Tribology in metal forming

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]

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]

Advantages of cold forming

Disadvantages of cold forming

No heating required

Better surface finish obtained

Superior dimension control

Better reproducibility and interchangeability of parts

Improved strength properties

Directional properties can be minimized

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

Advantages of hot forming

Disadvantages of hot forming

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

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 [7][8]

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

Friction in metal forming process [17]

In forming operations, at the interface between the workpiece and die/tool, two types of frictions can 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

  1. 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.
  1. 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 forming 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 [12][13]

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:

  • 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.

References:

  1. Metal forming process http://me-mechanicalengineering.com/metal-forming-process/
  2. Hot forming process http://www.themetalcasting.com/hot-forming-process.html
  3. Hot working https://en.wikipedia.org/wiki/Hot_working
  4. Lecture 1: Fundamentals of metal forming http://nptel.ac.in/courses/112107144/Metal%20Forming%20&%20Powder%20metallurgy/lecture1/lecture1.htm
  5. Hot Forming Tribology: Galling of Tools and Associated Problems by Leonardo Pelcastre https://www.diva-portal.org/smash/get/diva2:999073/FULLTEXT01.pdf
  6. Advantages and disadvantages of cold forming uploaded by Fadi Innocent https://www.scribd.com/doc/105807153/Advantages-and-Disadvantages-of-Cold-Working
  7. Azushima, Akira. “Chapter 2: Tribology in Metal Forming.” Tribology in Sheet Rolling Technology. Cham: Springer, 2016. 27-99. Google books preview only
  8. Altan, Taylan, and A. Erman. Tekkaya. “Chapter 9: Tube Hydroforming.” Sheet Metal Forming: Processes and Applications. Materials Park, Oh: ASM International, 2012. 179-211 Google books preview only
  9. Podgornik, Bojan, and Vojteh LeskovÅ¡ek. “Wear Mechanisms and Surface Engineering of Forming Tools.” Materiali in Tehnologije 49.3 (2015): 313-24
  10. Altan, Taylan, Gracious Ngaile, and Gangshu Shen. “Chapter 22: Die Failures in Cold and Hot Forging.” Cold and Hot Forging: Fundamentals and Applications. Materials Park, OH: ASM International, 2004. 295-318 Google books preview only
  11. Dohda, Kuniaki, Christine Boher, Farhad Rezai-Aria, and Numpon Mahayotsanun. “Tribology in Metal Forming at Elevated Temperatures.” Friction 3.1 (2015): 1-27
  12. Metal forming fluids http://www.ashburnchemical.com/metal-forming-fluids.html
  13. Choosing a Metal-Forming Lubricant http://www.machinerylubrication.com/Read/29495/metal-forming-lubricant
  14. Coating for stamping and forming tools written by Yury Madorsky, Matthew Thompson http://www.thefabricator.com/article/stamping/coating-for-stamping-and-forming-tools
  15. Triboform products http://www.triboform.com/software-overview/
  16. Triboform – software for simulation of friction and lubrication conditions http://www.autoform.com/en/products/triboform/
  17. Section 5.4, Friction in metal forming, Material behavior in metal forming. http://nptel.ac.in/courses/112106153/Module%201/Lecture%205/Lecture%205.pdf
  18. Scientific papers referred are open based and available online
  19. Image used is “Labelled for reuse with modification/for free and use them commercially

HARSHVARDHAN SINGH
About HARSHVARDHAN SINGH 19 Articles
Harshvardhan Singh is an Automotive Engineer and has good experience in lubrication science and experimental tribology. He loves to write about tribology and related fields such as coating technology, surface engineering and others.

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    • Thanks for your positive comment Celenor Gomez. Do read and share our other articles as well.

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