Magnesium alloys have poor ductility and it is difficult to manufacture magnesium alloy parts at room temperature. The authors have developed friction stir incremental forming method to form magnesium alloy sheets without heating and reported on the formability of AZ31-O sheets with a 0.5 mm thickness at room temperature. In this study, friction stir incremental forming was applied to the forming of AZ61 and AZ80 magnesium alloy sheets, which are more difficult to deform plastically at room temperature than AZ31, and the formabilities of AZ61 and AZ80 magnesium alloy sheets were investigated. Vickers hardness test and tension test of the formed parts were also carried out. From the experimental results, the AZ61 and AZ80 sheets were successively formed into a pyramidal frustum shape with minimum half apex angles of 30° and 40°, respectively. These results indicate 100% and 56% elongations although the fracture elongations of those sheets in tension test at room temperature were only 17% and 24%, respectively. The Vickers hardnesses and tensile strengths of AZ31, AZ61 and AZ80 formed parts were increased or decreased less than 20% compared with the unformed ones.
5000 series aluminum alloys do not have sufficient ductility and their applications are limited. To enhance the use of these alloys, it is necessary to develop new forming processes with high formability. The authors have developed a friction stir incremental forming method of forming magnesium alloy sheets without heating and reported on the formability of magnesium alloy sheets at room temperature. In this study, friction stir incremental forming was applied to forming A5052 aluminum alloy sheets, and the formability of these sheets was investigated. Tool rotation rate, tool feed rate and the half apex angle of the pyramidal frustum were changed. Hardness test and tension test on the formed parts were also carried out. A5052 sheets with a 0.5 mm thickness were successfully formed into a pyramidal frustum with a minimum half apex angle of 20°. This result coincides with an elongation of 192% although the fracture elongation of those sheets in the tension test at room temperature was only 14%. The Vickers hardness and tensile strength of the A5052 parts formed by the developed method were decreased compared with those of the unformed ones. Even with added strain by forming, however, the elongation of formed parts was greater than that of the unformed ones owing to the occurrence of grain refinement by dynamic recrystallization during forming.
The deformation behaviors of a 780MPa-class high-strength thin-walled tube (t/D = 3.7%) in a newly developed tube bending method (PRB) were examined, and compared with those in rotary draw bending (RDB). The results are as follows: (1) PRB enables 2.0D radius bending without the occurrence of defects such as wrinkling and fracturing even without the use of a mandrel and a wiper. (2) The absolute value of the thickness strain for PRB is smaller and the longitudinal strain is larger than that for RDB. (3) Shear strain with a circumferential distribution for PRB is different from that of RDB. The absolute value becomes maximum around the position of 90 degrees in PRB. On the other hand, it becomes zero at the same position in RDB. (4) Finite element method (FEM) simulation results show good agreement with experimental results in terms of strain and occurrence of defects. The FEM simulation is feasible for the prediction of formability and the examination of the mechanism in PRB.
Finite element method (FEM) simulation was carried out to clarify the deformation mechanism of thin-walled tubes in the new bending method PRB (Bending Method with Slight Reduction in Diameter by Push with Pressure Die and Rotation of Bending Die) in comparison with that in conventional rotary draw bending. Through this investigation, the following results are obtained. (1) By checking the focused points both inside and outside the bend, it was found that the plastic deformation was almost completed before the bent area in the case of PRB. (2) On the effect of the tube reduction with the pressure die on the deformation of tubes, circumferential compressive deformation involving longitudinal tensile deformation is enhanced outside the bend. The protrusion of the bending die toward a tube facilitates longitudinal compressive deformation inside the bend. (3) In the case of PRB, the tube material tends to be compressed along the longitudinal direction in a large area as a result of the protrusion of the bending die toward a tube inside the bend. This eases the circumferential compressive stress and restrains wrinkling inside the bend, even though the mandrel and wiper are not used in PRB.
This study deals with a surface modification phenomenon in a cylindrical surface finishing process using diamond burnishing tools. The burnishing process is a micro-plastic machining method, in which a spherical diamond point tool compresses the metal surface to obtain a mirrorlike surface roughness; it could improve surface layer hardness similarly to the shot-peening process. In this report, we try to evaluate the surface modification in the burnishing process using the nanoindentation technique and an X-ray residual stress measuring instrument owing to the extremely thin surface layer. From the experimental result, the hardness of the surface layer is improved (by at least 100 HV) and a significant residual stress (-700MPa) remains on the surface layer of the burnished workpiece.
A method of evaluating the workability of a tubing in cold pilgering was studied, including material deformability and process conditions. The circumferential compression test was confirmed as an effective evaluation method for deformability. The critical reduction in height at the initiation of the compression test proved to be a good measure of material deformability. A common mechanism of inner fissure in stainless steel, titanium alloy, and zirconium alloy during cold pilgering was proposed on the basis of the results of the compression test, cold pilgering test, and numerical analysis. The ratio of radial strain to circumferential strain during pilgering proved to be a good measure of cold-pilgering conditions for evaluating the workability of a tubing.
To enhance automobile weight reduction and to improve collision safety, the use of high-strength steel for press-formed parts has been expanding. However, dimensional accuracy defects occur because the use of a press-formed sheet metal induces springback behavior. In this paper, we propose a new forming method as a solution to this problem, here referred to as tension control (TC) forming. TC forming uses a technique that entails adding tension to a material after bending, it is effective for reducing springback. To implement this technique, the early stage of forming has some clearance between the die and the blankholder, then, the clearance is closed during the midst of forming, and then tension is added to the material during the latter half of the forming process. Experiments were carried out to investigate the effects of what on practical parts of high-strength steel. As a result, the proposed method is found effective for reducing springback in four types of high-strength steel. Furthermore, the effects of clearance and strokes with tension are clarified by elasto-plastic FEM, taking into account the Bauschinger effect.