The Role of Friction on Metal Forming Processes

*Luis Fernando Folle, Bruno Caetano dos Santos Silva, Gilmar Ferreira Batalha and Rodrigo Santiago Coelho*

#### **Abstract**

The friction that occurs in forming processes plays a fundamental role in the industry as it can be responsible for both manufacturing failure and its success. Scientific research has been done to try to understand this phenomenon as well as simulation software has been implemented aiming to predict the tribological behavior of the metallic pair in contact. Thus, this chapter is dedicated to the analysis of the main parameters that can influence the coefficient of friction, especially for metal manufacturing processes. Some simulation models that try to predict the behavior of friction under certain conditions of process speed, contact pressure and operating temperature will also be presented.

**Keywords:** metal forming process, friction coefficient, shear friction factor, Tribological Modeling and simulation, friction test, Tribological parameters

#### **1. Introduction**

The plastic deformation manufacturing area can be divided into two large sets, massive forming (which includes the processes of rolling, forging, extrusion, and wire drawing) and sheet forming (which includes bending, cup drawing, shearing and miscellaneous processes). This can be seen schematically in **Figure 1**. The main difference between these two sets lies in the dimensions of the final part after it is processed. In terms of dimensions in the Cartesian plane, a sheet has two dimensions much larger than the third and in massive forming there is no definable difference between the dimensions in the Cartesian plane. For each of these large groups, friction has a characteristic behavior due to both applied force levels and strain levels. Generally, when talking about sheet processes, considering the same material, both the strains and the applied forces are smaller because there is not much material in the thickness to generate a great resistance against the load application.

For all these processes illustrated above, friction will occur, since there will always be contact between two or more surfaces and eventually a lubricant between them. Friction occurs when there is contact between a (forming) tool and the material being deformed.

In general, friction is associated with a negative aspect of forming processes (energy consumption, tool wear, increased forming force, increased tool temperature, etc.). However, this is not always the case. In some processes such as rolling, friction is fundamental for the material to be "holding" by the cylinders. Even in sheet forming, friction plays a fundamental role in preventing defects such as wrinkling.

#### **Figure 1.** *Classification of metal forming operations. Source: Groover [1].*

So that excessive friction is not the agent causing defects, a lubricating film is used between the contact surfaces. In industrial processes, it is normal to use excess lubricating oils to avoid problems such as die wear. However, with the demand for cleaner manufacturing processes so as not to harm the environment, the use of lubricating oils must be reduced or replaced with another one with non-harmful components or even eliminated. In this way, there are studies which focuses on the lubricant free forming using tools with structured or textured surfaces (**Figure 2**), as can be understood in more detail in the studies carried out by [2–4].

In the case of cold forming, liquid lubricants are generally used. In the past, the use of animal fats and natural oils was common, nowadays the use is concentrated in mineral oils. The exact physical principle that governs the behavior of these lubricants is not yet fully known, but their application always brings improvements to the process, such as:


Consequently, the control of friction levels plays an important role in the distribution of stresses and strains. A very low friction coefficient can generate fixing *The Role of Friction on Metal Forming Processes DOI: http://dx.doi.org/10.5772/intechopen.101387*

**Figure 2.** *Conceptual model of different structured test tool surfaces [2].*

problems, promoting the appearance of geometric defects such as shape distortions. In sheet forming, for example, the friction force between the sheet press and the die must be high enough to obtain the desired plastic deformations, avoiding the wrinkling of the sheet. On the other hand, a very high friction force will promote wear of the surfaces in contact, which can lead to the appearance of cracks in the final product. Therefore, it is very important to control the friction levels in forming operations.

In the forming processes, it is well known that the success to obtain a part depends on three main factors: the geometry of the tools, the material properties of the piece and the interaction between the contact surface of these two materials. It is also known that the costs associated with the third factor represent around 5% of the final production value of the part. As such, any friction-related improvement in sheet forming can generate immediate payback for manufacturers. Decreasing the friction on the forming process can contribute to a lesser wear on the tools, thus increasing their useful lives, as well as representing a reduction in the required presses forces and, consequently, an increase in the energy efficiency of the forming process.

With the emergence of stricter environmental laws and the tendency to manufacture parts with zero waste, it will be necessary to create more efficient manufacturing methods that operate with highly reduced wastes. In this aspect, numerical simulation can contribute considerably, offering fast solutions that are very close to reality, that is, they can predict failures in the manufacture of parts without these being physically created. Within this context, a critical area is the measurement of

friction in forming, where the methods created so far sometimes fail to adapt to what happens in practice.
