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The general trend in the car body manufacturing industry is towards low-series production and reduction of press lubricants and car weight. The limited use of lubricants, in combination with the introduction of high and ultrahigh-strength sheet materials, continuously increases the demands on the forming tools. The major cause for tool failure during the forming process is transfer and accumulation of sheet material on the tool surfaces, generally referred to as galling. The adhered material creates unstable frictional conditions and scratching of the tool/sheet interface. To provide the means of forming new generations of sheet materials, development of new tool materialswith improved galling resistance is required, which may include tailored microstructures introducing specific carbides and nitrides, coatings and improved surface finish. In the present work, the galling wear mechanisms in real forming operations have been studied and emulated at a laboratory scale by developing a test equipment. The wear mechanisms, identified in the real forming process, were distinguished into a sequence of events. At the initial stage, local adhesive wear of the sheets led to transfer of sheet material to the tool surfaces. Successive forming operations resulted in growth of the transfer layer with initiation of scratching of the sheets. Finally, scratching changed into severe adhesive wear, associated withgross macroscopic damage. The wear process was successfully repeated in the laboratory test equipment in sliding between several tool materials, ranging from cast iron and conventional ingot cast tool steels, to advanced powder metallurgy tool steel, sliding against medium and high-strength steel sheets. By use of the test equipment, selected tool materials were ranked regarding galling resistance. The controlling mechanism for galling in sheet metal forming is adhesion. The initial sheet material transfer was found to occur, preferably, to the metallic matrix of the tool steels. Hence, the carbides in the particular steels appeared less prone to adhesion as compared to the metallic matrix. Therefore, an improved galling resistance was observed for a tool steel comprising a high amount of small homogeneously distributed carbides offering a low-strength interface to the transferred sheet material.Further, atomic force microscopy showed that nanoscale adhesion was influenced by temperature, with increasing adhesion as temperature increases. A similar dependence was observed at the macroscale where increasing surface temperature led to initiation of severe adhesive wear. The results were in good agreement to the nano scale observations and temperature-induced high adhesion was suggested as a possible mechanism.

HUDOUSEK Jiří: Drawing of a tin-coated metal sheet. The project elaborated in frame of master studies is submitting design of the given part production - business card case from tin-coated metal sheet by deep drawing technology. From a variations of the design of the product has been chosen option, when the case is assembled from two identical drawn parts which form the bottom and the lid of the component, the connection of these two parts is arranged by a spacer in form of a plastic insert on which the metal parts are attached. Based on the literary survey and calculations drawing in die with blankholder was designed for metal parts production, according to the force needed and tool size a hydraulic press ZH 100 was selected. Part of the work is the process parameters determination, technical and economic evaluation and drawing documentation.

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<p>Metal forming processes have been widely applied in many industries. With the severe competition in the market, a reliable and robust metal forming process becomes crucial for the manufacturer to reduce product development time and cost. For the purpose of supplying engineers with an effective tool for a reliable and robust design of metal forming process, this research investigates the application of traditional reliability theory and robust design methods in metal forming processes for the ultimate goal of increasing quality and reducing cost in manufacturing.</p> <p> A method to assess the probability of failure of the process based on traditional reliability theory and the forming limit diagram (FLD) is presented. The forming limit of a material is chosen as the failure criteria for analysis of reliability.</p> <p> A study of prediction of forming limit diagrams using finite element simulation without pre-defined geometrical imperfection or material imperfection is presented. A 3D model of the dome test is used to predict the FLD for AA 5182-0. The FE predicted forming limit diagram is in good agreement with the experimental one. The uncertainty sources for the scatter of forming limits are categorized and investigated to see their effects on the shape of FLD.</p> <p>A novel method of improving the reliability of a forming process using the Taguchi method at the design stage is presented. The thickness-thinning ratio is chosen as the failure criteria for the reliability analysis of the process. A Taguchi orthogonal array is constructed to evaluate the effects of design parameters on the thinning ratio. A series of finite element simulations is conducted according to the established orthogonal array. Based on the simulation results, Taguchi S/N analysis and ANOVA analysis are applied to identify the optimal combination of design parameters for minimum thinning ratio, minimum variance of thinning ratio, and maximum expected process reliability.</p> <p> A multi-objective optimization approach is presented, which simultaneously maximizes the bulge ratio and minimizes the thinning ratio for a tube hydroforming process. Taguchi method and finite element simulations are used to eliminate the parameters insignificant to the process quality performance. The significant parameters are then optimized to achieve the multiple optimization objectives. The optimization problem is solved by using a goal attainment method. An illustrative case study shows the practicability of this approach and ease of use by product designers and process engineers.</p> / Thesis / Doctor of Philosophy (PhD)

<p>In sheet metal forming processes, friction has decisive effects on the strain distribution in the deformed sheets and the quality of the final product due to the large surface/thickness ratio of the blank sheets. It is well known that friction in sheet forming operations is dependent on local contact conditions such as surface roughness, contact pressure and sliding velocity. Adding complexity to this frictional behavior, some rolled sheets have oriented surface roughness and show considerable frictional anisotropy. A constant friction model without consideration of these relevant phenomena is regarded as the reason why sheet metal forming simulations often fail to produce satisfying results despite the well developed material models. </p> <p>To develop a friction model which considers both of the varying conditions of local contact and the frictional anisotropy was the aim of this thesis. For this purpose, the analysis method of the friction test (draw-bend test) had to be examined for the capability to evaluate these parameters independently. Through careful study using finite element simulations, it was found that the conventional method has shortcomings in addressing pressure dependent friction due to the pressure non-uniformity existing in the test. Therefore, a new analysis method, which can evaluate pressure dependency of a friction coefficient, was developed. In the new method, contact pressure maps obtained from simulations were included in the analysis of test data.</p> <p>The new analysis method was applied to friction measurement of aluminum sheets with known anisotropic mill finish, and friction coefficients were obtained as functions of contact pressure, sliding velocity and sliding direction. In the obtained friction model, a friction coefficient is a continuous surface over the domain of contact pressure and sliding velocity. Lastly, the new friction model was implemented into a finite element code and the model was validated through circular cup drawing experiments and simulations. The comparisons showed good agreements in the aspects of punch force, cup size and failure location. Thus, the newly developed model can accurately predict the effects of anisotropic friction in sheet metal forming processes. </p> / Thesis / Doctor of Philosophy (PhD)

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