The Role of Friction on Metal Forming Processes

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.

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.

Figure 1.

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

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.

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, 3, 4].

Figure 2.

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

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:

• Decrease in the total forces needed for the forming operation; friction force is less on lubricated surfaces than on dry ones;

• Better flow of workpiece material within the dies that improves the distribution of strains and facilitates the fabrication of more complex parts;

• Prevention of die wear caused by adhesion or abrasion of surfaces;

• Quality assurance of products where surfaces are better preserved, free from “scratches”.

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 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.

2. Types of lubrication conditions

In 1902, Stribeck was the first to report the dependence of the coefficient of friction on the shaft speed in bearings. In his work he presented a curve with three different lubrication regimes. This curve was named “Stribeck Curve”. Subsequently, the coefficient of friction was presented as a function of the combination of the following parameters: lubricant viscosity, number of bearing shaft revolutions and normal shaft pressure. Figure 3 schematically illustrates the Stribeck curve where the initiation of various types of lubrication mechanisms as a function of lubricant viscosity, η, slide velocity, ν, and normal pressure, p.

Figure 3.

Stribeck diagram for different states of lubrication, with g dynamic viscosity, v sliding speed, p pressure/surface pressure. Source: Klocke [5].

Friction effects are always related to energy dissipation, so they are sometimes considered undesirable in manufacturing processes. In the case of metal forming, metal flow is limited by the contact pressure between die and part. Thus, friction can generate internal and surface defects, in addition to influencing the tensions in the tools, load and energy required. The purpose of the lubricant is to separate the tool and workpiece surfaces, thus shifting the friction conditions from the boundary friction area in the direction of hydrodynamic friction as shown in Figure 3.

There are four distinct lubrication conditions that determine the influence of friction on metal forming:

• Dry Friction, where there is no lubricant present, and the friction coefficient is governed by the contact between the surface roughness peaks of each part. Condition used, for example, in hot rolling of sheets, V-bending of sheets and extrusion of aluminum alloys.

• Boundary Lubrification, is a condition very similar to dry friction with the difference that the surfaces in contact may eventually have the presence of oxides that act as an intermediary element between the parts. This is the case with aluminum and carbon steel, which will always have an oxidized layer when in contact with air. Another situation in this condition would be a thin layer of lubricant that acts as an interface between the parts along with air pockets between them.

• Mixed lubrification is the condition in which a film of lubricant surrounds all the material to be formed. This situation results in an intermediate lubrication between the Boundary Lubrification and Hydrodynamic conditions. It is the situation most widely found in metal forming where the presence of the lubricant generates an efficient friction in the production of pieces.

• Hydrodynamic Lubrification, which exists when a thick layer of lubricant is present between the die and the component. This subject is dealt with in fluid mechanics, where the stresses generated are related to the fluid’s viscosity.

Figure 3 also shows the separation between the contact surfaces, as friction approaches Hydrodynamic Lubrification, there will be an increasing separation between work material and tools. This is not only in macroscopic conditions but can also occur in microscopic level where the lubricant will be retained in the roughness valleys and will be able to act when the pressure is greater. This will be explained further in the next item.

In addition to the conditions described above, two main factors can occur in the contact between two surfaces, the first is a friction by material adhesion that is caused by micro-welding at the contact interfaces and the second is a penetration friction. In the first case (Figure 4a), there is usually the pullout of the material with a lower hardness (workpiece) and this material ends up working as an interface material that promotes scratching of the shaped parts. In the second case (Figure 4b), there is a very large approximation between the parts that the roughness ends up being interpenetrated. In this case, it is very likely that material will be pulled out of the workpiece, promoting abrasive wear on the pieces and tools. In either case, a good lubrication condition can resolve.

Figure 4.

Schematic illustration of the components of the coefficient of friction present in metal/metal contact. Source: Folle and Schaeffer [6].

3. Models used to describe friction

To describe the friction phenomenon in forming, it is important to use a model that reasonably describes the reality, especially when the analysis surface is large. For this case, the friction force makes a relevant contribution to the total force required in the operation. Despite the great development in models that describe the behavior of materials, conventional computational numerical simulations generally do not provide correct results regarding friction. This is due to the use of very simplified friction models.

Two models are generally used to describe friction at the interface between tool and work material depending on the process being considered. The first is the Amontons-Coulomb model shown in Eq. (1), where there is a linear relationship between normal pressure and shear stress.

$$\tau =\mu \cdot p$$E1

where μ is the coefficient of friction (COF), p is the normal pressure and τ is the frictional shear stress.

Eq. (1) is valid only for relatively small shear stresses, because when τ exceeds the shear strength, k, of the workpiece material, the second model must be used. This second model was proposed by Orowan [7] and is shown in Eq. (2).

$$\tau =m\cdot k$$E2

where m is the shear factor, where its value ranges from 0 to 1 and k is the maximum shear strength of the work material.

Figure 5 represents the combination of the Amontons-Coulomb model and the limit shear stress model proposed by Orowan [7]. Shear stress is shown as a function of normal pressure. The first part of the figure is considered to be the Coulomb part. The relationship between the frictional force and the normal force, defined as the friction coefficient μ, is constant in this part of the curve.

Figure 5.

Relationship between contact pressure and frictional shear stress. Source: Based on Rodrigues and Martins [8] and Altan and Tekkaya [9].

When the normal pressure increases, the lubricant pockets start to leak and promote a decrease in friction visualized by the decrease in the slope of the curve, until reaching a constant value that is given by the maximum shear stress of the material. At this point, the friction coefficient no longer makes sense, and the concept of friction factor appears. This phenomenon is represented by the horizontal part of the curve (Figure 5).

In terms of metal forming processes, the coulombian friction coefficient is more used in sheet metal fabrication since the pressures cannot be so great as there is not enough material in the thickness to be deformed. As for the forming processes in bulk deformation, the shear friction factor must be used because in this case the pressures are always close to the maximum shear stresses of the work material.

4. Friction measurement tests for forming processes

There are several tests that were developed to measure the friction coefficient and friction factor in order to both understand how each process variable influences it and also to be loaded in finite element software for simulation of part manufacturing. The most common test still used today is the pin-to-disk test. Figure 6 schematically shows how these tests work. This test consists of placing a specimen in the form of a ring or disc in a testing machine and forcing a pin against the specimen, making a rotary movement along the axis of the specimen. During the test, the tangential force and the normal force are measured in the contact area. The test can be done with a flat or spherical pin, depending on the condition being analyzed. The analysis is quite versatile, as friction can be determined for different sliding speeds, lubrication conditions, pressure forces and even working temperatures, depending on the complexity of the machine. Another advantage of this test is that some machines are marketed solely and are already adapted according to the most used standards. However, there are some disadvantages of this test that are related to the working pressures, which cannot be very large and in relation to the manufacturing processes, not being applied to some cases such as forging and sheet bending.

Figure 6.

Schematic illustration of pin-on-disc test. Source: Trzepiecinski and Lemu [10].

In processes where the contact pressure reaches values high enough to get close to the maximum shear stress of the work material, there is another test called the ring upsetting test, shown in Figure 7. This test has good applicability, reproducibility, and reduced cost. Therefore, it is often cited in the literature to determine both the friction coefficient and the friction factor for different lubrication states, both hot and cold. The sample is produced in a cylindrical ring shape with a hole in the center (Figure 7 in the upper left corner). The sample is pressed axially between two parallel plane dies and after a specific strain, the geometry is measured and compared to the initial geometry. Thus, depending on the relationship between the start and end diameters, there will be a different friction on the part. If there is too much friction, the inside diameter gets smaller and the outside diameter larger. In case of little friction, the internal diameter becomes larger. To determine the friction values in this test, calibration curves are needed (Figure 7 on the right), which can be obtained in different ways, but the most common is with the help of numerical simulations. The graph in Figure 7 on the right shows the results of the ring upsetting test with different lubrication states. It should be noted that the friction coefficients determined are not absolute values, but average values over the entire contact surface between the specimen and the tool that were also determined along the entire path traveled.

Figure 7.

Specimen geometry in ring upsetting and a graphic calculated via FEM simulation for the evaluation of ring upsetting tests. Source: Klocke [5].

For sheet metal forming process, the most used test in the literature today is the Strip Drawing Test or Bending Under Tension (BUT) Test shown in Figure 8. The test consists of bending a sheet metal strip through a pin of predetermined radius and on this pin to make the sheet slide. For this, a force is applied at one of the sheet ends so that there is relative movement between the sheet and the pin. At the other end, a force is applied against the movement in order to tension the sheet and to be able to vary the contact pressure incident on the pin. The force that generates the movement is F1 and the force that is applied in the opposite direction is F₂. The radius pin r has the function of simulating the friction in the passage of the stamping die radius, since it is in this region that the tensions are greater.

Figure 8.

Bending under tension test. Source: Folle and Schaeffer [11].

In this test, there are two forces required to make the sheet slide on the pin, one is the friction force between the contact surfaces and the other is the force required to bend and unbend the sheet. As the purpose of the test is to know the friction force between the contact surfaces, it is carried out in two steps. In the first one, the pin through which the sheet passes can freely rotate through its axis, so that there is no relative movement in the pin/sheet interface. This creates a condition of minimal friction, as the force required to make the sheet move is due solely to the sheet’s bending and unbending force. In the second step, this same pin is fixed on its axis, preventing any movement. The force required to make the sheet move is then made up of the bending force plus the friction force. Thus, the bending force, measured in the first step, can be deducted from the second, and only the friction force is obtained as a result.

The BUT test described above was conceived from the idea of knowing the friction in the passage of the die radius and is the traditional way of performing this test. However, some authors [12, 13, 14, 15] have proposed some variations on this test in order to facilitate its construction or generate results closer to the deep drawing process. This is primarily due to the fact that to run the BUT test, it is necessary to build specific equipment for this, which is often difficult to perform. Thus, authors [13, 14, 15, 16] have proposed some systems in which it is possible to adapt to a universal testing machine. In other works [17, 18], a measurement is proposed in which a torque is obtained on the pin through which the sheet passes. The idea of this variant of the BUT test aims to eliminate the test carried out with the free pin (which can rotate on its axis), since the torque measured on the pin is generated solely by the friction force, which is the purpose of the test. This test has the advantage of being able to represent the most critical region of sheet metal forming processes, which is in the die radius, however, it is only valid for sheet metal, not applying to bulk forming. Another disadvantage is that there is still no marketable machine that can be purchased for this purpose, all available models were built in research institutes, requiring a project for their implementation.

5. Parameters that influence friction in forming process

Friction depends on several parameters such as lubrication, normal pressure, workpiece and tool surface roughness, type of contact pair materials, slip speed and temperature. In forming at high temperatures, the coefficient of friction is generally higher than in cold forming. This is because strain at high temperatures can increase adhesion at the contact interface and because the best lubricants cannot withstand high temperatures, causing performance to suffer. There are other factors that strongly influence friction, such as the oxide layers that form debris that promote wear on the tools. Researchers already know that friction is more influenced by factors such as temperature, contact pressure and lubrication, but other factors can also be listed and have been summarized in Figure 9.

Figure 9.

Parameters that can influence the friction in metal forming. Source: Trzepiecinski and Lemu [10].

The first factor to be considered in metal forming processes is the shape and finish of the surfaces. Figure 10 shows work done by Tillmann et al. [19] where the surface type and hardness were varied in a pin-in-disk test. The idea of modifying the surface of tools is commonly used to generate lubricant entrapment pockets that when surface pressure increases, lubricant is released from these pockets, thus decreasing friction. This effect is also desired for large displacements between the tools and the workpiece to avoid wear.

Figure 10.

3D images and mean roughness Rz values of the bionic structures a) St1, b) St2, c) St3, d) St4 e) St5, and f) flat reference surface. Source: Tillmann et al. [19].

The friction results from Figure 10 are shown in Figure 11 where you can see that the surfaces that generate the least friction are those with lubricant storage cavities as in Figure 11(a) and (b). Figure 11(c) does not give good results as the surface peaks are too high causing there to be very punctual contact between the dies and the sheet and the lubricant not even being reached by the opposite surface.

Figure 11.

Friction coefficient results obtained by Tillmann et al. [19].

As previously mentioned, lubrication is one of the main factors influencing friction as it can act as an efficient separation element between the parts in contact. It is already known that Teflon represents one of the lubricants that generates less friction at work interfaces, since it acts almost as a hydrodynamic lubrication, taking the friction coefficient to values below 0.05. Figure 12 shows a comparison made through the BUT test with 3 lubrication conditions, dry, with grease and with Teflon, with 2 surface conditions, only machined and chromed, with 2 pin sizes and for 5 different working materials. In this study, it is possible to see that the friction conditions were lower when Teflon was applied. For the other conditions, friction changes around an average of about 0.2.

Figure 12.

Friction coefficient for different finishes, lubricants, and materials. Source: Fratini et al. [20].

A very low friction condition is sometimes not the best option for the forming process, as the flow of the working material must be controlled so that the piece is free from defects. Figure 13 shows two pieces made by sheet forming with different lubricants and the figure on the right was made with a Teflon sheet. It is possible to see that a very low friction promoted a freedom of strains in the material that caused the appearance of wrinkles on the edges, and this is considered a process defect.

Figure 13.

Pieces stamped with different lubricants under the same conditions. Left figure: Liquid lubricant. Right figure: Teflon sheet. Source: Folle and Schaeffer [6].

In terms of numerical simulation, there are several efforts to adapt the friction variables in software. In the work of [21] a Pin-on-Disk System was used to evaluate the contact pressure in relation to friction, where a decreasing variation curve was constructed as shown in Figure 14. This curve was loaded into the software to evaluate the spring back, and the error with respect to the real piece was 7.4% while the error with constant friction was 31.1%.

Figure 14.

Variation curve used in the work of [21].

In the work of [22], a friction model was developed that considers the change in surface texture at the microscale and its influence on the friction behavior at the macroscale. This friction model was implemented in finite element code and applied to a full-scale sheet metal forming simulation. The results in Figure 15, showed a higher friction coefficient distribution compared to the constant friction model, however, this work was not compared with a real part, but the friction values were within acceptable levels.

Figure 15.

Simulation result with constant friction model (a) and texture change model (b). Source: Hol et al. [22].

Efforts to develop more efficient friction models are still current. In the work of [23], a model was also developed that considers the measured surface topographies of the sheet metal and the tool to determine the pressure distribution of the lubricant. Lubricant pressure distribution is used to estimate the load carried by solid-to-solid roughness contacts and the lubricant to calculate the overall coefficient of friction. The new mixed lubrication friction model is used in the FE code to make simulations of the real part shown in Figure 16. The results of the simulated and real major and minor deformations were very close for the presented model (Figure 17), although the friction was not simulated constant to check the differences between the 2 models.

Figure 16.

Part geometry that was simulated by Shisode et al. [23].

Figure 17.

Comparison of major and minor strain distributions between experiments and simulations. Source: Shisode et al. [23].

In the work of [24], the researchers made the evaluation of friction in relation to the sliding speed and the contact pressure for an aluminum alloy and obtained the curve in Figure 18. With these results, they performed a numerical simulation of a profile U-shaped and evaluated both sheet thickness (Figure 19) and spring back. For the thickness of the sheet, the variable friction model was perfectly suited to the real result. As for the spring back, there was a noticeable reduction in the error level, from 25–8% in one of the sheet’s inclination angles.

Figure 18.

Friction coefficient curve with different sliding speeds, with five loads. Source: Dou and Xia [24].

Figure 19.

(a) Thickness measurement points; (b) thickness comparison between numerical result and actual measurements. Source: Dou and Xia [24].

Recent advances in friction measurement research have supported the development of software that is able to simulate the variation of the friction coefficient based on properties such as surface roughness and calibration tests. Figure 20 shows two surfaces that were scanned and loaded into the TriboForm® software to estimate friction and Figure 21 shows the result that the software generated from the data provided. From Figure 21, it is possible to see that the behavior of the friction coefficient in relation to the sliding speed, the contact pressure and the level of deformation agree with the literature. This generates more precision in predicting the material’s behavior against these variables and, consequently, also generates a better scenario for decisions of those involved in the fabrication of the piece.

Figure 20.

Impression 3D surface texture sheet (left) and tooling (right). Source: Sigvant et al. [25].

Figure 21.

Simulated friction behavior for different strain levels in the sheet material. Source: Sigvant et al. [25].

To illustrate these improvements being applied in software, Figure 22 shows two simulations made under different friction conditions, the first with constant friction and the second with variable friction. In this case, the simulation with a friction coefficient more faithful to reality proved to be more efficient in predicting process problems and this has been gaining strength in simulation software.

Figure 22.

Simulation with constant friction and variable friction compared to a real piece. Source: Sigvant et al. [25].

It can be seen in the Figure 22, the results of the finite element modeling and simulation with constant friction and variable friction compared to a real production sheet metal part. The figure shows that a simulation with constant friction masked a problem that appeared in the real part and with the use of variable friction (TriboForm), the simulation can predict the defect circled in green in the photo on the right. Thus, it is important that friction is well characterized so that production defects can be avoided.

Other studies are being conducted to further improve the understanding of the characteristics of the tribological system in relation to the forming process, especially for sheet metal, also using dedicated software such as TriboForm and TriboZone. The work of [26], for example, aimed to identify the influence of the heterogeneity of roughness along the tool in the material forming process using TriboZone software. In Figure 23, it is possible to see that a roughness heterogeneity model adds more reliability to the results, however, there was a great similarity in relation to the TriboForm variable friction model, which makes this model equally effective. On the other hand, models that only depend on pressure and constant models can generate more dispersed deformation data, which should not correspond to reality.

Figure 23.

FLD of the part considering the different types of friction model. Source: Sigvant et al. [26].

Another work using the same simulation platform, but with the objective of testing the influence of the amount of lubricant on the piece’s deformations was done by Tatipala et al. [27]. In this study, an automotive door side was chosen, and the amounts of lubricant applied in each zone were measured. Figure 24 shows the variation in lubricant amounts in relation to the part’s blank position.

Figure 24.

Variation of the amount of lubricant measured in each region of the blank. Source: Tatipala et al. [27].

The information on the variation of lubrication amounts in each region of the part was loaded into the software and the strains were evaluated. Figure 25 shows that with a variable amount of lubricant, there will also be a different strain on the sheet, which makes the result of strains on the sheet more accurate.

Figure 25.

Variation of major strain for blank with constant lubrication (left) and the variable amount of lubrification (right). Source: Tatipala et al. [27].

As shown so far, friction in the forming process is a major contributor to the deformation performance of a piece. Research has indicated that it is important to have an adequate surface, both for the work material and for the tools, so that there is a correct lubrication system. It is equally important to know the main mathematical models that can be applied to each manufacturing situation by the forming process, predicting with more precision the results for the manufacturing of a piece or product. These friction models have been studied more intensively to be loaded in numerical simulations and thus predict possible manufacturing defects. The main results of the studies showed that there is a considerable improvement in predictability with respect to spring back, wrinkle defects, strains in the workpiece and final sheet thickness.

Previously, it was enough to use constant friction in the simulations and the results for those materials were enough, so that a simple test could obtain the most adequate friction coefficient. However, with the advancement of new materials, such as advanced high strength steels, high plasticity aluminum alloys with high strength, magnesium alloys for cold forming, duplex stainless steels, dissimilar materials joined by welding, among others, have required that simulations be increasingly detailed in the amount of information to be collected. It is already possible, for example, to load in the software, information such as the topological profile of the surfaces in contact, the viscosity of the lubricant, the variation with pressure, temperature, sliding speed, level of strains and amount of lubricant applied in each region of the piece. This brings more accuracy to the failure prediction models, but also generates more work to be surveyed.

6. Final remarks and future trends

Tribology, including the study of surface phenomena such as contact, friction, lubrication and wear, plays a fundamental role in metal forming processes, as it directly influences the final quality of the parts produced and the useful life of the tools, which generally have a very high cost. In this sense, the use of mathematical models that represent the adequate friction conditions and the use of computational simulation tools play a fundamental role in reliably predicting the tribological aspects of the forming processes.

Generally, industrial case studies focused on sheet metal forming simulation using finite element methods, includes a constant coulombian friction model and cannot achieve good results when comparing numerical and experimental results, in terms of the prediction of wrinkling as well as ruptures, as presented in this work. However, further improvements are going towards spring back prediction, which includes surface texturing parameters and variable friction approach. All, aiming to a better reliability on tribological modeling of the friction influences for the metal forming process.

The present work shows that there is a growing concern regarding friction phenomena in metal forming. Numerical simulations are already at a very advanced stage in their ability to predict the behavior of the material being manufactured. However, the amount of information that must be collected for the software to be able to simulate with precision is increasing and this will require a greater volume of data associated with the materials of the pieces, tools and metal interfaces. This may seem problematic in terms of costs, however, it must be taken into account that the failure and scrap rates will be practically zeroed, leading to more gains than the efforts to obtain all the necessary properties.