Metal Forming and Tribology

Introduction

Metal forming is a manufacturing process in which forces are applied on raw material such that stresses induced in the material are greater than yield stress and less than ultimate stress. The material experiences plastic deformation to change the shape of the component and converted to the desired shape of the component. [1][2][3][4][5][6]

Cold forming process is the plastic deformation of metals below the recrystallization temperature. In most cases of manufacturing, such cold forming is done at room temperature. Sometimes, however, the working may be done at elevated temperatures that will provide increased ductility and reduced strength, but will be below the recrystallization temperature. [1][2][3][4][5][6]

Advantages:

• No heating required
• Better surface finish obtained
• Superior dimension control
• Better reproducibility and interchangeability of parts
• Improved strength properties
• Directional properties can be minimized

Disadvantages:

• Higher forces required for deformation
• Heavier and more powerful equipment required
• Less ductility available
• Metal surfaces must be clean and scale-free
• Strain hardening occurs (may require intermediate anneals)
• Imparted directional properties may be detrimental • May produce undesirable residual stresses

Hot forming process is widely used in manufacturing components used in the aerospace, power generation or metal forming industries. In this process, heat is applied to soften the piece of metal. Then some form of pressure is used to alter the shape of the metal. In hot working, refinement of grain size occurs, thus, improving mechanical properties. [1][2][3][4][5][6]

https://www.youtube.com/watch?v=f5JpJERRdeo

Advantages:

• Negligible wastage of metal during metal forming process
• Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities
• Favorable grain size is obtained leading to better mechanical properties of material
• Pores may reduce in size or close completely during deformation
• Decrease in yield strength hence lesser amount of force is required
• Greater ductility of material is available, and therefore more deformation is possible

Disadvantages:

• Poor surface finish
• Poor reproducibility and interchangeability of parts
• Handling and maintaining of hot metal is difficult and troublesome
• Poor accuracy and dimensional control of parts
• Undesirable reactions between the metal and the surrounding atmosphere
• Huge amount of electricity required for obtaining high temperatures
• The components with cross holes cannot be produced easily using metal forming process

Characteristics of tribology in metal forming 

• Boundary lubrication regime is generally found in metal forming process. [7][8]
• At the tool and workpiece interface, the pressure is very high and the contact region is significantly wide. [7][8]
• Higher interfacial temperature. [7][8]
• The relative speed between tool and workpiece changes in the contact region. [7][8]
• Lubricants greatly influence workpiece surface. [7][8]
• Plastic deformation gives rise to virgin surfaces and change in surface morphology. [7][8]

Friction in metal forming process

In forming operations, at the interface between the workpiece and die/tool, two types of frictions can be arise – sliding friction or sticking friction. Sliding friction arises due to surface shear stress opposing the metal flow. High coefficient of friction leads to a situation called sticking friction (surfaces adhere to each other).

Wear mechanisms in metal forming processes

Cold forming: Tool failure is caused by five main wear mechanisms [9]

• Abrasive wear: Forming hard particles like carbides with hard materials can give rise to abrasive wear. Abrasive wear can be two – body or three – body which ultimately leads to deterioration of tool surface (scratch marks).
• Adhesive wear: Welding of asperities between the tool surface and the work-piece followed by the gradual transfer and accumulation of the work material on the tool surface can give rise to adhesive wear. This problem is also referred to as galling.
• Fatigue wear: Repeated stress cycles can give rise to fatigue wear. This type of failure is more prevalent in punching, stamping and fine blanking operations.
• Crack propagation: Stress concentrations in certain regions and the tensile stress fields in the tool material can give rise to crack initiation followed by crack propagation.
• Plastic deformation: Caused by high contact pressure exceeding compression yield stress of the whole tool material.

Hot forming: Tool failure is caused by four main kinds of loading [9][10][11]

• Thermal: Heat transfer from work-piece, repeated cyclic heating and cooling of the tool surface give rise to thermal loads.
• Mechanical: Crack is induced and propagates through the tool due to mechanical loading.
• Tribological: At higher temperatures, oxide layers are formed on the work-piece surface which can be a source of abrasive wear as these films are often brittle in nature.
• Chemical: High temperatures also gives rise to chemical reactions on the tool surface like oxidation or corrosion.

In order to provide proper wear prevention for hot and cold forming tools, it is necessary to optimize the combination of hardness and toughness. Also proper use of solid lubricants and surface coatings can also reduce wear and fatigue. Working environment should also be considered as a key factor.

Metal forming fluids 

Metal forming fluids are used to provide lubrication and cooling in metal bending, stretching and shaping operations. Metal-forming fluids generally are categorized into four main types: [12][13]

• Water-based or soluble oils
• Oil-based lubricants
• Synthetic and semisynthetic
• Solid lubricants

For cold forming operations, fats, fatty acids, mineral oils, soap emulsions are generally used. For hot forming, glass, graphite, mineral oils can be used as lubricants.

Coatings for metal forming tools [14]

Thin hard material coatings deposited by the PVD or CVD process are usually used to improve the wear resistance of sheet-metal forming tools. Experts insist to match coatings with substrate to avoid delamination. Material selection, heat treatment, surface preparations greatly influences coatings performance.

Tribometers used to evaluate friction in metal forming [11]

Pin-on-disc test, Ball-on-disc test, Block-on-disc test, Block-on-cylinder test, upsetting sliding test, sliding compression test, ring compression test, double cup extrusion test are some of the test that can be used to evaluate friction. Manufacturing of smaller prototypes of metal forming machines and equipping them with friction transducers could provide results closer to real time scenario.

Software for metal forming tribology [15][16]

Many institutes and companies have started using simulation software’s which provide realistic consideration of tribological effects in metal forming. “TriboForm” for example, is a high-impact software solution for the simulation of tribology, friction and lubrication in metal forming processes. The user can quickly simulate the effects of tool coatings, lubricants, material surface characteristics or new sheet materials on friction.