heating and cooling rates of a part. It is also used to predict the stresses and deformations induced in a part during the entire heat treating process. This is done using ANSYS FEA software that has multiphysics capabilities. Both thermal and structural analyses can be performed, so the heating and cooling rates, thermal gradients, and any induced stresses and deformations can be analyzed and predicted. These analyses can predict the heating of a part in a fluidized bed and compare this with that of a convection oven. One of the first phases of applying the software was to accurately model the thermal process. To date, the typical margin of error between the FEA results and lab testing is +/-2% throughout the process (Fig. 3). This makes it possible to model the fluidized bed heat treating process and demonstrate its advantages on a customer's part even before preliminary lab tests can be performed.
This capability is being continually refined as part of a scientific study through a NIST-ATP grant. As part of this and other research, the company built a lab and demonstration facility in Plymouth, Minn., with a full time metallurgist on staff. This facility consists of two full fluidized bed-based systems (Figs. 4 and 5) capable of solution heat treatment, quenching, and aging. The facility also can demonstrate the automatic and in-line nature of the process. In addition, the lab has all the equipment necessary to determine aluminum alloy microstructures and the tensile properties of parts before and after heat treatment. With the lab facility, it has been possible to validate FEA models, and to use ANSYS to accurately predict the thermal cycle and temperature profiles in various parts, from simple test specimens to complicated castings such as engine blocks. The combination of a full function heat treatment lab, a metallurgical testing facility and advanced FEA software makes possible an understanding of the process that spans the thermal and metallurgical effects of process variables, as well as the industrial equipment required to implement the process. It allows the process to be tailored to the requirements of a specific component, and provides the flexibility to modify resulting material properties via slight adjustments in the process parameters.
The solution heat treating process is a necessity in the processing of aluminum alloys, and there is currently a great opportunity for improvement in process times and finished part quality. The new fluidized-bed technology provides a solution to the problems of long cycle times, batch processing and nonuniform heat transfer, and offers improved temperature control. This technology makes it possible to move the process in-line with production and at the same time utilize short heat treating cycles. Due to short cycle times, the efficiency of the beds and the ability to receive hot parts directly from casting, there is potential for a tremendous reduction in energy consumption. This is in addition to reduced operating costs because of the automated nature of the material handling. Finally, the tight temperature control improves the quality and repeatability of the resulting material properties. Technomics' fluidized beds provide the technology to greatly improve the efficiency, quality, and speed of heat treating.