Forming journals

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Hydroforming

Application of Hydroforming:

Hydroforming benefits include weight savings, strength improvement, part reduction, and additional design options. Hydroformed component features can include changing cross sections with varying corner radii and curved sections. Engineers and part designers must consider material properties such as yield strength, ultimate strength, and allowable elongation in the design of a component. Materials commonly formed are brass, stainless steel, carbon steel, and aluminum. These materials vary widely in formability characteristics.

The demand for lighter component solutions remains a primary driver in the automotive industry. Hydroforming is considered as one of the potential enabling technologies to deliver lightweight components.

Hydroforming enables manufacturing of closed sections with non-uniform cross-sectional areas along the length by using a circular tube as the input material. While conventional stamped and welded closed sections need a flange area to facilitate welding, hydroformed closed sections enables weight saving by avoiding the flange area. The capacity of the Hydroforming process enables a designer to replicate the profile of brackets / child parts directly on the components, thus reducing the number of brackets / child parts.

The traditional sheet metal components have been replaced by hydroformed tubes, leading to lesser components, weight saving and an excellent crash performance.

Importance of FEM in Hydroforming:

Although tube hydroforming is a long known technology, the applications of hydroformed tubular components has increased in recent times, because of the more advanced pressing control technology, but also because of the availability of reliable Finite Element Models (FEM), that eliminate the expensive trial and error process in the development of the tools and components. 


Examples of Hydroforming:

Figure. Conical hydroformed component

Definition of Tube Hydroforming:

The principal of tube hydroforming is given in figure. A pre-bended tube is placed in a tool set in a press which applies the closing force. At the ends of the tube two cylinders are placed that can apply axial feeding. The tube will be filled with fluid and the tube will be formed under pressure.

Figure.Tube hydoforming.

In figure a typical sequence is shown for tube hydroforming of a simple T-component. First the tube will be positioned in the die set, the tools will close and the tube will be filled with water. Axial feeding will build up pressure and enable the inflow of material into the T-shape. After releasing the pressure, the dies can be opened and the part be removed.

Figure. Tube hydroforming sequence for a T-shape

Definition of Sheet Hydroforming:

In figure the sheet hydroforming is shown, where the hydraulic fluid is used as a flexible die in a deep drawing process of cylindrical components.

Figure. Sheet hydroforming

Apart from sheet hydroforming as an enhancement of the classical deep drawing process, a special process has been developed especially for automotive outer components called the Hydromec process. In figure the principle of this method is shown where a sheet is pre-formed in one direction generating a uniform strain in the sheet. After that a mechanical punch will press the sheet in the other direction giving the component its final shape, supported by the hydraulic fluid. In this way a component with a lot of pre-strain will be pressed, which will give the component excellent dent resistance, especially when so called bake hardening steels are used that raise the Yield strength of a pre-formed component considerably during the paint cycle.

Figure. The Hydromec process.

In figure another special form of sheet hydroforming is shown, the so called pillow hydroforming, where two stacked sheets are presses together in one toolset into a an outer and inner panel of, in this case a bonnet, reducing cycle time, tool costs and increasing dent resistance.

Figure. A Hydromec bonnet

Figure. Pillow sheet hydroforming

Control of Material Properties

Metals and alloys may not posses all properties required in a finished product. The metal properties can be controlled by following methods:

1. Alloying

2. Heat Treatment

3. Mechanical Working and Re-crystallization.

Behavior of metal when subjected to various types of stresses depends upon the following factors:

1. Magnitude of stress

2. Nature of stress

3. Method of application of stress

4. Time of application of stress

5. Temperature

6. Composition of metal

Properties to be considered for forming process

Following properties of metals play an important role in forming of metals:

1. Elasticity

2. Plasticity

3. Hardness

4. Strength

5. Ductility

Elasticity is the property of an object or material which causes it to be restored to its original shape after distortion.

A rubber band is easy to stretch, and snaps back to near its original length when released, but it is not as elastic as a piece of piano wire. The piano wire is harder to stretch, but would be said to be more elastic than the rubber band because of the precision of its return to its original length. A spring is an example of an elastic object - when stretched, it exerts a restoring force which tends to bring it back to its original length.

In other word, elasticity is the property of solid materials to deform under the application of an external force and to regain their original shape after the force is removed is referred to as its elasticity. 

The external force applied on a specified area is known as stress, while the amount of deformation is called the strain. 

It is also defined as the ability of material to return to its original dimensions after suffering a deformation or strain due to stress resulting from some applied force.

Plasticity is the property of material to be deformed repeatedly without rupture by the action of a force, and remain deformed after the force is removed.

In other words plasticity is the ability of a solid body to permanently change shape (deform) in response to mechanical loads or forces. Deformation characteristics are dependent on the material from which a body is made, as well as the magnitude and type of the imposed forces.

It is also defined as its ability to retain any deformation produced in it without fracture under the action of external load. 

Hardness is the resistance of a material to localized deformation.

It is also defined as the property by virtue of which a material resists the penetration of other bodies into it.

The term can apply to deformation from indentation, scratching, cutting or bending. In metals, ceramics and most polymers, the deformation considered is plastic deformation of the surface. For elastomers and some polymers, hardness is defined at the resistance to elastic deformation of the surface. 

The lack of a fundamental definition indicates that hardness is not be a basic property of a material, but rather a composite one with contributions from the yield strength, work hardening, true tensile strength, modulus, and others factors.

Strength is defined as the resistance offered by the material when subjected to external load. 

It is defined as the resistance offered by which the material opposes the deformation.

Higher the strength the higher is the capacity of the material to withstand load without rupture or failure. 

Depending upon the type of load the strength can be tensile, compressive, shear, bending and torsional. 

Resistance offered by the material is stress, so the strength can be measured in terms of stress. 

The stress required to cause permanent deformation is called yield strength and the maximum stress before fracture is called ultimate strength.

Ductility is a property of material by virtue of which it can be drawn into wires without rupture and without loosing much strength. 

It is measured by the percentage elongation or percentage reduction in the cross sectional area before rupture.

Some other properties to be considered:

Stiffness of a material is defined as its resistance to elastic deformation.

Toughness is the ability of a material to resist fracture under impact loads, i.e., suddenly applied loads.

Brittleness is the ability of a material to which they shatter before much strain has applied.

Tenacity is defined as the ability of a material to resist fracture under the action of a tensile force.

Deformation

Deformation is the change in dimensions or forms of material under the action of applied forces. 

Metals are aggregates of crystals or grains. The characteristics of a metal depend upon the properties of crystals or grains. 

Within each grain the atoms may be imagined as packed together in a regular geometric pattern (called space lattice) which is repeated indefinitely in three dimensions to build up the solid grains. 

Because of the regular pattern of atoms they can be considered to lie in various parallel planes.

The deformation of metals is necessary to form various types of shapes without rupture. 

Deformation is based on type of strain produced due to loading of metal. 

The resistance of a crystalline body to deformation is dependent upon the bonding forces between the atoms. 

There are mainly two types of deformations such as elastic & plastic deformation. 

Elastic Deformation: When force is applied on the metal within a limit, atoms move along the direction of force and occupy the new position, but on the removal of force, the atoms come back to their original position is called elastic limit and this type of deformation is called elastic deformation.

Plastic Deformation: When force is applied on the metal beyond the elastic limit, atoms does not come back to their original position on removal of applied force and occupy the new position. This type of deformation is called plastic deformation.

Introduction to Forming

1. Metal forming is the process in which the desired size and shape of components are obtained through the plastic deformation of metals. 
2. The stresses induced in metal by applied forces during the forming process are greater then yield strength but less than the fracture strength of the metal.
3. Metal forming is more economical process and less wastage of material.
4. Some of the properties like elasticity, plasticity, hardness, strength and ductility plays an important role in metal forming process.
5. If the working temperature is higher than the re-crystallization temperature of the metal then the process is called hot forming otherwise cold forming.
6. During hot forming a large amount of plastic deformation can be imparted without significant strain hardening. In cold forming strength and hardness are increased due to strain hardening. 
7. Material failure in tension and compression failure plays a vital role in metal forming.
8. In forming stress-strain diagram for various cases need to understand.
9. The study of control of material properties are to be identified.
10. The special properties like creep,fatigue,bending,fracture and failure need to be understood clearly.
11. Factors affecting mechanical properties need to be stressed.
12. Selection of materials for forming process must be watched closely.
13. High level focus need to be given on yield strength and strain hardening.