Kuniaki Dohda

Microforming: Study of Friction Conditions and the Impact of Low Friction/High-Strength Die Coatings on the Extrusion of Micropins

Microforming is a relatively new realm of manufacturing technology that addresses the issues involved in the fabrication of metallic microparts, i.e., metallic parts that have at least two characteristic dimensions in the sub-millimeter range. The recent trend towards miniaturization of products and technology has produced a strong demand for such metallic microparts with extremely small geometric features and high tolerances. Conventional forming technologies, such as extrusion, have encountered new challenges at the micro-scale due to the influence of ‘size effects’ that tend to be predominant at this length scale. One of the factors that shows a strong influence is friction. This paper focuses on the frictional behavior observed at various sample sizes during micro-extrusion. A novel experimental setup consisting of forming assembly and a loading stage has been developed to obtain the force-displacement response for the extrusion of pins made of brass (Cu/Zn: 70/30). This experimental setup is used to extrude pins with a circular cross-section that have a final extruded diameter ranging from 1.33 mm down to 570 microns. The experimental results are then compared to finite-element simulations and analytical models to quantify the frictional behavior. It was found that the friction condition was non-uniform and showed a dependence on the dimensions (or size) of the micropin. The paper also investigates the validity of using high-strength/ low friction die coatings to improve the tribological characteristics observed in micro-extrusion. Three different extrusion dies coated with diamond-like carbon with silicon (DLC-Si), chromium nitride (CrN) and titanium nitride (TiN) were used in the micro-extrusion experiments. All the coatings worked satisfactorily in reducing the friction and correspondingly, the extrusion force with the DLC-Si coating producing the best results.Copyright

FEM simulation of surface smoothing in the ironing process

Abstract The purpose of this paper is to simulate surface smoothing in the ironing process by elasto-plastic FEM. The simulated results show that surface smoothing is largely developed during a very short contact length from the entrance of the contact zone where the material is in the elastic state and can be remarkably promoted by the plastic deformation in the surface layer. With increase in the reduction in thickness, in the die angle and in the friction coefficient on the punch side, the surface smoothing of the workpiece can be accompanied at the shorter contact position from the entrance of the contact zone. The simulated results are in good agreement with experimental results.

Effects of Average Lubricant Velocity and Sliding Velocity on Friction Behavior in Mild Steel Sheet Forming

A series of experiments was carried out using a rolling-type tribometer to investigate the frictional dependence on the average velocity of the lubricant V at the contact zone inlet and the relative sliding velocity ΔV between the roll and workpiece during deformation. Experiments using mild steel strips with a dull surface of 0.63 μm Ra showed that the friction coefficient decreases with increasing forming velocity, especially at high forming velocity. This variation in the friction coefficient may be due to the increase in ΔV derived from the increase of the forming velocity, because the friction coefficient decreases with increasing ΔV but varies little with increasing V. It was also confirmed that an increase of ΔV or a decrease in V promotes flattening of the workpiece asperities.

Investigation Into Relationship Between Friction Behavior and Plastic Deformation Using a Newly Devised Rolling-Type Tribometer

A new rolling-type tribometer has been devised to investigate the effects on friction behavior of the average velocity of the lubricant at the inlet to the contact zone and the relative sliding velocity during deformation independently. This paper shows the principle of operation of the tribometer and deformation mode of the workpiece, especially considering the relationship between friction and bulk deformation. Experiments using mild steel strips with a dull surface showed that the friction stress on the friction roll, which has a smaller velocity than that of the traction roll, is mainly controlled by lubricating conditions on the friction roll side. The contact pressure is mainly decided by the friction conditions on the traction roll side. It was also revealed that, with an increase of the reduction in the workpiece thickness, the friction stress on the friction roll side remains constant when the friction roll is rotated. The friction stress increases when the friction roll is stopped after a certain reduction in the workpiece thickness.

Research into mirror surface finishing by the ironing process

Abstract The object of this paper is to clarify the mechanism of mirror surface finishing by the ironing process. The mirror surface is in demand for electro-optical parts such as optical drums and magnetic discs, which hitherto have been finished by diamond bite cutting, lapping, etc. In the ironing process, the metal surface on the die side becomes a rubbed surface ( Ra = 0.05 μ m for high-viscosity oil) which is markedly inferior to the die surface ( Ra = 0.006−0.011 μ m ); when oils of lower viscosity are used galling often occurs. The metal surface on the punch side becomes a near replica of the punch surface, the degree of replication increasing with larger reductions and lower viscosity oils or when the ironing is carried out dry. In these experiments, the metal surface was finished to Ra = 0.004 μ m under the following conditions: dry on the punch side; high-viscosity oil on the die side; and 50% reduction.

FEM analysis of contact mechanism in press-forming of lubricant pre-coated steel sheet

Abstract This paper describes the contact situation between the die and the lubricant pre-coated steel sheet in the press-forming by using FEM simulation. The FEM simulation is carried out by supposing that the lubricant pre-coated steel sheet consists of the lubrication layer and thick galvanized layer. Both the lubrication layer and galvanized layer are assumed as rigid–plastic material. The variations in the contact situation between the die and the lubricant pre-coated steel sheet are investigated by changing the friction coefficient between the die and lubrication layer, the thickness and hardness of lubrication layer, the velocity ratio of the relative sliding velocity to pressing velocity, etc. The simulated results show that the contact area ratio is influenced largely by the average contact pressure, the velocity ratio, the thickness of lubrication layer, and friction coefficient.

Lubricity of volatile lubricants in sheet metal rolling

Abstract A series of experiments is carried out by using a rolling type tribometer to investigate the lubricity of the volatile lubricants at high speed forming. The roll material is the die steel alloy SKD11, and the workpiece material is the mild steel SPCE with a rough surface and the aluminum alloy A3004 with a smooth surface. Experimental results show that the friction coefficient decreases with increasing working velocity for both SPCE and A3004, in any lubricant. With an increase of reduction in thickness, the friction coefficient decreases for SPCE, but increases for A3004. Some volatile lubricants have the same lubricity as the generally used mineral oil with low-viscosity by judging from the value of friction coefficient, the surface appearance of rolled workpiece and the roll surface damage.

Effects of Friction Models, Geometry and Position of Tool on Curving Tendency of Micro-Extrusion 6063 Aluminum Alloy Pins

Micro-extrusion process is one of the micro-forming technology for fabrication of micro-parts such as micro-gear shaft for microelectromechanical system (MEMS) and micro pins for electronic parts. This paper presents the friction models effects and geometry effects on curving tendency of micro-extrusion 6063 aluminum alloy pins. Three friction models were considered: (1) Coulomb friction, (2) plastic shear friction, and (3) combined (Coulomb & plastic shear) friction. The finite element simulation was carried out and the results showed that the combined friction model accurately predicted the micro-extrusion results. Then, four tool geometry and position effects were investigated: (1) punch shift length, (2) die angle, (3) die shift length, and (4) bearing length. The finite element simulation was carried out to determine these tool geometry and position effects on the curving tendency of micro-extruded pins. The results showed that punch shift length and die angle did not affect the curving tendency. However, die shift length caused the micro-extruded pins to curve. The increase in bearing length helped straighten the micro-extruded pins.

Tribological effects of chlorine-free lubricant in strip drawing of advanced high strength steel

This article investigated the tribological performance of the specially formulated chlorine-free lubricant in strip drawing of advanced high strength steel. Four different lubrication conditions (dry, chlorine-free lubricant, chlorine additive lubricant, and mineral base lubricant) at two sliding speeds (10 and 100 mm/min) were carried out to observe the friction coefficients of the die-workpiece interface in the strip drawing test. The main difference among these lubricants was the contents of chlorine and sulfur additives. The die and workpiece materials were SKD11 and JSH780R, respectively. The results showed that the combination of chlorine and sulfur additives provided the best tribological behaviors. In addition, only the small amount of sulfur content could establish a bond with metal surfaces. However, the higher sulfur content could interact with metal surfaces, because it was influenced by the increased temperature (higher sliding speed) and adsorption.

Influences of contact pressure, sliding velocity, lubricant, bending angle and surface texture on friction behaviours in stainless steel strip drawing

Abstract This article investigates the tribological effects of stainless steel strip drawing by considering the influences of contact pressure, sliding velocity, lubricant, bending angle and surface texture. The die and sheet materials used to carry out the strip drawing tests were SKD11 and AISI 304, respectively. Different values of the contact pressure, sliding velocity and bending angle were tested. Various types of lubricants and die surface textures were also investigated. The results show that there was an optimum value of friction coefficient for each condition. The minimum friction coefficient was found at the condition having 9.375 MPa contact pressure, 100 mm/min sliding velocity, chlorinated oil, 45º bending angle, and smooth coated die surface.