Ovens designed for age-hardening processes must have precise temperature uniformity, proper airflow, energy efficiency, low maintenance and provide consistent metallurgical results for a variety of part and workload shapes and sizes.
Design for Aging
It is a fact of life that most ovens are designed for general heat treatment and not specifically designed for aluminum aging. This drawback can create a number of issues, including inconsistent results, higher maintenance and reduced productivity, because the user is often unaware that a general-purpose oven may not be best suited for running critical processes such as age hardening. By designing heat-processing equipment specifically for aging, cycle times can be reduced and productivity increased.
Convection is the Key
Aluminum aging is a relatively low-temperature operation. The parts are heated using recirculated air and then held at temperature. Convection (forced air) is especially effective in aluminum processing because of the very high thermal conductivity (over four times that of steel). Aluminum will accept heat virtually as fast as the oven can deliver it via convective heat transfer (i.e., hot air). By using increased convection and heat input, heat transfer to the parts will be optimized. This reduces the oven cycle time. The same principal applies during the cooling cycle. Hot aluminum will release heat extremely quickly when cooling air is sufficiently and properly delivered, thereby reducing the cooling time.
More Airflow Delivers Better Performance
The biggest factor in the design of any oven, especially age ovens, is the air recirculation rate, which is measured in CFM (cubic feet per minute) and often expressed in “air changes per minute.” This refers to the volume of air circulated within the oven by the recirculation blower. The CPM reflects the number of times per minute all the air is completely recirculated through the heating chamber. For example, if an oven work chamber is 8 feet wide x 10 feet long x 7 feet high, its volume is 560 cubic feet. If the recirculation rate is 28,000 CFM, the CPM is 50 (28,000/560).
A high-performance age oven should deliver approximately 40-60 CPM. This is in contrast to general-use ovens, which are often 5 CPM (or 2,800 CFM in the above example). Considering that a 28,000 CFM oven operating at 350˚F will circulate 82,300 pounds of air per hour, versus 8,230 pounds per hour for a 2,800 CFM oven, it is understandable why airflow has such a big impact on oven performance.
Let’s look at the theoretical heat-up rates of aluminum castings (Fig. 1) at both 5 and 50 CPM. The 50 CPM design heats the castings to 350˚F in approximately 15 minutes in contrast to 30 minutes for the 5 CPM design, a savings of 15 minutes.
After aging is complete, the cooling cycle will compare similarly, with an additional 15-minute savings for the 50 CPM design. This results in a total savings of 30 minutes over the entire cycle. If the soak time is 2 hours, for example, this savings translates into 20% higher efficiency over each aging cycle (Fig. 2). Therefore, the oven will process 20% more product using the higher recirculation rate, which will shorten the payback period and increase the productivity of the equipment by thousands of dollars over its life.
When selecting an oven to perform age-hardening processes, be sure that it has excellent temperature uniformity, preferably in the order of ±3ºC (±5ºF). This will ensure that it has adequate airflow to handle aluminum processes. When investigating an age-oven purchase, consider a high-performance unit designed specifically for aging. It will provide faster payback and increased productivity.