TRIBOLOGY IN SHEET METAL FORMING: A MODIFICATION OF TRIBOMETER FOR STRIP DRAWING TEST

In the pursuit to produce the best product at the lowest cost, much can be done to reduce the cost of production. One of these methods, which is nowadays inseparable from the development part of production, is numerical simulation. However, the real world cannot be accurately modelled. The use of simplified models for numerical simulation can lead to different results between reality and simulation. One of these models is a coefficient of friction used in sheet metal forming simulations. Nowadays modern simulation software used for simulating the sheet metal forming process uses simplified model of friction coefficient using variables including surface roughness, topography, lubrication, and others. One of the variables not included in these models is temperature. In deep drawing, temperatures of sheet metal can be over 120 °C. Temperature can greatly change tribological properties, thus changing the friction between the tool and the sheet metal. The aim of this paper is to review a method of measurement of tribological properties in sheet metal forming and to design a modified tribometer for strip drawing test, focusing on the effect of temperature on tribological properties, predominantly for materials used in the automotive industry.


INTRODUCTION
In recent years, trends in the automotive industry have been towards the production of lighter components to reduce emissions and production costs while maintaining product quality.Achieving these conditions can be achieved by research into materials and production technologies, etc.One of the important parameters in the context of surface forming is the discipline of tribology, which influences several properties in forming processes.
Modern simulation software uses formulas based on Amontons laws of friction to calculate simulations of tribological conditions in forming processes, describing the relationship between friction force and coefficient of friction, where friction force is directly proportional to compressive force.The ratio between these forces is expressed as the coefficient of friction.The coefficient of friction used in the numerical simulations is determined under ideal conditions.During forming processes, deformation of the material occurs, which causes a change in parameters that affects the tribological properties of the material (temperature, surface topography, oil viscosity, etc.).Following the change in tribological properties, the parameters of the forming operation, in particular the forming force, also change.[1][2][3] This article focuses on the search for a tribometer for use in testing tribological properties at various temperatures.The data from this research would be used in numerical simulations to further increase the accuracy of the sheet metal forming simulations.

TRIBOLOGICAL TESTS IN THE SHEET METAL FORMING PROCESS
Coefficient of friction in sheet metal forming is measured by myriad of ways.Tribological test methods can be distinguished into six categories as shown in Figure 1.Categories 1-3 are from the perspective of testing forming processes during production of an actual product.Major advantages of this approach is relative accuracy of results, since the real forming operation is used during trial.Main disadvantage is limitations to change the parameters of the trial, mainly load and the implicit measurement of variables like coefficient of friction.Furthermore, these tests require special dies and are therefore quite expensive.[1] On the other end of the spectrum, we have the general tribometers described in Figure 1 as category 6. Main advantages of general tribometers are their standardisation (DIN 51350, DIN 51834, ASTM D5707, ASTM D1894, ASTM G99) these standards assure, the repeatability of the tests.Disadvantage, with regard to sheet metal forming, are different contact conditions therefore low applicability to forming processes (relatively small contact area, lubricated condition, etc.).[1].Category 4 are mainly used for study of tribological systems, particularly lubricants.Model tests performed by drawing a cylindrical cup often lack systems for direct measurement of force or friction coefficients.Therefore, these tests are mainly used in studies focused on qualitative comparisons and proof of feasibility [1] Tests of category 5 utilize strip of sheet metal as a test sample.Methodology behind model tests of category 5 is to simulate conditions that occur in sheet metal forming operations by simulating frictional behaviour under specific conditions (contact pressure, sliding velocity, temperature, topography, etc.).Such tests mainly aim at quantitative results in form of friction coefficients.From methods mentioned in Figure 2, the strip drawing is most suitable for the purpose of measuring the difference of tribological properties, in this case changes dependant on temperature.Device for strip drawing test is relatively simple to build in an already established laboratory.Universal testing machine can be easily modified for this purpose.Therefore, this paper is focused on strip drawing test.

STRIP DRAWING TEST
Strip drawing test (Figure 2a) is one of the methods used for measuring coefficient of friction in sheet metal forming due to its relative simplicity and adaptability for various universal testing machines.Main principle of strip drawing test is to use one or two dies, that apply pressure on strip of sheet metal.In case of using two flat dies the equation of coefficient of friction is described in (1) and pictured in Figure 3. where: -the coefficient of friction (-) FNthe normal force (N) FT -the pulling force (N) The jaws can be flat, but it is also possible to use a cylindrical or other shape.It is important to note that the resulting coefficient of friction is influenced by the tribological conditions (contact pressure, relative velocity of the friction bodies, temperature, amount and type of lubricant, topography of the surface of the jaws and sheet metal), as well as the shape of the jaws and the orientation of the sheet metal between the jaws.For consistent results, it is important that the tribometer design is as rigid as possible.[1-3]

STRIP DRAWING TRIBOMETER:
Design incorporates the universal testing machine LabTest 5.100SP1 as a source of the pulling force with realtime monitoring, shown in Figure 4. Parameters of the universal testing machine are shown in Table 1.Guide rods are placed inside precision bronze bushings to ensure smooth movement of operation and accuracy of die alignment.The compression length is adjustable by two screws located on the sides of the plates, but main movement is done by trapezoidal screw located in the middle of the plate.Trapezoidal screw is mounted inside of the front frame in a bronze bushing and kept in place by nut, which is held in place by a pin.Adjustment of clearance is done in such a way to assure a smooth operation.Movement screw ensures quick clamping of the test sample, due to springs being preloaded, the distance that needs to be cleared is minimal.3D model of the tribometer is shown in Figure 5 Figure 5 3D model of the proposed tribometer Heating of the dies is done by a heating element embedded in a square aluminium rod which is clamped between the jaws.Temperature reading will be carried out by thermocouples located on the back of the jaws.
The desired temperature will be reached when the jaws are heated to the desired temperature.The expected temperature range is from 20 °C to 120 °C.
Test sample is a strip of sheet metal 0.8 mm thick, 30mm wide and 300mm long.Sliding length of the sample is 200mm.Maximum normal force is determined by size of linear springs used, assumed maximum normal force is 10kN.Material of the test sample will be from commonly used steels in automotive industry such as: HCT490X, DX57D.[4,5] The methodology of the test is as follows.Firstly, test samples are cleaned and degreased, same applies to the dies.Springs are preloaded to the given amount of normal force by tightening of screws, at this step all other parameters of the test are set (temperature reading, speed, etc.).Then heating device is inserted into the jaws and clamped (Figure 6a).When goal temperature is reached heating device is removed from tribometer and set aside.Test sample is then clamped in the jaws of the universal testing machine and dies of the tribometer clamp the test sample (Figure 6b).Universal testing machine is then turned on and sample is pulled through the dies of a tribometer.Test is done when sample is pulled through (Figure 6c).Then steps are repeated for the next test.Further research activities into tribological properties under realistic forming temperature conditions would contribute to the refinement of simulation software results, resulting in cost reduction in the pre-production phase of sheet metal forming manufacturing.

Figure 3 Figure 4 •
Figure 3Schematic of strip drawing test with flat dies[1]

Figure 6
Figure 6 Test methodology: (a) Tribometer with heating device, (b) tribometer with clamped test sample, (c) Tribometer after the test

Table 1
Technical parameters of universal testing machine LabTest 5.100SP1