Three basic types of forging presses, namely screw presses, hydraulic presses, and mechanical crank presses, can meet any challenge posed by forging of aluminum for automotive applications. Mller Weingarten AG offers presses of all three basic types. The following article will detail the use of a hydraulic press and a mechanical forging line for producing modern automotive powertrain components of aluminum.
Forgeability of materials
Materials used for forging are rated by forgeability, a rating of deformation capacity, which indicates the sustainable deformation without fracture. This capacity is dependent on the yield stress in the different temperature ranges. Forgeability of forged materials is ranked as follows:
1. aluminum alloys
2. magnesium alloys
3. copper alloys
4. steels, including carbon steels, alloyed steels, martensite-hardened steels, austenitic and non-corroding steels.
Each material has to be forged within its specific temperature range. The lower limit of the temperature range refers to the recrystallization temperature. The upper limit is determined by oxidation, coarse grain formation, and possible phase transformation. Within the suitable forming temperature range the yield stress Kf changes considerably. By heating the metallic work piece the yield stress is lowered while the necessary forming force and the needed forming work is also reduced.
High-grade aluminum is suitable for forging stock. If a higher strength is required, so-called naturally hardened wrought alloys like AlMg5 should be used for forging. For hot forging, wrought alloys such as AlMgSi or AlCuMg can be used; these are alloys that are applied in a hardened condition.
The usual forging temperature of AlMgSi1 is from 470 to 520° C. The heat quantity supplied is, however, considerably higher than the forming work. In general, 85% of the forming energy transforms into heat.
Due to the very high thermal conductivity of aluminum it is necessary to forge aluminum in heated dies. The tool temperature should be from 250 to 300° C. Also, a 5-kg aluminum billet must have gone through all forming operations of hot forming within 40 seconds; otherwise coarse grain formation begins.
Coarse grain formation beginning on the outer skin due to heat absorption results in aluminum components having a far lower dynamic loading capability than steel components. However, by intermediate heating or artificial heating, coarse grain formation is kept under 1 mm on the edge surface.
Hot forging always results in producing a certain amount of excess material, or flash. Due to a higher deformation capacity of aluminum, more extensive flash formation occurs.
In view of the cost difference of aluminum material in relation to steel (approximately 2:1; 1 ton of steel at the amount of C= 500, and 1 ton of aluminum AlMgSi1 at C= 1,650), it is particularly important to keep the material excess as low as possible. Therefore Mller Weingarten has developed a process of aluminum cross-rolling (in 2001) and aluminum reducer rolling (in 2002).
Cross-rolling is a continuous forming process during which a cylindrical billet is formed on cylinder jackets between two tool segments operated continuously.
Reducer rolling is multistage forming achieved by partial pre-forming in different roll grooves discontinuously.
During hot forging, both procedures can be used to produce coordinated pre-forming that results in the lowest possible material excess during flash formation.
Basically pre-forming technology is utilized to put mass in the right positions for different cross sections to assure a properly formed forging piece, and to keep the press forces and the forming energy as low as possible at different cross sections.
Both pre-forming technologies are being used in hot forging aluminum powertrain components.
When mass production pre-forming machines are integrated into fully automated forging lines, loading is performed by robots. Robots or manipulators are also used to handle work pieces during production.
The loading or unloading operation is monitored by temperature sensors, and frequent measurement ensures a fully automated process that enables only work pieces in good condition to get to the main forming press after pre-forming.
For hot forging aluminum, forging machines such as screw presses, crank presses, hydraulic presses, and hammers are used.
The forging machine to choose depends on the forming process, the work piece (geometry and material), and quantities to be produced. The machine selection also depends on the potential degree of automation as well as on economic features. The machines’ types can be classified as energy-related machines (screw presses, hydraulic double-acting hammers), course-related machines (crank presses), and force-related machines (hydraulic presses).
Hydraulic presses can be controlled very well in terms of force and speed, so they are suitable for pre-forming or open die forging due to their high flexibility.
Also, due to their high flexibility, hammers can be adapted to different working conditions. They are the least expensive forming machines; however, they can produce excessive noise and shock, which affects working conditions.
Mechanical presses are course-related machines that can generate only a certain forming energy via a defined course and thus with a limited force. The following presentation will be limited to mechanical and hydraulic presses only.
Mller Weingarten manufactures hydraulic presses with force ratings between 0.2 MN to 100 MN and mechanical crank presses from 12.5 MN to 63 MN.
Mechanical versus hydraulic
Hydraulic presses are force-related machines that work more slowly overall but they are more flexible. With a relatively high energy requirement, hydraulic pressures are built up via electrically driven pumps that produce ram movement. By using an accumulator drive it is possible to achieve a quicker ram speed. Without an accumulator drive, a direct drive can achieve a higher ram speed by consuming more energy.
Due to their flexibility, hydraulic presses can be automated very effectively. Multistage forming procedures can be designed and controlled far better on hydraulic presses than on mechanical presses, which deliver only a certain amount of force and thus only a certain amount of energy.
The flexibility of hydraulic presses, however, requires a greater investment. Furthermore their higher flexibility can result in higher operating and maintenance costs.
A new generation of crank presses introduced by Mller Weingarten in 2000 has been designed for multistage forming and mass production. These machines are suitable for a fully automated forging operation with material supply carried out either by robots through press windows or by a mechanical or electronic transfer. Due to the large die space, multistage forming is possible with the following forming operations: pre-forming, pre-forging, finish forging, trimming, punching, and calibrating.
On a mechanical press, energy is supplied to a flywheel by an electric motor, with approximately 15% of this energy needed for each stroke. As mechanical presses are course-related machines, a limited forming energy for a certain ram course and thus a limited forging force in the bottom dead center is available. Because of this, the process design has to be coordinated. Therefore it is necessary, for complex pre-forming operations, to allow for the limited forming energy through pre-forming or by multistage pre-forming in two steps.
Forging lines for spindles
For an aluminum spindle of approximately 5.8 kg final weight, a complete forging line has been planned and manufactured, beginning with the supply of raw material by sawing and ending up with artificial aging. The forging line includes the primary material stock, sawing the raw material sections, pre-heating, pre-forming, and a three-stage main forming operation, as well as a separate hydraulic press for the trimming, calibrating, and artificial aging operations.
The complete forging line consists of two forming presses connected to two pre-heating furnaces and one heat treatment furnace. The work piece transfer is done in a fully automated operation with help of five-axis robots that check the work piece temperature prior to its transfer to the next process step. The production line operates with a cycle time from 12 to 16 sec, corresponding to an output of 1,200 pieces per shift.
The investment for such a comprehensive forging line from raw material magazine to artificial aging is approx. 8.5 million Euro. A machine of this type has been installed in the French-speaking region of Switzerland and was commissioned in 2000.
The same spindle is being produced on a mechanical forging line, but with a completely different production concept. In this case, the starting point is a special extrusion profile matching the pre-form. The bent billet is taken to the pre-form furnace, followed by a pre-forming operation on two hydraulic presses. After pre-forming the work piece passes through intermediate heating, before it reaches the main forging press of 3,150 t (31.5 MN). The forging operations on the main forging press carry out a contemporaneous pre-forging with trimming and calibrating and, as a single forging operation, the finish forging in the center of the die holder.
After hot forging and trimming the forged piece is aged and treated thermo-mechanically.
The entire work piece transfer is carried out by eight forging robots on the complete line.
The investment for such a line is 8 million Euro, 1.8 million of which is for the main forming press. The cycle time to manufacture such a powertrain component is 8 sec on a mechanical production machine resulting in 2,400 pieces per shift.
Optimized production concept
Mller Weingarten has developed an optimized production concept for aluminum spindles by using both hydraulic and mechanical forming presses.
The concept is to heat the extruded billet, to supply it via robot handling to a hydraulic pre-forming press that carries out a two-stage pre-forming operation as well as a bending operation. A mechanical transfer device carries the work piece through the hydraulic press in a cycle time of 8 sec. At the exit of the hydraulic pre-forming press the billet is handed over to a shuttle that delivers the billet to a mechanical crank press. This 31.5 MN mechanical forging press is equipped with a mechanical transfer that carries the work piece through three forming operations, pre-forging, finish-forging, trimming, punching, and calibrating in a continuous process.
At the exit of the press the finished hot-forged spindle is handed over to a robot that passes the spindle onto a final thermo-mechanical treatment.
The advantages of the concept are:
- Reduced investment due to omitting intermediate heating
- Higher productivity in pre-forming
- Increased overall productivity due to mechanical transfer through pre-forming and main forming, thus shorter cycle times per piece
- Improved process capability due to shorter cycle times per piece
- Less work piece handling, lower investment
- Lower risk of coarse grain formation.
This article is based on a presentation at Forge Fair 2003. Dr.-Ing. Thomas B. Herlan is General Manager, Forging Division, and Dr.-Ing. Marc Fleckenstein is Technical Marketing/Sales, with Mller Weingarten AG.
For further information on hot forging of aluminum components contact Ken Setze at Mller Weingarten Corp., 599 East Mandoline, Madison Heights, MI 48071, Tel 248-588-7800, Fax 248-588-7820. Or, e-mail: [email protected]; Or visit www.mueller-weingarten.de