The Big 6 of Aluminum Heat Treat
Over 2,400 years ago, Chinese metalcasters were using heat treatment, including quenching, to improve hardness, and, archeologists have learned, even 1,000 years before that, a people group in modern-day Turkey knew how to create surface hardening by heating a sword to forging temperature and then rapidly cooling it in water. The science behind these practices is well understood today, and the specific temperatures and processes for castings made from wide ranging alloys are available to foundry operations that offer heat treating as a value-add to their customers. In ferrous alloys, the heat-treating cycle known as a quench and temper (Q&T) treatment is used to alter the microstructure of the cast partensure air flows evenly around the castings), fast door movement, and a manipulator for load transfer. (B) Alternatively, drop-bottom furnaces are made specifically for heat treating—the bottom literally drops out after solution treating so parts fall directly into the quench with a transfer time of under one second. These furnaces can process 50–100 lbs. of parts at a time, Dossett explained. Following solution treating, the box furnace with circulating fan is again required for aging.
To guarantee accuracy of furnace temperature and control of heat uniformity, foundries must follow the standards from the Society of Automotive Engineers (SAE) to conduct uniformity surveys, initially on a quarterly basis and then after any repairs are performed. Operators must use certified thermocouples, and the temperature
. In nonferrous alloys such as aluminum, metalcasters employ a similar process called solution treatment and aging, which is designed to closely control the formation of tiny strengthening particles in the alloyed aluminum.
Broken down to its core, heat treating aluminum castings, also referred to as precipitation hardening, involves two basic steps: (1) solution heat treating the castings in a furnace and (2) quenching them, often in water, to cool the parts—together, these steps change the final mechanical properties of the aluminum casting, such as electrical conductivity, tensile yield, and elongation, explained process metallurgist and materials engineer Jon Dossett. Heat treating creates hardness, as the ancients knew, but heat treating aluminum can also produce malleability and ductility.
Though its name would suggest some kind of immersion, solution heat treating is dry heating that occurs with either conduction, convective, or radiation heat transfer. When the casting is heated to within 10F–25F below eutectic/melting temperature of the aluminum alloy, microstructure is altered. Various heterogeneous elements become homogenized—and as compounds are precipitated out, the atomic lattice becomes locked, Dossett explained, making the casting harder. Quenching, he said, which can be rapid or slow depending on desired outcome, has the effect of “freeze-framing” what the solution heat treat has accomplished.
Not all aluminum grades respond to solution treating/precipitation hardening, said Dossett, but the following are good candidates:
In-house vs. Outsourced
Aluminum heat treating is a fairly straightforward and simple process, according to Dossett, but one that demands strict adherence to the AMS 2750 standard. Without a doubt, it’s a highly time- and temperature-sensitive process—that’s because the most critical and difficult aspect heat-treat operators encounter is maintaining correct temperature uniformity. Consequently, the equipment to do so must be properly and precisely installed and positioned, handled, and maintained.
Running a heat-treat operation inside the foundry is very much an economic decision—on one hand, it’s not a very expensive process, and the greater the volume a company does, the lower their cost per part will be. But a sufficient number of exacting requirements, as well as the accompanying maintenance program, make it a job some foundries choose to outsource to heat-treat specialists. In fact, Dossett, a 50-year manufacturing veteran, cautions foundries that do their own heat treating to carefully weigh the quality demands, noting distinct advantages to using third-party providers, despite the additional cost.
“I was doing some commercial heat treating—I had the equipment to do it, I had the people, and I had the instrumentations to assure these temperatures, but I still had a lot done by outside people. The problem with keeping it in-house is, sometimes internal errors do get covered up. If you do your own, to me, it doesn’t have the validity as having somebody from the outside. That’s a cautionary thing that I would say for anybody doing their own heat treating.
“Aluminum is so sensitive to temperature,” he added, “so if you decide to do heat treating in-house, you should have an outside audit on your internal operation periodically; at least once a year.”
Six for Success
For those who accept the challenges, successful in-house heat treating is hinged on the foundry’s expertise with six key factors: furnace types, temperature control, quenching, fixturing, aging, and quality control testing.
1|Furnace types. For the heat treat process itself, two furnace options are available: (A) Foundries can use a box furnace with a circulating fan (to
control system must be crosschecked weekly. The survey of heat treat furnaces requires a planned thermocouple replacement program as well as a preventive maintenance plan for the equipment.
2| Temperature control. Furnace audits are so critical because of the tightrope-thin margin of temperature at at which solution treating takes place: within 10–25 degrees of the melting point of aluminum. Exceed that window and operators will be cleaning a puddle of melted aluminum, Dossett said—a fact he knows from first-hand experience, he confessed.
“I can’t emphasize too much the criticality of temperatures in this heat treating process,” he said. “If you’re not careful, the castings can get soft on you, or in the worst case, they melt.”
To stay within the correct temperature for electric-sourced heating, operators use PID tuning to set the rate of approach, the proportional band, and the integral and derivative functions of the control instrumentation. These programmed electric instruments sense when the temperature is getting close to the set-point, said Dossett, helping operators prevent temperature overshoot.
In Table 1, a listing of heat treat and eutectic/melting temperature is listed for wrought materials as an example of how solution treating occurs for most alloys within 15 degrees of the melting point.
3| Quenching. Because particles begin to rapidly drop out of solution from the heating process, getting the parts transferred into a cooling quench tank quickly is essential for halting the particle fallout and “freezing” the modified microstructure in place. That’s what creates the hardness. For parts that are 0.5–1 in. thick, operators have 15-20 seconds to transfer them to the quench. (Hence the benefit of a drop-bottom furnace with one-second transfer time.)
Plan for 1.5 gallons of liquid for every lb. of product to be quenched—this ensures the hot castings don’t raise the temperature of the bath more than 10–20 degrees during immersion.
“If you have a 50-lb. part in a five-gallon bucket, the water is going to boil, right?” said Dossett.
Quench temperature is important—if water (the most common of quenching liquids) is heated 100 degrees, about 60% of the cooling power is lost, he said. “For water, you really want to control the temperature plus or minus 10F, and it’s normally used in the range of 60F–150F,” Dosset said. “Now, the 60 degrees obviously is faster, and the bath that’s uniformly 150 degrees is slower. Those are things you have to decide on given your work mix.”
A fast quench—with highest cooling rates—will produce the highest tensile properties, in other words a harder, stronger, and more brittle material. A slower quench will result in a material of less strength but greater ductility.
“You have to be aware that when you heat treat and then quench, a very violent change is happening in the structure of the part and it can cause distortion,” Dossett cautioned. “And so, depending on distortion factors, you may want to either fixture the work so that it stays in a particular orientation when it goes into the quench, or you may want to have the water be warmer.”
4| Fixturing. A rule of thumb Dossett has used over the years is: If length of the part divided by diameter is greater than six then the parts should probably be fixtured vertically with respect to the direction they’re entering the quench. Fixturing, done easily and inexpensively using steel separator screens/wire mesh baskets, keeps the parts separated during the solution treating so they don’t become “nested,” he said. If stacked like logs, parts won’t be evenly exposed to the circulating heat flow on all surfaces. Fixturing also facilitates uniformity of cooling during quenching.
“Multiple stacking baskets can be used for similar parts,” Dossett said. “Again, when you do your temperature uniformity survey, you survey nine points: the four corners at each end of the load and the center of the load. So, if you survey with three stacking baskets high, for example, then that’s the size load you can run in that furnace … You really have to survey the area that you’re going to run in.”
5| Aging. Following solution treating and quenching, operators will next do either a natural aging—room temperature four to 12 hours, depending on the specification—or artificial aging in a furnace at 250F–450F depending on the alloy.
In the case of a T7 alloy, these aluminum castings are used in high operating temperature applications, such as near motors or transmissions, so they’re frequently over-aged, said Dossett, to make sure the hardness doesn’t change during those operating conditions.
Temperatures of solution treating and aging for typical heat treat cycles are shown in Table 2.
6| Quality control tests. The final stage of the aluminum heat treating process is to conduct a test that indicates either success or failure in achieving the customer’s specifications, Dossett said. The most commonly used test is a hardness test, he added.
“Because the aluminum is relatively soft compared to steel, you use a 500 kilogram Brinell test, he said. “For more sophisticated results, you may use tensile testing. There can also be fracture toughness testing and stress corrosion testing. And some alloys require you to do conductivity testing, which is more representative of the strength than the hardness. Your customer will need to tell you which tests they want you to use.”
Boiling It Down
Nothing trumps temperature in the delicate game of heat treating, so operators must be conscious about making any changes that will potentially alter the precise temperature required for the job at hand. Dossett cited a case in which a foundry was experiencing difficulty achieving desired properties, and the supervisor decided to relocate the thermocouple used for controlling temperature.
“Those are well thought-out places designed by the people who make and install the equipment,” he said, “and you really shouldn’t move anything.
“This whole thing is all about temperature. And if you do something that influences the temperature, you’re going to get into trouble.”