Metal Forming Simulation: FEA Based Design and Optimization

Cold forming

The most important manufacturing processes are cold heading and extrusion processes, but also punching, hobbing, thread rolling, and drawing processes. Cold forming is restricted to easily formable materials, or rather materials which can easily be transferred into a formable microstructural state.

Cold forming results in strain hardening, meaning both the strength and resistance to forming increase with ongoing deformation. Thus, cold formed components can withstand greater operational loads. At the same time, strain hardening results in reduced formability (ductility) of the material. If the component needs to be formed further, the strain hardening of the component has to be removed via recrystallization annealing. Cold forming and annealing are often part of a multi-stage process.  

The high yield stress and strain hardening result in very high press forces in the manufacturing process, and shift the focus to the used forming dies and die materials. The die life essential for cold forming is often only achievable through prestressed dies, which means that the simulation of cold forming has to consider the effect of the stress rings in addition to the material flow. The realistic mapping of forces in the forming processes, and the consideration of the effects of spring back, while taking into account the elastic-plastic material law, are indispensable for a high precision simulation of cold forming processes.

sheet metal forming

Sheet metal components are highly suited to lightweight constructions. Different methods of sheet metal forming can be used depending on the geometry of the desired part. Based on the characteristics of each deformation process, the forming engineer can choose between: Deep drawing, ironing, punching, bending, stamping, and a variety of other manufacturing processes. Due to the geometric complexity of the parts being manufactured, additional multistage forming that combines different processes is frequently required within a phase of production. Therefore, production is usually achieved by automated transfer or stage pressing, or by progressive tools.

Using advanced FEA tools for sheet metal forming design and simulation, the feasibility of the project at hand is evaluated including predictions of the geometrical accuracy, cracking behavior, risk of cracking, and formability. The simulation results show the product’s characteristics such as wall thickness distribution, edge curvature, and the hardness distribution in forming steps.

Rolling

Rolling is a continuous or non-continuous pressure forming process with one or more rotating rollers. Additional tools can be used, such as plugs, mandrels etc. Molding occurs either through motorized rollers or by pulling rollers .

Rolling is one of the most diverse forming processe in the forming technology. It is used for the production of semi-finished as well as finished products. Rolling processes are used in all areas of forming technology, both in hot forging and cold forming, and of course in sheet metal forming. There is a multitude of rolling processes. Some are named here: flat rolling, profile rolling, tube rolling, roll forming, forge rolling, cross-wedge rolling, wire rolling, and cross-row rolling.

Open Die Forging

Open die forging is used for large and critical products which cannot be forged by basic forging processes due to high deformation load or massive dimensions. Technological processes of open die forging are applied to produce individual, low-volume parts for die blocks, rings for further rolling, blanks of crankshafts for marine engines and other large parts requiring the characteristics and durability of a forging.

The shape of the workpiece does not result from the shape of the dies used, but from repeated local forming with geometrically simple dies that are typically moved relative to the workpiece. The shape of the workpiece is created incrementally, i.e. step by step. Usually, this is done when hot forging very large components for the machine and plant building industries, or for the densification of casted ingots. Open die forging is also used where shaping dies cannot be used for economic reasons. For special component shapes, techniques such as rotary swaging processes are used during cold forming of large production volumes.

When designing open die forging processes, the pass schedule, including possible intermediate heating, must be planned in such a way as to reach the required final geometry and the required material properties with as little effort as possible. High performance materials such as titanium and nickel based alloys can only be forged within a narrow temperature range.

Heat Treatment

Most of the technologies for the production of forged parts are required heat treatment after deformation to obtain the desired properties, both on the surface and through the entire volume of the detail. Heat treatment is a process or a combination of processes to treat metallic components. The components, commonly made of steel(s), are temporarily heated to specific temperatures.

Taking into account the rate of heating and cooling, the material properties of a component can be altered and improved. The presence of certain agents can lead to changes in the carbon or nitrogen content of the component. In all heat treatment processes, there are several decisive and important factors: Time (heating and holding time), temperature, atmosphere, and quenching or cooling conditions.

In principle, there are two kinds of heat treatment processes: processes resulting in a thorough change of the microstructure and processes that result in merely changing regions close to the surface of the component. Examples of the former would be thermal processes, such as annealing and hardening. Examples of the latter, thermochemical processes, would be diffusion and coating processes, such as carburization, case hardening, nitrating, boriding.

Simulating the process reduces the amount of experimentation required during process design and process optimization. It also helps to smooth trial runs and the prototyping process. Through simulation, typical mistakes such as too much or too little hardening, cracks from residual stress or too much distortion can be discovered before they are made.

Simulation Features:

• Prediction of phase transformations, phase composition of the workpiece, both at the deformation and heat treatment processes.
• Diffusion and martensitic phase transformations at cooling and heating of material while accounting the latent heat of phase transitions and volume changes in the transformation process.
• The heat treatment for simulation of different production processes: quenching, tempering, annealing etc.

Hot Forging

Hot forging comprises all forming processes that occur above the recrystallization temperature of a metal. The characteristic effect of hot forming, is the significant material strength reduction (yield stress). Hot forging is used when the goal is to achieve complex 3D geometries via forming. In addition, it enables the processing of difficult-to-form materials, which can be formed only with limitations when cold. Due to the strength reduction under hot forging conditions, the force and work demand of the processes can be lowered in comparison to cold forming.

The recrystallization is responsible, through the complete reformation of the microstructure, possibly multiple times, for the formation of a relatively fine-grained microstructure. It exhibits the optimal combination of strength and ductility. This circumstance qualifies hot forging as one of the most important manufacturing processes for the production of highly stressed safety components.

Ring Rolling

Ring rolling is a forming process for the production of rings. The ring rolling processes enable the production of rings with diameters ranging from ten centimeters to ten meters. Because of very high demands in accuracy and reliability required for production of ring and wheel rolling, efficient simulation is needed for detail investigation and verification in extremely high-performance finite element (FEA) simulation software.