Key Engineering Materials Vol. 716

Abstract: The essential purpose of the work was to determine the phenomena that occur in multipass wire drawing process of high carbon steel wires with high speed in hydrodynamic dies and to assess their influence on moulding the wire properties after the drawing process. The multiparameter analysis of the issues has involved the theoretical dissection of the phenomena arising in high speed wire drawing process in hydrodynamic dies with the usage of the finite element method supported by the experimental multipass drawing process in industrial conditions. On the basis of numerical analysis the influence of drawing speed on wire temperature was estimated. For final wires the investigation of mechanical properties, topogrhaphy of wire surface, the amount of lubricant on the wire surface, the pressure of lubricant in hydrodynamic dies were determined.

Abstract: Commercially pure titanium (CP Ti) has been actively used in plate heat exchangers due to its light weight, high specific strength, and excellent corrosion resistance. However compared with automotive steels and aluminum alloys, only limited research has been conducted on the plastic deformation characteristics and press formability of CP Ti sheets. In this study, the mechanical properties, including the anisotropic property and the stress-strain relation, of the CP Ti sheet are clarified in relation to press formability. A new proposed strain hardening model, Kim-Tuan equation, is successful in perfectly describing the stress evaluation for strain increment of this material during strain path. The forming limit curve (FLC) of the CP Ti sheet as a criterion for press formability was experimentally evaluated by punch stretching testing and analytically predicted via Hora’s modified maximum force criterion. The predicted FLC based on the Kim-Tuan strain hardening equation and the appropriate yield function correlates well with the experimental results of the punch stretching test.

Abstract: The incremental forming of titanium alloy sheets combines the advantages of this advanced flexible manufacturing process, that allows to produce complex components without using dedicated tools, with the interesting properties of the material under consideration. In this study, thin sheets of grade 1 titanium were incrementally formed to evaluate their formability and surface quality by varying the tool-sheet contact conditions. Experimental tests and surface analyses highlight dependence on the contact conditions of the surface quality rather than of the formability. Moreover, they emphasize that the tool-sheet contact conditions mainly affect the repeatability of the process due to the occurrence of galling.

Abstract: Forming limit diagrams (FLDs) of AA6082 at warm/hot stamping conditions were determined by using a specially designed test rig. The tests were carried out at various temperatures from 300 to 450°C and forming speeds ranging from 75 to 400 mm/s. The strain was visualized and measured using ARGUS software provided by GOM. The results clearly show that the formability of AA6082-T6 sheet metal, in terms of the limit major strain, increased by 38.9 % when the forming temperature was increased from 300°C to 450°C at a speed of 250 mm/s, and increased by 42.4 % when the forming speed was decreased from 400 to 75 mm/s at a temperature of 400°C. It was verified that hot stamping is a promising technology for manufacturing complex-shaped components.

Abstract: The paper presents the results of physical modelling of the plastic deformation of the Mg/Al bimetallic specimens using the Gleeble 3800 simulator. The plastic deformation of Mg/Al bimetal specimens characterized by the diameter to thickness ratio equal to 1 was tested in compression tests. The aim of this work was determination of the range of parameters as temperature and strain rate that mainly influence on the plastic deformation of Mg/Al bars during metal forming processes. The tests were carried out for temperature range from 300 to 400°C for different strain rate values. The stock was round 22.5 mm-diameter with an Al layer share of 28% Mg/Al bars that had been produced using the explosive welding method. Based on the analysis of the obtained testing results it has been found that one of the main process parameters influencing the plastic deformation the bimetal components is the initial stock temperature and strain rate values.

Abstract: The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Abstract: The article presents a comparison of the hole-flange factors for four steel sheets which have different strength parameters. Three types of punches were applied in the research: cylindrical, spherical and conical. The holes in the samples were made with the use of the following cutting technologies: laser cutting, electrodischarge method using wire and punching. The coefficient of the hole expansion shows that the shape of the used tools has a great impact on the final diameter of the hole. The greatest enlargement of the diameter was obtained for the conical punch, a smaller enlargement for the spherical one and the smallest enlargement for the cylindrical punch. The results of the hole expansion tests using the conical punch diverge from the general rule saying that the greater the plastic deformation of the sheet metal during tensile tests, the more you can enlarge the hole before cracking appears.

Abstract: Ti₂AlNb based alloy has been paid more and more attention in recent years because of their high application potential in jet engines for good mechanical properties at high temperature. However, the control of microstructure and mechanical properties for components in sheet metal forming is very difficult because the complex phase transformation. In this paper, a cylindrical part was produced by hot gas forming and the formability of a Ti-22Al-24.5Nb-0.5Mo sheet with thickness of 2mm was studied at 985°C. It is found that the parts could be formed with small bottom-corner radius of 4mm with outer diameter of 60mm and the depth of 20mm. The strain distribution and thinning ratio of the parts were analyzed. The maximum thinning ratio was 56.3% near to the small corner. The microstructures of the original blank and the cylindrical parts were observed by optical microscopic (OM). It is found that the orthorhombic (O) phase and α₂ phase significantly reduced during the forming process. On the other hand, at different position of the parts, different microstructures appear because of different strain values.

Abstract: High performances of tool materials and lubricants have been necessarily developed to extend die life for hot forming of steel. To promote the developments, economical and simple evaluation testing methods have been required. Tribological performances of tool materials, surface treatments, and lubricants are examined by this test. In this paper, a tapered-plug penetration test has been devised for hot forming of steel. It was found that the combination of an appropriate non-graphite lubricant and a nitride die was effective in reducing in galling and friction between a die and a work by the tapered-plug penetration test.

Abstract: Friction has a significant influence on almost all metal forming processes. An in situ measurement of the friction stress within the forming process is in general difficult. Therefore, different experimental setups based on the indirect measurement of a friction dependent value are used to determine the friction conditions in laboratory experiments. For example the ring compression test and the conical tube-upsetting test are using the change of the geometrical shape of a specimen to investigate an averaged friction coefficient within the process. The essential advantages of conical tubes are the prevention of sticking friction and a homogeneous displacement and relative velocity along the contact surface depending on the friction conditions and the used cone angle. However, in both methods the development of the friction conditions during the upsetting process and the relative velocity between tool and workpiece are unknown. In this paper an extended setup of the conical tube-upsetting test is presented. The development of the specimen profile is detected by a laser sensor during the process at elevated temperatures. Experiments are conducted for different cone angles and the measured data is compared to FE-simulations. The time-dependent geometric data is used for the calculation of the relative displacement and relative velocity between tool and workpiece at the edge of the contact zone. A comparison with classical nomograms indicates a change of the friction conditions during the upsetting process. Finally, simulations are fitted to the experimental results by using a variable friction coefficient.