Development and analysis of microstructures for the transplantation of thermally sprayed coatings

Thermally sprayed coatings and tribological surfaces are a point of interest in many industrial sectors. They are used for better wear resistance of lightweight materials or for oil retention on surfaces. Lightweight materials are often used in the automotive industry as a weight-saving solution in the production of engine blocks. For this, it is necessary to coat the cylinder liners to ensure wear resistance. In most cases, the coating is sprayed directly onto the surface. Previous research has shown that it is possible to transfer these coatings inversely onto other surfaces. This was achieved with plasma sprayed coatings which were transplanted onto pressure-casted surfaces. These transplanted surfaces exhibited better adhesive strength, smoother surfaces, and lower form deviation compared to directly coated surfaces. Additionally, it was shown that even microstructures of a surface coated by plasma spraying can be transferred to pressure-casted surfaces. This paper presents the development and micromilling of different microstructures for transferring thermally sprayed coatings onto pressure-casted surfaces. In the development process, microstructures with different shapes and aspect ratios as well as thin tribological surfaces are designed in order to evaluate the advantages and limitations of the transplantation process. In subsequent experiments, the micromilling process and a simulation of the coating transplantation are presented and analyzed.DFG/Mo 881/9-1DFG/Bi 498/6-

Advances in CAD/CAM/CAE Technologies

CAD/CAM/CAE technologies find more and more applications in today’s industries, e.g., in the automotive, aerospace, and naval sectors. These technologies increase the productivity of engineers and researchers to a great extent, while at the same time allowing their research activities to achieve higher levels of performance. A number of difficult-to-perform design and manufacturing processes can be simulated using more methodologies available, i.e., experimental work combined with statistical tools (regression analysis, analysis of variance, Taguchi methodology, deep learning), finite element analysis applied early enough at the design cycle, CAD-based tools for design optimizations, CAM-based tools for machining optimizations

Contact pressure and sliding velocity ranges in sheet metal forming simulations

In the last few years many efforts have been carried out in order to better understand what the real contact between material and tools is. Based on the better understanding new friction models have been developed which have allowed process designers to improve numerical results in terms of component viability and geometrical accuracy. The new models define the coefficient of friction depending on different process parameters such as the contact pressure, the sliding velocity, the material strain, and the tool temperature. Many examples of the improvements achieved, both at laboratory scale and at industrial scale, can be found in the recent literature. However, in each of the examples found in the literature, different ranges of the variables affecting the coefficient of friction are covered depending on the component analysed and the material used to produce such component. The present work statistically analyses the contact pressure and sliding velocity ranges achieved during numerical simulation (FEM) of sheet metal forming processes. Nineteen different industrial components representing a high variety of shapes have been studied to cover a wide range of casuistic. The contact pressure and sliding velocity corresponding to typical areas of the tooling have been analysed though numerical simulation in each case. This study identifies the ranges of contact-pressure and sliding velocities occurring in sheet metal forming aimed to set the characterization range for future friction studies

CRYOGENIC BURNISHING OF Co-Cr-Mo BIOMEDICAL ALLOY FOR ENHANCED SURFACE INTEGRITY AND IMPROVED WEAR PERFORMANCE

The functional performance of joint implants is largely determined by the surface layer properties in contact. Wear/debris-induced osteolysis and aseptic loosening has been identified as the major cause of failure of metal-on-metal joint implants. A crucial requirement for the long-term stability of the artificial joint is to minimize the release of debris particles. Severe plastic deformation (SPD) processes have been used to modify the surface integrity properties by generating ultrafine, or even nano-sized grains and grain size gradients in the surface region of many materials. These fine grained materials often exhibit enhanced surface integrity properties and improved functional performance (wear resistance, corrosion resistance, fatigue life, etc.) compared with their conventional coarse grained counterparts. The aim of the present work is to investigate the effect of a SPD process, cryogenic burnishing, on the surface integrity modifications of a Co-Cr-Mo alloy, and the resulting wear performance of this alloy due to the burnishing-induced surface integrity properties. A systematic experimental study was conducted to investigate the influence of different burnishing parameters on distribution of grain size, phase structure and residual stresses of the processed material. The wear performance of the processed Co-Cr-Mo alloy was tested via pin-on-disk wear tests. The results from this work show that the cryogenic burnishing can significant improve the surface integrity of the Co-Cr-Mo alloy which would finally lead to advanced wear performance due to refined microstructure, high hardness, compressive residual stresses and favorable phase structure on the surface layer. A finite element model (FEM) was developed for predicting the grain size changes during burnishing of Co-Cr-Mo alloy under both dry and cryogenic conditions. A new material model was used for incorporating flow stress softening and associated grain size refinement caused by the dynamic recrystallization (DRX). The new material model was implemented in a commercial FEM software as a customized user subroutine. Good agreement between predictions and experimental observations was achieved. Encouraging trends are revealed with great potential for application in industry

CRYOGENIC MACHINING AND BURNISHING OF AZ31B MAGNESIUM ALLOY FOR ENHANCED SURFACE INTEGRITY AND FUNCTIONAL PERFORMANCE

Surface integrity of manufactured components has a critical impact on their functional performance. Magnesium alloys are lightweight materials used in the transportation industry and are also emerging as a potential material for biodegradable medical implants. However, the unsatisfactory corrosion performance of Mg alloys limits their application to a great extent. Surface integrity factors, such as grain size, crystallographic orientation and residual stress, have been proved to remarkably influence the functional performance of magnesium alloys, including corrosion resistance, wear resistance and fatigue life. In this dissertation, the influence of machining conditions, including dry and cryogenic cooling (liquid nitrogen was sprayed to the machined surface during machining), cutting edge radius, cutting speed and feed rate, on the surface integrity of AZ31B Mg alloy was investigated. Cryogenic machining led to the formation of a featureless layer on the machined surface where significant grain refinement from 12 μm to 31 nm occurred due to dynamic recrystallization (DRX), as well as increased intensity of basal plane on the surface and more compressive residual stresses. Dry and cryogenic burnishing experiments of the same material were conducted using a fixed roller setup. The thickness of the processed-influenced layer, where remarkable microstructural changes occurred, was dramatically increased from the maximum value of 20 μm during machining to 3.4 mm during burnishing. The burnishing process also produced a stronger basal texture on the surface than the machining process. Preliminary corrosion tests were conducted to evaluate the corrosion performance of selected machined and burnished AZ31B Mg samples in 5% NaCl solution and simulated body fluid (SBF). Cryogenic cooling and large edge radius tools were found to significantly improve the corrosion performance of machined samples in both solutions. The largest improvement in the material\u27s corrosion performance was achieved by burnishing. A finite element study was conducted for machining of AZ31B Mg alloy and calibrated using the experimental data. A user subroutine was developed and incorporated to predict the grain size changes induced by machining. Good agreements between the predicted and measured grain size as well as thickness of featureless layers were achieved. Numerical studies were extended to include the influence of rake angle, feed rate and cutting speed on the featureless layer formation

Project for the analysis of technology transfer Quarterly reports, 1 Jul. - 31 Dec. 1970

Summary of research activities of Project for Analysis of Technology Transfer for period 1 July - 31 Dec. 197

Friction and Wear Behavior of Thermally Sprayed Oxide Coatings

Conventional alloys and composites are widely used as coating materials in various contacting interfaces within gas turbine engines. These alloys form in-situ lubricious oxides during sliding at high temperatures, which help to reduce friction and wear. However, the formation of an oxide film on the contact zones requires time and depends on the chemical nature of the materials as well as the contact conditions and environment. In most cases, the running-in period for traditional alloys or composites is relatively long, which ultimately causes an overall increase in wear. Since the lubricious oxide is responsible for low wear and steady-state friction coefficient at high temperatures, it could be beneficial to replace the conventional alloys/composites and use such oxides instead. Based on prior work, ionic potential, and interaction parameters, which emphasis the lubricity at high temperatures, examples of such oxides include CuO, Ta2O5, CoO, NiO, Co-Ni-O. In this dissertation, CuO, Ta2O5, CoO, NiO, Co-Ni-O oxides were sprayed to produce thick coatings with dense, homogeneous microstructures using Suspension Plasma Spray (SPS) and High Velocity Oxygen Fuel (HVOF). The effects of spray parameters on the composition and microstructure of the coatings were investigated. The CuO and NiO coatings produced by SPS partially reduced to Cu2O, Cu and Ni, respectively. On the other hand, CoO, Ta2O5, and Co-Ni-O coatings remained single phase. The thermally sprayed coatings were tested on a ball vs flat tribometer with dry sliding reciprocating condition at various temperatures (i.e., 25°C, and 450°C) against an alumina counterface. CuO and CoO were found to have low coefficients of friction at high temperatures compared to other oxides (i.e., Ta2O5, NiO, Co-Ni-O). On the other hand, CoO and Co0.75Ni0.25O were found to be superior in terms of wear resistance at high temperatures. Scanning electron microscopy (SEM), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman analysis, and focused ion beam (FIB) were used to characterize the coatings and the corresponding wear tracks to determine the dominant wear mechanisms. In general, brittle fracture, cracking, and tribofilm delamination were found to be the main wear mechanisms leading to high wear of the oxides at room temperature. In contrast, the formation of a relatively ductile, smeared tribofilm, grain refinement, and amorphous layer closer to the wear track surface contributed to friction and wear reduction at high temperatures. In addition, a low interaction parameter of the oxides, regardless of the microstructure of the oxide coatings, led to the low friction. Such a correlation was not observed with the high interaction parameter and ionic potential. Furthermore, the high sintering ability or diffusion coefficient of the oxides could play a role in reducing friction and wear at high temperature

THE INFLUENCE OF CRYOGENIC MACHINING ON SURFACE INTEGRITY AND FUNCTIONAL PERFORMANCE OF TITANIUM ALLOYS FOR BIOMEDICAL AND AEROSPACE APPLICATIONS

The excellent properties of titanium alloys such as high strength, as well as good corrosion and fatigue resistance are desirable for the biomedical and aerospace industry. However, the same properties that make titanium alloys desirable in high-performance applications also make these space-age materials “difficult-to-machine” materials, as the titanium alloys exhibit high cutting temperatures because of their high strength and low thermal conductivity. Cryogenic machining is a severe plastic deformation (SPD) processes which uses liquid nitrogen as the coolant to take away the heat generated during machining in a relatively short time. Cryogenic machining can significantly reduce the cutting temperatures at the tool-workpiece interface, thereby improving the surface integrity of the manufactured components. This dissertation presents the results of experimental and numerical investigations of the effects of different cooling conditions on the machining performance and machining-induced surface integrity of Ti-6Al-7Nb and Ti-5553 alloys. Surface integrity and residual stresses induced by cryogenic machining are studied and compared with dry machining. Corrosion tests were also conducted to study the influence of machining parameters on the corrosion resistance of machined Ti-6Al-7Nb alloy. The results of the numerical and experimental studies show that compared with dry machining, cryogenic machining generates superior surface finish, along with higher surface layer hardening. The sub-surface residual stress profile is more compressive after cryogenic machining, and evidence of nanostructured grains is also observed in the influenced surface layer under both cooling conditions. Also, cryogenically-cooled machined sample showed better corrosion resistance compared with dry machined sample