Drawing is a manufacturing process that consists of indirect deformation by pulling the material through a tool with a conical geometry. The process is usually performed at room temperature (cold forming), so the selection of effective lubricants is critical. If lubrication is inadequate, there is a high risk that both, tool and the manufactured wire, will fail. In this study, annealed AISI 1020 steel rods were drawn and the effectiveness of three different industrial lubricants was tested. During the process, the drawing force values were recorded and used to determine the friction coefficient that developed under each lubrication condition. Numerical simulations were performed to further understand the process. Based on the experimental and numerical results, qualitative and quantitative analysis were performed for each condition. Among the different lubricants used in this study, zinc stearate showed the lowest value for drawing force, 18.8 kN, followed by Lub A and B with values of 20 kN and 20.6 kN, respectively. The numerical models showed excellent approximation to the force values determined in the tests. The values for the coefficient of friction obtained by both the numerical analysis and the empirical model indicate that zinc stearate has the highest lubricating effect among the lubricants focused on this study.
El estirado es un proceso de fabricación que consiste en la deformación indirecta tirando del material a través de una herramienta de geometría cónica. El proceso generalmente se lleva a cabo a temperatura ambiente (formado en frío), por lo que la selección de lubricantes efectivos es fundamental. Si la lubricación es inadecuada, existe un alto riesgo de que tanto la herramienta como el alambre fabricado fallen. En este estudio, se estiraron varillas de acero AISI 1020 recocidas y se probó la efectividad de tres lubricantes industriales diferentes. Durante el proceso, los valores de la fuerza de tracción se registraron y utilizaron para determinar el coeficiente de rozamiento desarrollado bajo cada condición de lubricación. Se realizaron simulaciones numéricas para comprender mejor el proceso. Con base en los resultados experimentales y numéricos, se realizaron análisis cualitativos y cuantitativos para cada condición. Entre los diferentes lubricantes utilizados en este estudio, el estearato de zinc presentó el menor valor de fuerza de tracción, 18,8 kN, seguido de Lub A y B con valores de 20 kN y 20,6 kN, respectivamente. Los modelos numéricos mostraron una excelente aproximación a los valores de fuerza determinados en las pruebas. Los valores del coeficiente de rozamiento obtenidos tanto por análisis numérico como por el modelo empírico indican que el estearato de zinc tiene el mayor efecto lubricante entre los lubricantes objeto de este estudio.
Drawing is a forming technique aimed to reduce the cross-section and thus increasing the material in the longitudinal direction. The technique consists in drawing the raw material at the exit of a die, which determines the geometry of the final product (
Knowledge of the geometric parameters shown in
Many process variables affect the quality of the manufactured wires. Various researchers have studied the effects of variables such as the reduction ratio (r), the half-angle of the mould (α), and the friction coefficient (µ) (
The friction coefficient presents between the working material and the surface of the tool has a direct influence on the final result of the product (
Residual stresses in the drawn products have a significant influence as they affect the occurrence and development of surface cracks in the wire (
Quantifying the friction generated in the process by defining the coefficient of friction allows qualitative analysis of different lubricants and optimization of the process (
Several authors have already used inverse finite element analysis to determine the coefficient of friction that arises during the mechanical forming processes (
In the experimental procedure, specimens were drawn from the annealed AISI 1020 carbon steel. The chemical analysis of the material obtained by optical spark emission spectrometry is shown in
C | Mn | P | S | Si | Cr | B | Ti | Al |
---|---|---|---|---|---|---|---|---|
0.23 | 1.24 | 0.02 | 0.002 | 0.21 | 0.19 | 0.0039 | 0.046 | 0.037 |
In the annealing treatment, the material was heated in a resistance furnace at 850 °C for 1 hour for complete temperature homogenization and then slowly cooled in the furnace. The final microstructure, after annealing, is shown in
To evaluate the microstructure obtained, metallographic specimens were prepared according to standard procedures (
Drawing tests were carried out on a universal testing machine EMIC DL6000, with a load capacity of 600 kN. The process speed was set at 60 mm·min-1. During the tests, displacement and force measurements were recorded.
To perform the experiments, the test specimens (
Lubricant | Characteristic | Application status |
---|---|---|
Zinc Stearate | Soap | Power |
Lub. A | Polyglycol - Synthetic lubricant | Oil |
Lub. B | Paraffinic - Mineral lubricant | Oil |
As it is presented in
Here, Fsim is the value obtained from the drag force via the FEM model. Thus, the objective function in this study is defined as the sum of the squares of the errors between the experimental drag force and the via FEM, where the objective is to obtain the lowest values of the Tobj function.
For this, the values for the friction coefficients were determined to satisfy the condition for the difference between the values of the experimental and numerical force, equation two.
To reduce the computational effort, only the behavior of the specimens was classified as viscoplastic and the tools were kept as rigid. The yield curve determined experimentally in the tensile test was introduced into the software to characterize the mechanical behavior of the material.
Type analysis | 2D viscoplastic |
---|---|
Number of elements | 852 |
Node number | 3408 |
Element size | 0-9837 mm |
Element type | Quadrilateral |
Tool temperature | 20 °C |
Initial temperature of specimen | 20 °C |
Drawing speed | 60 mm·min-1 |
Heat transfer coefficient | 10000 W·m²K-1 |
The average value of the drawing force monitored in the experiments is used for the inverse analysis strategy. The friction coefficient for each test is determined by this value.
The flow curve of the annealed steel AISI 1020 determined in the uniaxial tensile test is presented in
In addition,
Yield stress (MPa) | Ultimate Tensile stress (MPa) | Elongation A (%) | Reduction in area Z (%) |
---|---|---|---|
218.6 | 341.0 | 37.0 | 21.8 |
The drawing force values were subject to fluctuations caused by the surface condition of the specimens. This behavior is due to the change in the lubrication regime, which initially migrates from static friction to sliding friction. In the experiments where stearate was used as a lubricant, it was found that the drawing force values were lower than those obtained with lubricants A and B. Regarding the curve of force versus time, it was noted that the drawing force remained constant until approximately 80 seconds of drawing, after which, there was a gradual increase until reaching the maximum value at the end of the drawn bar. In the tests carried out with lubricants A and B, it was observed that the drawing force presented significant oscillations, with an initial peak in both cases, due to the beginning of the process in which the fluid does not completely cover the work surface, resulting in a dry lubrication regime, affecting the state of application of lubricants. After the initial peak of force, the lubrication regime changed from static to mixed, leading to a reduction in force value. With regard to lubricant A, the force value remained reduced after the start until about 80 seconds, from which the drawing force increased until the end of the process. In the case of lubricant B, a more pronounced increase in the force was observed up to about 90 seconds, after which the force value dropped again. Such oscillations and changes in the lubrication regime can be explained by the variation in the contact pressure, which is a highly sensitive factor in liquid lubricants, causing a change in the thickness of the lubricating film at high contact pressures, something frequent in cold drawing processes.
The region of the curves where the oscillations were smaller was selected as the analysis area for the determination of the coefficients of friction.
Condition | Force, |
Average Yield Stress, |
Tool Angle, α [Rad] |
Initial area, |
Final area, |
---|---|---|---|---|---|
Zinc Stearate | 18899.2 | 218.6 | 0.0698 | 126.7 | 81.7 |
Lub A | 20035.4 | ||||
Lub B | 20662.7 |
Using the data in
The value of the actual stress for each case can be seen in
The maximum stress and strain values were observed on the drawn surface. The maximum true strain observed in the three conditions is equal and corresponds to 0.36. The magnitude of the maximum equivalent voltage is different under each condition. This demonstrates that the lubrication has an effect on the stresses developed in the process. In condition 1 (stearate) a maximum stress value of 478.4 MPa was observed. In condition 2 (Lub A), 397.3 MPa and in condition 3 (Lub B) 392.2 MPa. The average value of the 3 conditions is 422.6 MPa. The stress-strain relationship verified in the simulations coincides with the stress-strain curve shown in
In
An analysis of the maximum temperature during drawing can be seen in
The coefficients of friction obtained by the analytical and numerical approach are shown in
Lubricant | Coefficient of friction [µ] | Relative error [%] | |
---|---|---|---|
Siebel | Numerical Model | ||
Zinc Stearate | 0.08493 | 0.1075 | 26.57 |
Lub A | 0.09789 | 0.125 | 27.69 |
Lub B | 0.10401 | 0.135 | 26.44 |
Zinc stearate was the lubricating method that gave the lowest drawing force in the experiments and had a maximum force value of 18,899 kN. Forming with lubricants A and B resulted in maximum forces of 20,035 and 20,662 kN, respectively. Force is a process-dependent variable, and the maximum value obtained during forming depends on independent variables such as friction. The lower force required for forming with zinc stearate indicates that this lubricant presents a higher effectiveness in reducing friction between contact surfaces.
The Siebel Model was able to reproduce the variations in the values of the drawing force, and among the lubricants, zinc stearate was the one that had the lowest average value of the coefficient of friction of 0.08493.
The numerically determined tensile force values showed excellent correlation with the experimental values with maximum deviations of 0.5%.
Among the analytical and numerical methods, the numerical method presents a conservative behaviour, defining higher values for the friction coefficients when compared to those determined by the numerical model.
The results calculated with the finite element method and the Siebel model show a maximum relative error of 27.69%. However, from a qualitative point of view, both approaches show convergences that define zinc stearate as the most suitable lubrication method, followed by lubricant A and B, respectively.
The insufficient performance shown by lubricants A and B may be related to their formulation, composition and their physical and chemical characteristics, which may not be consistent with the drawing process in question. In addition, there is the possibility of compatibility problems between the lubricant and the material to be drawn, culminating in unsatisfactory performance of the lubricant. In this sense, it is imperative to continue the research and development of new lubricants, with a view to finding alternatives that are more compatible with the drawing process.
This study was financed in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brasil (CNPq) - Finance Code CNPq/MCTI/FNDCT nº 18/2021 (Process: 404196/2021-7). The authors are recipients of fellowships from the CNPq (research productivity - PQ1-4/2021; PDJ - 25/2021; GD - 2019) and CAPES (PROEX-IES-2020).