"Problem 1 Extrusion The direct extrusion process is used"
Problem 1: Extrusion The direct extrusion process is used for the production of a round bar with a diameter of d1=12 mm. The diameter of the container is d0=80 mm. The velocity of the press plate is v0=15 mm/s = const. a) Calculate the velocity of the extruded bar in cross-section C within stationary state. b) Draw qualitatively the distribution of the axial velocities at the cross-sections A,B,C into the provided diagrams. Mark the known velocities in your plots! Problem 2: thixoforming a) The process technology of thixoforming combines characteristics of the conventional process technologies as forging and casting. Please describe the advantages of thixoforming relating to following aspects in keywords: • shape of components • process temperature • required forces. b) The word “thixotropic” is a description of a typical material behaviour and has his origin in the Greek language (“thixis” = touch and “tropos” = turning/ transformation). Please exemplify the material behaviour in the face of the viscosity when a thixotropic material is sheared e.g. in a forming process. c) Please explain in a few words how the characteristic of the solidus-liquidus interval influences the operation of thixoforming regarding the liquid phase fraction and the process window. Problem 3: wire drawing a) The formula of the drawing force according to Siebel is given in the next row: µ 2α F = A1 ⋅ k fmϕ l 1 + + α 3ϕ l Explain shortly the main terms and the variables of the equation. b) What would happen in the drawing process regarding the drawing force, if the fraction of shear and friction have the same values and how could this be achieved? c) A wire shall be drawn from diameter d0 to diameter d1. Which of the following cases would require more work : i. Drawing a wire in one pass ii. Drawing a wire in two pases Consider that for both cases the initial and end diameter of the wire are the same and that all dies have the same drawing hole angle. Give a short explanation of your answer. Problem 4: Ring Rolling Name following form defects, give reasons for their appearance and specify two methods for their correction. Name Reason Methods Problem 5: rolling a) What is meant by the term „neutral point“ in flat rolling? Specify your answer regarding the rolling speed and the local speed of the material within the compressed length esp. at the entry line and the exit line. b) What are the advantages of rolls with a small diameter during rolling and what disadvantages result from that? c) Although the use of hot rolled strip is more cost-efficient, cold rolled strip is nevertheless used very frequently in industry. Please explain this fact. To do that name at least two advantages of hot rolling and two advantages of cold rolling. Problem 6: deep drawing a) The following figures are showing four of the most typical failures in deep drawing. Please name the failure as well as one of its possible reason. Figure 1: Figure 2: Figure 3: Figure 4: b) The deep drawing feasibility of a material is often described by the limit drawing ratio βmax. Please write down the equation of the limit drawing ratio of axially- symmetrical as well as non-axially-symmetrical deep drawn parts and list all variables by their correct names. Problem 7: Forging a) Please explain the reasons for surface defects called overlapping. You can also use a drawing for explanation. b) Which characteristic conditions are more critical to create these defects? Please give the correct definition of the bite ratio and state shortly if overlapping appears at high or low bite ratios and high or low thickness reductions. c) Please write down three reasons why a product is formed using the open die forging process.