Defect and Diffusion Forum Vol. 414

Abstract: To improve the sensitivity of the steady combined forward and backward extrusion test proposed in previous work, an optimization job based on the finite element simulations was carried out. A raw material of 0.45% carbon steel was tested under different stain rates from 0.001s^-1 to 1s^-1 and different temperatures from 30°C to 400°C, and the material flow stresses were modelled by Hensel-Spittel equation. The deformation degree of the forward extrusion was set as 50%. The key parameters including the deformation degree of the backward extrusion, the ratio between the radius of the punch nose and the radius of the punch, the taper angle of the punch, the die angle, the sizing lands of the punch and the die were optimized. The sensitivity of the optimal design is improved about 20% compared with previous design when the friction factor is assumed as 0.03~0.15. The new group of calibration curves presents more scatter than the old group. The sensitivity improvement is also validated by the experimental works.

Abstract: This paper describes the tribological investigation of the cold forming and ejection process during ironing of hollow components with helical internal geometry. The experimental tests on a test tool are intended to evaluate the tribological system by varying the surface pretreatment of the blanks and the lubricant. For the evaluation of the tribological effects the ironing and ejection force are measured. In addition, the die filling as well as the surface quality of the pressed parts are determined, allowing the correlation of the test results with the tribological conditions. Furthermore, a numerical investigation of the process was performed. The numerical results are presented and compared with the experimental results.

Abstract: Forward rod-backward can combined extrusion test was used to investigate the tribological properties of Ti-6Al-4V titanium alloy with oxide film at cold and warm forging conditions. 0.08 and 0.10 µm-thick oxide films were formed on the test piece by atmospheric oxidation treatment. A friction test was performed without using lubricants. As a result, the oxide film was effective to prevent seizure at cold forging condition, where the friction coefficient was estimated as about 0.15. However, at warm forging condition, a decohesion of the oxide film and seizure occurred, where the estimated friction coefficient was larger than that at cold forging condition. Furthermore, thickening the oxide film was effective against the decohesion and to reduce the friction coefficient.

Abstract: Rolling operations with flat dies allow the highly efficient production of cylindrical parts in large quantities. [1] Latest trends towards shorter lifecycle times and increasing product complexity [2] strengthen the need for an efficient design process of flat die rolling operations. Finite element modelling plays a major role in establishing an efficient design process. Precise modelling of the friction conditions during rolling operations with flat dies is still posing a challenge today. [3] The present work aims to experimentally identifying the limits of rolling of an axisymmetric part, using flat dies. By gradually increasing the stroke rate, the process limits caused by excessive slippage are determined for six different tribological systems. The workpiece rotation and tool movement is measured optically by using a digital camera system. Large differences in slippage are observed, depending on the tribological system. While systems with polymer based lubricants show excessive slippage at low stroke rates, systems using oil or no lubricant bear higher stroke rates before process limits are reached. Furthermore the optical measurements reveal that slip may also occur without complete process failure leading to the assumption that the process limit of excessive slippage in rolling operations is rather fluent than binary. Based on this experimental data, future investigations will focus on methods to predict these process limits through FEM using advanced friction modelling and tribometer tests.

Abstract: To fabricate sandwich-structure composite of porous metal with nonporous surface layer, the nonporous skin layer was formed on surface of open-cell type nickel foam from aluminum powder by friction stir powder incremental forming (FSPIF) process. In this process, the surface pores of the foam were filled with the powder, then the powder and the cellular matrix near the surface of the foam were incrementally hammered by a rod-shaped tool without rotation. After that, the hammered surface of the foam was incrementally stirred by the tool at a very high rotation rate. The formed skin layer was composed of two layers; the friction stirred layer (relative density: above 0.90) in the upper part and the compact layer (relative density: 0.60–0.90) in the lower part. The friction stirred layer with a maximum thickness of 0.8 mm was formed on the surface of the foam without deforming the cellular matrix of the inside of the foam under the forming conditions; a tool rotation rate of 8000 rpm, a tool feed rate of 60 mm/min, a tool pushing pitch of 0.1 mm, and a total forming depth of 5.0 mm.

Abstract: In this study, the impact of tool pin profile on the dissimilar joining characteristics of aluminum alloy AA2017 and polycarbonate in friction stir welding (FSW) was examined. The tool pins made of SKD11 tool steel were used in the experiments of the FSW to join AA2017 and polycarbonate plates in a butt configuration at a tool rotation speed of 1320 and 1760 rpm and tool transverse speed of 60 mm/min. The tool shoulder diameter and tool pin height were set to 10 mm and 2 mm. The shapes of tool pin were cylindrical with and without groove, tapered cylindrical with and without groove, bobbin shaped with and without groove, square, triangular, pentagonal, hexagonal, oval, and ellipsoidal. The measurement of axial force was performed using a dynamometer. The changes in workpiece temperature at six locations during the process were measured using K-type thermocouples embedded in the plates. Microstructures of welded joints were examined by an optical microscope. Vickers Microhardness mapping was performed in and around the weld nugget zone, which is formed by the material flow between the two different base materials. From the result, the grooved tool pins generated less heat; while cylindrical, square, and oval type pins are also result in the superior properties of the welds during joining processes. Each tool pin had unique material flow behavior and weld formation, and the weld defects such as tunnel/voids occurred at the joint interface. The oval tool pin gives rise to the highest joint efficiency at a higher tool rotation speed, and the ellipsoidal tool pin generates the highest joint efficiency at a lower tool rotation speed was noticed.