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In the automotive industry, the development of electrically powered vehicles has become a major forward-looking topic. For improving the range and thus the efficiency of electric cars, lightweight construction has gained even more importance. In this regard, hot stamping has been established as a suitable and resource efficient process to manufacture high-strength and lightweight body-in-white components. This method combines hot forming and quenching of boron-manganese steel 22MnB5 in a single process step. As a result, complex structures with thin sheet thicknesses and high ultimate tensile strength up to 1500 MPa are generated. However, the use of lubricants is not possible at elevated temperatures, which subsequently leads to high thermo-mechanical tool stresses. As a side effect, high friction and severe wear occur during the forming process, which affect the resulting part quality and maximum tool life. Therefore, the aim of this study is to improve the tribological performance of hot stamping tools by using a laser implantation process. This technique is based on manufacturing highly wear resistant, separated and elevated structures in micrometer range by embedding hard ceramic particles into the tool material via pulsed laser radiation. As a result, highly stressed areas on the tool surface can be modified locally, which in turn influence the tribological and thermal behavior during the forming process. In this regard, laser implanted and conventionally tool surfaces were investigated under hot stamping conditions. A modified pin-on-disk test was used to analyze the friction coefficient and occuring wear mechanisms. Furthermore, quenching tests as well as hardness measurements were carried out to gain in-depth knowledge about the cooling behavior of the modified tool surfaces and its impact to the resulting mechanical part properties.
Laser implantation aims at reducing friction and wear on highly stressed surfaces in forming processes. Especially the hot stamping process that is used as a resource efficient process for manufacturing geometrical complex and high-strength structures, exhibits severe wear and high friction during the forming operation. The laser implantation process addresses this problem by combining two different approaches (surface modification and surface structuring) in surface technology by creating elevated, highly wear-resistant micro-features to influence the tribological performance. Pure TiB2 implants have shown to reduce tool-sided wear significally and improve the part formability by reducing local necking in deep drawing tests. Within the scope of this work, TiB2-TiC and TiB2-TaC hard material mixtures are implanted on X38CrMoV5-3 hot work tool steel. The aim is to investigate how the implant material properties can be influenced by the application of different mixing ratios of hard material mixtures under the specific variation of the process parameters. Distinct implant formations are tested on a novel test apparatus to examine the influence on the tribological properties.
From the analyses of the implant properties by hardness measurements, light microscopic images, EDX and XRD analyses process parameter ranges are identified to produce defect-free dome- and ring-shaped implants. The specific process parameters (pulse power, pulse duration, mixing ratio and coating thickness) can be used for the determination of the implant geometry (height, width and depth). The tribological tests exhibit improved friction and wear properties. Based on these results, a tribosystem manufactured by this additive surface treatment technology shows great potential to enhance the effectiveness of the hot stamping process.
Within the scope of this work, a laser implantation process has been used, in order to improve the tribological performance of hot stamping tools. This surface engineering Technology enables the generation of dome-shaped, elevated and highly wear resistant microfeatures on tool surfaces in consequence of a localized dispersing of hard ceramic particles via pulsed laser radiation.
As a result, the topography and material properties of the tool and thus the tribological interactions at the blank-die interface are locally influenced. However, a suitable selection of hard ceramic particles is imperative for generating defect-free surface features with a high share of homogenously disturbed particles. For this purpose, different niobium (NbB2 and NbC) as well as titanium-based (TiB2 and TiC) materials were embedded on hot working tool specimens and subsequently analyzed with regard to their resulting shape and mechanical properties. Afterwards, modified pin-on-disk tests were carried out by using conventional and laser-implanted tool surfaces, in order to evaluate the wear and friction behavior of both tooling systems.
The control of friction and wear is a major concern in many industrial applications. A promising method for tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in a deterministic pattern where needed on highly stressed surfaces, i.e., on forming or cutting tools.
However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free metal matrix composite with a high share of homogeneously distributed particles and, especially, high implant hardness. In this study, TiN, TiC, and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel were investigated.
Although all powder materials had grain sizes smaller than 10 μm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (∼64%) was observed for TiB2. By scanning electron microscopy and energy dispersive x-ray spectroscopy, it was also shown that a significant share of the preplaced particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By x-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in situ formation of TiC particles, which was proven by electron backscatter diffraction.
In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown
the most promising results.
The aim of this study is to increase the tribological performance of hot stamping tools by using a laser implantation process. This technique allows the fabrication of separated, elevated and dome-shaped microfeatures on the tool surface in consequence of a localized dispersing of ceramic particles via pulsed laser radiation. Hence, the topography and material properties of the tool are modified, which influences the tribological interactions at the blank-die interface. However, an appropriate selection of ceramic particles is an essential prerequisite, in order to obtain tailored and highly wear resistant surface features. In this regard, different titanium-based hard particles (TiB2 and TiN) were laser-implanted on hot working tool specimens and subsequently tested by means of a modified pin-on-disk test regarding to their wear and friction behavior.
Over the last years, the weight of modern car bodies has risen significantly due to the increasing customers’ demand for comfort and safety equipment. However, this ongoing trend leads to an increasing fuel consumption and thus to higher carbon dioxide emissions. In order to counteract these problems, hot stamping has been established in the automotive industry as a key technolo-gy for lightweight construction, regarding the manufacturing of safety-relevant car body compo-nents. Hot stamped parts are commonly made out of boron-manganese steel 22MnB5, which is initially austenized and subsequently formed and quenched in one process step. As a result, geo-metrical complex structures with an ultimate tensile strength of 1500 MPa are generated. The surfaces of the workpieces are coated with an Al-Si layer to avoid oxide scale formation and to ensure corrosion protection. However, the coating system leads to an increased adhesive wear on the tool surface due to the high thermo-mechanical tool stresses. Therefore, a time and cost con-suming rework of the hot stamping tools is required. The aim of this study is to increase the tribological performance of hot stamping tools by using a laser implantation process. This tech-nique allows the ma-nufacturing of separated, elevated and dome-shaped microstructures on the tool surface in consequence of a localized dispersing of hard ceramic particles by pulsed laser radiation. The generated surface features offer great potential for reducing the tribological load, due to their high hardness and wear resistance. For this purpose, the friction coefficient of un-modified and laser implanted tool surfaces were examined and compared by using a modified pin-on-disk test. In addition, the surfaces were analyzed by optical measurements in order to quantify the amount of wear.
The control of friction and wear is a major concern in many industrial applications. A promising method for a tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in deterministic pattern where needed on highly stressed surfaces, i.e. on forming or cutting tools. However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free Metal Matrix Composite with a high share of homogeneously distributed particles and especially a high implant hardness.
In this study TiN, TiC and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of the pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel was investigated. Although all powder materials had grain sizes smaller than 10 µm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (~64 %) was observed for TiB2. By scanning electron microscopy and energy dispersive X-ray spectroscopy, it was also shown that a significant share of the pre-placed particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in-situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 HV1 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By X-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in-situ formation of TiC particles, which was proven by electron backscatter diffraction. In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown the most promising results.
The control of friction and wear is a major concern in many industrial applications. A promising method for a tailored surface modification is the so-called laser implantation technique. This method combines surface texturing and material optimization in one processing step by a localized dispersing of hard ceramic particles using pulsed laser radiation. Wear resistant, protruding micrometric features (implants) with defined geometry can be created in deterministic pattern where needed on highly stressed surfaces, i.e. on forming or cutting tools. However, in order to maintain the implants over the tool’s lifetime, a suitable selection of hard ceramic particles is a prerequisite. They must provide a defect-free Metal Matrix Composite with a high share of homogeneously distributed particles and especially a high implant hardness.
In this study TiN, TiC and TiB2 hard particles were compared as implant materials for the first time. By a systematic variation of the pulse power and pulse duration, their dispersing behavior and influence on the material properties of AISI D2 tool steel was investigated. Although all powder materials had grain sizes smaller than 10 µm, it was possible to disperse them by pulsed laser radiation and to obtain defect-free protruding implants. The highest share of dispersed particles (~64 %) was observed for TiB2. By scanning electron microscopy and energy dispersive X-ray spectroscopy, it was also shown that a significant share of the pre-placed particles was dissolved by the laser beam and precipitated as nanometer sized particles within the matrix during solidification. These in-situ formed particles have a decisive influence on the material properties. While the TiN and TiC implants have shown maximum hardness values of 750 HV1 and 850 HV1, the TiB2 implants have shown the highest hardness values with more than 1600 HV1. By X-ray diffraction, it was possible to ascribe the lower hardness values of TiC and TiN implants to high amounts of retained austenite in the metal matrix. By implanting TiB2, the formation of retained austenite was successfully suppressed due to the in-situ formation of TiC particles, which was proven by electron backscatter diffraction. In conclusion, all the implant materials are basically suitable for laser implantation on AISI D2 tool steel. However, TiB2 has shown the most promising results.
Laser beam welding of aluminum die casting is challenging. A large quantity of gases (in particular hydrogen) is absorbed by aluminum during the die-cast manufacturing process and is contained in the base material in solved or bound form. After re-melting by the laser, the gases are released and are present in the melt as pores. Many of these metallurgic pores remain in the weld seam as a result of the high solidification velocities. The natural (Archimedean) buoyancy is not sufficient to remove the pores from the weld pool leading to process instabilities and poor mechanical properties of the weld. Therefore, an electromagnetic (EM) system is used to apply an additional buoyancy component to the pores. The physical mechanism is based on the generation of Lorentz forces, whereby an electromagnetic pressure is introduced into the weld pool. The EM system exploits the difference in electrical conductivity between poorly conducting pores (inclusions) and the comparatively better conducting aluminum melt to increase the resulting buoyancy velocity of the pores. Within the present study, the electromagnetic supported degassing is investigated in dependence on the laser beam power, welding velocity and electromagnetic flux density. By means of a design of experiments a systematic variation of these parameters is carried out for partial penetration laser beam welding of 6 mm thick sheets of wrought aluminum alloy AlMg3 and die-cast aluminum alloy AlSi12(Fe) where the wrought alloy serves as a reference. The proportion of pores in the weld seams is determined using X-ray images, computed tomography (CT-) images and cross-section images. The results prove a significant reduction of the porosity up to 70 % for both materials as a function of the magnetic flux density
The potential of lowered surface features as well as the application of wear resistant coatings have been known for many years to improve the tribological behavior of forming tools. More recent studies also discuss the capability of protruded microfeatures for adjusting the tribological behavior between contacting surfaces. The demand for a high wear resistance of such structures as well as their economical and reliable production, however, often limits the industrial application. The laser implantation process can overcome these limitations. In contrast to conventional cw-laser dispersing processes, where the formation of uniform metal matrix composite layers is intended, this surface engineering technique aims to improve the tribological behavior of contacting surfaces by a localized dispersing of pre-placed hard ceramic particles. This enables the formation of deterministic textures composed of separated wear resistant dome- or ring-shaped microstructures (implants).
Since TaC shows very promising material properties for improving the wear resistance of tools exposed to severe operating conditions, this paper analyzes its suitability for pulsed laser implantation on X38CrMoV5-3 tool steel for the first time. In the experiments, the influence of the particles and the laser parameters (pulse power, pulse duration and focal diameter) on the material properties of the localized dispersed zones was studied by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The composite´s (micro-) hardness was measured and calculated by using a rule of mixture. Additionally, the influence of the laser parameters and the TaC particles on the geometrical properties of the implants was studied by optical microscopy and white light interferometry.
The results showed that defect-free implants with hardness values of ~900 HV1 can be obtained at the focal spot, since a localized dispersing of the TaC particles is possible using a pulsed millisecond laser. However, in dependence of the laser intensity, also a partial dissolution of the initial particles occurs. This leads to the precipitation of new dendritic TaC nanoparticles and to varying contents of retained austenite in the matrix. Both effects have a strong influence on the implant hardness and must be considert by the rule of mixture. Regarding the geometrical response it was pointed out that protruded microfeatures with heights up to 10 µm can be created. In comparison to laser remelted zones, the implanted zones showed significantly altered weld pool profiles due to the influence of the particles on the melt convection. A transition of the implant shape from predominantly dome-shaped to predominantly ring-shaped was observed for intensities >1.7∙106 W/cm2 due to the onset of the keyhole effect.