Hot stamping of high strength steels has been continuously developed in the automotive industry to improve mechanical properties and surface quality of stamped components. One of the main challenges faced by researchers and technicians is to improve stamping dies lifetime by reducing the wear caused by high pressures and temperatures present during the process. This paper analyzes the laser texturing of hot stamping dies and discusses how different surfaces textures influence the lubrication and wear mechanisms. To this purpose, experimental tests and numerical simulation were carried out to define the die region to be texturized and to characterize the textured surface topography before and after hot stamping tests with a 3D surface profilometer and scanning electron microscopy. Results showed that laser texturing influences the lubrication at the interface die-hot sheet and improves die lifetime. In this work, the best texture presented dimples with the highest diameter, depth, and spacing, with the surface topography and dimples morphology practically preserved after the hot stamping tests.
In recent years, researches on tribology of metal forming processes for the automotive industry are focused on the improvement of products surface quality and process performance. In hot stamping the improvement of the die lifetime is a relevant factor for obtaining better products, lower costs, and high productivity, so it is essential to understand how the severe conditions of pressure and temperature influence the lubrication at the interface dies-sheet and, consequently, the wear mechanisms which can cause die lifetime reduction.
Hot stamping of hardenable steels consists in heating the blank above the austenitizing temperature and then transferring it to the press where it is stamped and finally cooled and tempered between the dies, improving productivity and the mechanical properties of the stamped part like ultimate strength and impact toughness [
Hot stamping is a relatively fast process if compared to other metal forming processes, so the manufacturing costs would be significantly increased if production has to be interrupted to premature and frequent changes of worn dies. To resist the severe working conditions of hot stamping, dies must present high hot hardness to minimize wear at high temperatures, low sensitivity to cracks caused by thermal shocks, low sensitivity to local tempering and softening, high mechanical strength to avoid mechanically induced cracks, low chemical affinity with the sheet alloy being stamped, and high oxidation resistance at high temperatures to avoid cracking by corrosion [
Therefore, many researches have been presented to improve dies lifetime by developing new materials for dies and blanks and coatings for the steel sheets [
The wear mechanisms at the tools-workpiece interface are very complicated and therefore have been studied for years by many researchers for several hot metal forming processes. Among the factors which may influence the way die surfaces can be damaged, the most studied are the process conditions (temperature, speed, and contact pressure), die surface modification (coating, texturing, and heat treatment), and workpiece surface improvement by coating and heat treatment [
Recently, several studies have been presented to evaluate the influence of coating steel sheets on the performance of hot stamping dies [
In the work of Boher et al. [
Finally, the third material called SG3, also nonnitrided, is fully martensitic with some fine and globular carbides rich in iron, tungsten, and silicon. From the results obtained with a deep-drawing process simulator (DDPS) they concluded that the most important wear mechanism was the material transfer caused by the adhesion of particles of the sheet coating during the hot sliding. The surface damage presented a rapid kinetics influenced by the hardness and particles shape and distribution of each die steel.
The modification of the surface texture by many manufacturing processes is being extensively studied [
Ibatan et al. [
Ramesh et al. [
Geiger et al. [
Andersson et al. [
In this work, we study the tribological effects of laser texturing the surface of hot stamping dies to evaluate the influence of the textures on the lubrication and wear by varying the diameter, the depth, and the density of the dimples and by evaluating the textures before and after the stamping tests by microhardness measurements, 3D surface topography, and scanning electron microscopy (SEM).
To define the conditions of laser texturing and lubrication, this work was based on some conclusions presented in the literature, which led to the hypotheses listed below considering the parameters shown in Figure
Parameters for the laser surface texturing of hot stamping dies.
As observed by Rapoport et al. [
Therefore, some hypotheses may be presented on the influence of the laser texturing parameters on the wear and lubrication: The increase in the number of dimples increases the retention of lubricant, so the density of dimples ( The distance between two adjacent dimples ( Larger dimple diameters ( To guarantee some lubricant to be expelled to the interface during sliding
the diameter of the crest ( the dimple depth ( the dimple depth (
To define the region of the dies to be laser textured, hot stamping of a steel part “U” shaped was simulated using the software Forge 2008 based on the Finite Element Method (FEM) in which the wear of the dies is modeled by modified Archard’s equations [
Figure
Inputs for the numerical simulation.
Material | Initial temperature | Type and number |
Mean size of | |
---|---|---|---|---|
Blank | Steel DIN 27MnCrB5—1.8 mm thick | 900°C | Tetrahedral |
2.46 mm |
|
||||
Tooling | Steel AISI H13 | 25°C | Triangular |
7.85 mm |
|
||||
Process parameters | Press speed 10 mm/s | Solid lubricant MoS2 | Heat transfer coefficients and |
Dimensions (in mm) of the dies (a) and punch (b) used in the hot stamping tests.
Numerical simulation—tooling and blank: before hot stamping (a) and after hot stamping (b).
As shown in Figure
Die wear in MPa·mm—numerical results.
The parameters to laser texturing the regions shown in Figure
Three distances between adjacent dimples (177, 266, 420, and 531
All the surfaces were textured with a solid-state lamp-pumped Nd:YAG laser, operating at 1,064 nm wavelength with a power of 100 W. The dimples dimensions were defined and controlled by the processing software available in the laser equipment.
Table
Dimensions of the texture parameters shown in Figure
Texture # | Dimple |
Distance |
Dimple |
Density |
---|---|---|---|---|
|
|
|
| |
1 | 30 | 177 | 100 | 25 |
2 | 150 | 266 | 150 | 25 |
3 | 30 | 420 | 150 | 10 |
4 | 30 | 266 | 150 | 25 |
5 | 150 | 531 | 300 | 25 |
Sketches and SEM images at the die radius of the five textures before stamping tests.
The punch and the dies were assembled in a hydraulic press with nominal capacity of 300 kN and work speed of 10 mm/s. Before each stamping test, the dies were cleaned and lubricated with solid MoS2.
The blank of DIN 27MnCrB5 steel (1.8 mm thick, 65 mm wide, and 82 mm long) was heated in an electric furnace at 900°C for 10 minutes and then transferred and hot stamped in the press to form a “U” shaped part. Each texture was tested 100 times.
The textured surfaces of the dies were analyzed before and after the hot stamping tests to evaluate the surface topography, the microhardness, and the surface integrity. After hot stamping all the surfaces were cleaned with ethanol in ultrasound equipment before the analyses.
The surface topography was analyzed with the 3D profilometer WYKO NT100 from Veeco Instruments. The microhardness Vickers was measured three times in four positions (① to ④ in Figure
The scanning electron microscope Zeiss EVO MA15 was used to evaluate the surface integrity of the textured dies, respectively, before and after the stamping tests. The surfaces were also photographed with a digital camera Sony DSC-W350.
Table
Microhardness Vickers of all textures before and after the hot stamping tests.
Texture # | Position | Before hot stamping tests | After hot stamping tests | ||
---|---|---|---|---|---|
Mean |
Standard deviation | Mean |
Standard deviation | ||
1 | 1 | 565 | 20 | 538 | 25 |
2 | 565 | 32 | 573 | 19 | |
3 | 597 | 19 | 592 | 30 | |
4 | 574 | 23 | 534 | 32 | |
|
|||||
2 | 1 | 486 | 40 | 590 | 32 |
2 | 597 | 18 | 572 | 16 | |
3 | 534 | 35 | 576 | 4 | |
4 | 525 | 23 | 568 | 23 | |
|
|||||
3 | 1 | 571 | 33 | 501 | 30 |
2 | 587 | 38 | 603 | 29 | |
3 | 581 | 17 | 581 | 19 | |
4 | 580 | 29 | 616 | 21 | |
|
|||||
4 | 1 | 607 | 19 | 563 | 12 |
2 | 591 | 22 | 563 | 9 | |
3 | 584 | 51 | 565 | 29 | |
4 | 530 | 20 | 569 | 32 | |
|
|||||
5 | 1 | 514 | 13 | 568 | 35 |
2 | 571 | 17 | 595 | 28 | |
3 | 564 | 34 | 568 | 19 | |
4 | 583 | 32 | 585 | 18 | |
|
|||||
As grinded | 1 | 548 | 26 | 592 | 28 |
2 | 570 | 40 | 595 | 28 | |
3 | 564 | 25 | 568 | 19 | |
4 | 568 | 27 | 585 | 35 |
Microhardness Vickers measured in point (2) near the die radius for all textures before (B) and after (A) hot stamping tests: maximum, mean, and minimum values.
Comparing statistically the means and standard deviations for each point of the six textures before and after the stamping tests it may be concluded that there is not a significant difference, assuming a significance level
To evaluate how laser texturing affected the dimples geometry and dimensions, the diameters of six dimples were measured twice for each texture, totalizing twelve measurements. The same six dimples were taken to measure three times the distance between adjacent dimples and dimples depth. Table
Nominal and measured dimensions of the texture parameters shown in Figure
Texture # | Dimple depth ( |
Distance ( |
Dimple diameter ( |
Density | ||||
---|---|---|---|---|---|---|---|---|
Nominal | Measured (mean/standard deviation) | Nominal | Measured (mean/standard deviation) | Nominal | Measured (mean/standard deviation) | Nominal | Measured (minimum/maximum) | |
1 | 30 | 25 ± 5 | 177 | 175 ± 5 | 100 | 106 ± 10 | 25 | 22/37 |
2 | 150 | 151 ± 10 | 266 | 265 ± 4 | 150 | 171 ± 8 | 25 | 29/37 |
3 | 30 | 27 ± 1 | 420 | 424 ± 6 | 150 | 206 ± 45 | 10 | 11/28 |
4 | 30 | 23 ± 7 | 266 | 269 ± 6 | 150 | 155 ± 10 | 25 | 25/31 |
5 | 150 | 93 ± 3 | 531 | 543 ± 17 | 300 | 363 ± 28 | 25 | 28/43 |
The mean and dispersion of each dimension near the nominal values, except for the dimple depth of texture #5 and the dimple diameter of textures #3 and #5, confirm that the conditions applied in the laser process allowed a controlled texturing in most of the conditions.
The dispersion of dimple diameter of texture #3 kept the minimum dimple density near the nominal 10% justifying the worst lubrication condition, as discussed in the analysis of the surface topography, and confirming the conclusions of Rapoport et al. [
Although texture #5 presented a mean measured dimple depth smaller than the nominal and a mean dimple diameter larger than the nominal, the minimum dimple density was not reduced and the maximum density was increased to 43% that is near the optimum density (40 to 50%) found by Rapoport et al. [
Figures
Surface topography—texture #1, die radius: (a) before and (b) after the stamping tests.
Surface topography—texture #2, die radius: (a) before and (b) after the stamping tests.
Surface topography—texture #3, die radius: (a) before and (b) after the stamping tests.
Surface topography—texture #4, die radius: (a) before and (b) after the stamping tests.
Surface topography—texture #5, die radius: (a) before and (b) after the stamping tests.
Surface topography—ground surface, die radius: (a) before and (b) after the stamping tests.
Bulges are evident in textures #1, #2, and #4 before the stamping tests, with the highest crests being observed in texture #2 (Figure
Textures #3 and #5 presented few small crests, while textures #1 and #4 presented bulges with similar height and distribution. Textures #1, #3, and #4 presented similar dimple depths before the stamping tests.
After the hot stamping tests, texture #1 presented a significant change of topography, and dimple depth was significantly reduced (Figure
Texture #2 also experienced a significant change: the topography was completely modified, the dimple depth presented the highest reduction among the five textures, and the bulges were completely deformed flattening the surface (Figure
Before the stamping tests, texture #3 (Figure
Otherwise, texture #4 (Figure
Texture #5, beyond presenting few bulges before stamping tests (Figure
The ground surface with average roughness of 2.0
Figures
SEM images of die radius—texture #1: (a) before and (b) after hot stamping tests.
SEM images of die radius—texture #2: (a) before and (b) after hot stamping tests.
SEM images of die radius—texture #3: (a) before and (b) after hot stamping tests.
SEM images of die radius—texture #4: (a) before and (b) after hot stamping tests.
SEM images of die radius—texture #5: (a) before and (b) after hot stamping tests.
SEM images of die radius—ground surface: (a) before and (b) after hot stamping tests.
SEM images are coherent to the results of the surface topography. Textures #1, #2, and #3 (Figures
Most dimples of all textures are filled with particles of the solid lubricant or oxides from the workpiece at elevated test temperature, as can be observed in Figure
SEM image of particles inside a dimple on the die radius of texture #5 after hot stamping tests.
Otherwise, textures #4 and #5 (Figures
Texture #5 practically presents the same dimples morphology after the stamping tests, except for the initial few bulges that were slightly flattened.
Finally, the images of the ground surface (Figure
The influence of dimples dimensions (diameter, depth, and spacing) on the performance of laser textured hot stamping dies has been studied, and the main conclusions are as follows: Laser texturing is a suitable process to improve the tribological performance of hot stamping dies, since all the textures evaluated in this work were better than the untextured surface to prevent metal-to-metal contact. The processing conditions used to laser texturing gave good results in terms of dimple dimensions and spacing for most of the tested textures. Increasing the number of dimples increases the retention of lubricant, so the density of dimples must be as high as possible to retain more lubricant. However, the distance between two adjacent dimples cannot be too small to be interfered by the bulges built up during laser texturing and deformed in the hot stamping tests. The dimple diameter must be the highest as possible with a high spacing to avoid metal-to-metal contact due to the high bulges. Textures with the smallest dimples diameter, depth, and spacing presented the worst performance with the topography completely modified by adhesive and abrasive wear. The best performance was obtained by the texture with the highest dimples density, near to 40%, as concluded by other researchers, with the highest dimple diameter and spacing, and with a depth of one-quarter of the dimple diameter. The surface topography and dimples morphology were practically preserved after the stamping tests, except for the bulges that were slightly flattened.
The authors declare that they have no competing interests.
The authors wish to thank FAPESP (Process 2011/12927-6) for the financial support to this work.