Gas-Fired Heat-Treatment Plant for Aluminum Structural Parts

This article presents a heat-treatment plant for lightweight structural aluminum parts.

Premium car manufacturers have considerable experience with the use of aluminum in body parts as a substitute for steel and for forming complex structures. Thin-walled aluminum parts have already been used in the aluminum body shells for several years. The use of lightweight aluminum parts significantly reduces the weight of a vehicle body, improving fuel economy and cutting emissions. These parts are heat treated in a continuous furnace plant (Fig. 1). Minimizing residual stress in the parts is critical for maximizing the potential part loading under working conditions. So, the treatment should result in minimal residual stress. This is dependent on the quenching media (Fig. 2), and air quenching offers the lowest residual stress.

Heat-Treatment Process

The plant heat treats aluminum components for cars, which are usually produced in a left-hand and a right-hand version. The purpose of the heat treatment is to reach mechanical properties such as tensile strength, yield strength and elongation at the rupture specified by the bodywork designers.

The aluminum parts – produced in die-casting machines – feature thin walls and complex geometries. In order to avoid the need for complex straightening following heat treatment, any distortion of the parts during the process (especially during quenching) must be minimized. For this reason, the quench facility is a key component of the plant. The quenching medium used is air. Air quenching ensures compliance with the specified mechanical properties at the same time as minimizing any distortion and the residual stress of the parts.

The solution-annealing temperature is 460-500°C (860-932°F) with a temperature tolerance of ∆T= ±3K. The temperature for the elevated-temperature age-hardening process is 160-240°C (±3K). During air quenching, the parts must be cooled to less than 240°C (464°F) within two minutes. For some alloys, this time may even be shorter. High reproducibility is achieved by automatic operation of the plant.

Charge Carriers

After removal from the mold and stamping, the aluminum castings are inserted into a charge carrier. It is possible to arrange several charge carriers in a charging rack. The charge carriers are specifically designed for the parts to be treated. They are produced after the optimum position of the aluminum parts in the cooling airflow and the charging rack have been determined by tests. The charge carriers support the aluminum castings in such a way as to prevent deformation during the solution-annealing process at 460-500°C.

The useful dimensions of the charging racks are dependent on the requirements of the specific production. Several parts are positioned in the charge carriers, which are designed to take up a sufficient quantity of aluminum parts to ensure adequate capacity utilization in the heat-treatment plant. At the same time, the air-quench process must be the same for all the parts, and reproducibility must be ensured (Fig. 3).

The charging racks are designed for handling by forklifts and roll conveyor within the production facility.

Heat-Treatment Plant Concept

The heat-treatment plant is a continuous roller-hearth furnace plant designed for solution annealing and elevated-temperature age hardening. The conveyor systems outside the furnace and in the quenching chamber include roller conveyors and cross conveyors designed to allow fully automated operation of the heat-treatment plant (Fig. 1).

All the conditions in the furnace zones and the transport operations are fully automatically controlled. A batch tracking system allows identification of each part and details the related heat-treatment process. Heating of the furnace can be accomplished by gas burners or by electrical heaters.

The loaded charging rack is positioned on the roller table upstream from the solution-annealing furnace using a forklift or a roll conveyor. After positioning the charging rack on the roller table, the worker acknowledges the rack on the plant control system and releases it for heat treatment. After a step in the cycle has elapsed, the charging rack is moved forward into the heat-up zone of the solution-annealing furnace. It may also be necessary to select a heat-treatment program on the plant control system. From this point, all the steps in the heat-treatment process are completed automatically.

Solution Annealing

The solution-annealing furnace is a continuous roller-hearth furnace operated in steps with an entry and an exit door. It is designed to handle several charging racks positioned behind each other and has control zones with recirculation fans. In the heat-up zone, the material is brought to the solution-annealing temperature, which is maintained until the completion of the step. When the step has been completed, the rack is transferred to the next zone, where the annealing temperature is also maintained. Before the rack is transferred to the last zone, the zone temperature must reach the temperature setpoint again after the opening and closing of the exit door. The next charging rack is then loaded into the heat-up zone by the roller table upstream from the furnace (Fig. 4).

Using the furnace control system, it is possible to set variable setpoints for the zone temperature and the recirculation fan speed in the heat-up zone. A separate temperature curve can be programmed in each of the control zones, and the heating is controlled continuously to obtain optimum heat-up and soaking curves for the parts to be treated.

Air Quench

Downstream from the solution-annealing furnace, the charge is quenched in air. Before the solution-annealing furnace exit door is opened, the controlled-speed cooling fans are started up. A charging rack with charge carriers is then discharged into the quench facility in only 10 seconds, and the parts are cooled to less than 240°C in two minutes. During the quenching process, the charging rack is reversed on the rollerway inside the quench facility in order to ensure homogeneous cooling of the charge.

Air nozzles are distributed evenly over the side walls and floor of the quench chamber. The quench airflow rates can be varied using the speed control of the quench-air fans. Manual butterfly valves in the various nozzle zones allow adjustments for the optimum quench airflows for different aluminum parts.

Quench air is routed to and from the quench facility via a duct system. The temperature of the quench air is controlled by a motor-operated butterfly valve system in the ducting. In the winter, the cold air drawn in from outside the building is heated slightly by the spent quench air to ensure reproducible quench-air temperatures.

The two roller gates at the quench facility entry and exit are closed during the quenching process. An extraction fan is installed on the roof of the quench facility to remove the quench air from the chamber downstream from the charge. Silencers are installed in the air intake line. Together with the insulated casing of the air-quench chamber and the roller gates, these silencers help to limit the noise level of the plant.

Following the completion of quenching, the charging rack is transferred to the entry roller table of the elevated-temperature age-hardening furnace.

Age Hardening

When the heat-up zone of the elevated-temperature age-hardening furnace has become vacant, the furnace door is opened and the charging rack is charged into the furnace. As with the solution-annealing furnace, the first furnace zones are designed for heating up and soaking. In view of the longer soaking time required in this furnace, the elevated-temperature age-hardening furnace has more rack stations with more control zones and recirculation fans. The design temperature range is 160-240°C (320-464°F).

For the elevated-temperature age-hardening furnace, the heat-treatment program may define a shorter step time than for the solution-annealing furnace. Once again, the control system allows a variable setpoint to be selected for the zone temperature and the speed of the heat-up zone recirculation fan to be changed during the step. A temperature curve that has been optimized for the parts being treated can be set for each control zone. As in the solution-annealing furnace, the gas-burner output is continuously controlled.

When the step has been completed, the exit door is opened and the charging rack is discharged onto the downstream roller table. After the exit door has closed and the temperature in the exit zone has reached the setpoint again, the charging racks in the furnace are moved forward by one station.

The roller table downstream from the elevated-temperature age-hardening furnace is also equipped with a cross conveyor that moves the charging rack to an unloading station. It ensures that the roller table is empty so that another charging rack may be discharged from the furnace when the next step has concluded.

The temperature curves and times for the entire process are documented. In the event of temperature tolerance or step-time infringements during the heat-treatment process, the charging racks in the appropriate furnace zones are shown with a “not-OK” symbol on the plant display. The worker must eliminate and reset the malfunctions. The material tracking system ensures that the affected charges are marked as “not OK” until they leave the heat-treatment plant. Warning signals then alarm the workers, who must move the rack concerned to a quarantine position and carry out additional material tests if appropriate.

Summary

This plant for heat treating aluminum body parts was based on a plant design already used successfully to treat similar body parts. The plant features a number of benefits:

• Flexible setting of temperature curves in each furnace zone
• Variable recirculation flow rates in heat-up zones
• Tight temperature tolerances in control zones
• Flexible air-quench facility
• Charging racks with replaceable charge carriers
• Minimum distortion of aluminum parts and good mechanical properties thanks to air quenching
• Controlled quench-air temperature for reproducible quenching conditions
• Low noise level thanks to encapsulated air-quench facility
• Expansion possible by extension of furnace length
• Extension by parallel heat-treatment line with transferable quench facility also possible (Fig. 5)
• Automatic material and process tracking system

The heat-treatment plants for aluminum parts offered by Tenova LOI Italimpianti are tried and tested systems with an air-quench facility that can be used flexibly for large and small quantities of parts in the automobile industry. IH  

For more information: Contact Adolf Hanus, LOI Thermprocess GmbH, Essen, GERMANY; tel: +49 201 / 1891 846; e-mail: e-mail: adolf. hanus@loi.de