• Transactions of Materials Processing
• /
• v.22 no.2
• /
• pp.86-92
• /
• 2013

When stamping ultra-high-strength steel (UHSS), the phenomenon of galling, which corresponds to a transfer of material from the sheet to the tool surface, occurs because of the high contact pressure between tool and workpiece. Galling leads to increased friction, unstable interface conditions, scratches on the sheet and the tool surfaces and, eventually, premature tool surface failures. Therefore, a simple and accurate evaluation method for tool scratching is necessary for the selection of tool material and coating, as well as for a better optimization of process conditions such as blank holder force and die radius. In this study, the pin-on-disc (PODT) and pin-on-flat surface (POFST) tests are conducted to quantitatively evaluate scratch-related tool life for stamping of UHSS. The variation of the friction coefficient is used as an indicator of scratch resulted from galling. The U-channel ironing test (UCIT) is performed in order to validate the results of the friction tests. This study shows that the POFST test provides a good quantitative estimation of tool life based on the occurrence of scratch.

• Transactions of Materials Processing
• /
• v.14 no.8 s.80
• /
• pp.696-703
• /
• 2005

In the modern days, a galvannealed sheet steel (GA) instead of a cold rolled steel sheet has been widely used as an alternative to extend the life of automotive body. Accordingly, the mechanical properties of GA for automobiles were taken into account and studied by examining their variation with annealing temperature. To clarify the effect of surface features on the mechanical and frictional properties of GA, the several tests such as nanoindentation, Vickers hardness and nano scratch test were executed. The frictional characteristics of coating layers of GA were examined through nano scratch test in this study. The friction coefficient of coating layers on the surface was obtained from the nano scratch. The variation of friction coefficient versus velocity and pressure was taken into consideration in this paper. Hardness and elastic modulus of coating layer were increased as increasing annealing temperature.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2005.10a
• /
• pp.113-117
• /
• 2005

In the modern days, a galvannealed sheet steel (GA) instead of a cold rolled steel sheet has been widely used as an alternative to extend the life of automotive body. Accordingly, the mechanical properties of GA for automobiles were taken into account and studied by comparing with the temperature variation on annealing in this study. To clarify the effect of surface features in the mechanical and frictional properties of GA, the several tests such as nanoindentation, victors hardness and nano scratch test were executed. The developed neural networks apply also to obtain reliable mechanical properties of the thin films. Load-displacement curve was computed by the analysis procedure and compared with experimental results. The frictional characteristics of coating layers in GA were verified though nano scratch test in this study. The friction coefficient of coating layers on the surface was obtained from the nano scratch. The variation of friction coefficient versus velocity and pressure was taken into consideration in this paper.

• Transactions of Materials Processing
• /
• v.24 no.6
• /
• pp.405-410
• /
• 2015

Sheet-forming of Mg alloys is conducted at elevated temperatures (250℃) due to the low formability at room temperature. The high-temperature process often gives rise to surface damage on the alloy (i.e. galling.) In the current study, the frictional characteristics of DLC coating slid against an AZ31 Mg alloy at various temperatures were investigated. The coating has been used widely for low-friction processes. Dry-sliding friction and galling characteristics of an AZ31 Mg alloy (disk), which slid against uncoated and a DLC-coated STD-61 steel (pin), were investigated using a reciprocating-sliding tribometer at room temperature and 250℃. To represent the real sliding phenomena during a sheet metal forming process, single-stroke tests were used (10mm stroke length) rather than a reciprocating long sliding-distance test. The DLC coating suppressed adhesion between the alloy and the tool steel at room temperature, and exhibited a low friction coefficient. However, during sliding at 250℃, severe adhesion occurred between the two surfaces, which resulted in a high friction coefficient and galling.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2004.08a
• /
• pp.137-144
• /
• 2004

The drawbead is one of the most important factors in sheet metal forming for automotive parts. So clarifying the friction characteristics between sheets and drawbead is essential to improve the formability of sheet metal. Therefore in this study, drawbead friction test was performed at circular shape bead and rectangular shape bead. The results show that the tendency of drawing force for rectangular bead is nearly similar with circular bead and the drawing force is nearly proportional to friction coefficient.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2009.10a
• /
• pp.250-253
• /
• 2009

In this study, the friction test was performed to find friction characteristics of advanced high strength steel (AHSS) sheets and the multiple regression method was employed to obtain friction models. The friction coefficients associated with the lubricant viscosity, drawing speed, and blank holding pressure are measured. Differently from GA steel sheets, the effects of the lubricant viscosity and pulling speed are a little, which are explained by a theory of adhesion and wear as well as a deformation of friction surface. In addition, the effects of friction parameters are numerically represented by friction regression models.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2006.05a
• /
• pp.251-252
• /
• 2006

Tensile and anisotropy test were performed to evaluate the mechanical properties of coated and uncoated steel. These results were used to predict the deference of formability between two sheets. Cup-drawing test was performed to verify formability of two sheets. Also, Cup-drawing test could predict the coefficient of friction and the forming limit. Finite Element Method of cup-drawing was performed to assess the deference between two sheets considering frictional characteristics. This result was compared with the former results.

• Transactions of Materials Processing
• /
• v.19 no.7
• /
• pp.435-440
• /
• 2010

The creep life of 9Cr1MoVNb steel, in terms of Larson-Miller parameter(LMP), was evaluated by small punch(SP) creep simulation and verified by uniaxial creep test. By employing the elastoplastic FEM(finite element method), the small punch creep behaviors associated with various friction coefficients were simulated to identify a real friction phenomena. The friction coefficient, ${\mu}$=0.7, determined by comparing deflection history was used in the small punch creep simulation to find the equivalent stresses with which the relationship between punch load and uniaxial creep stress was found. The creep life was then predicted by the LMP, which was the relationship among the rupture time, temperature, and stress. Finally, the LMP calculated by SP-creep simulation was compared with that had computed by the uniaxial creep test and fairly matched LMPs were found.

• International Journal of Ocean System Engineering
• /
• v.3 no.3
• /
• pp.147-151
• /
• 2013

This study was to correctly estimate the friction and wear behavior of carbon fiber and PEEK sheet composites, and the validity of using them as alternatives to the metal-based materials used for artificial hip joints. Moreover, this work evaluated the friction coefficient according to the fiber ply orientation, along with the fractured surfaces of the carbon/PEEK composites. The unidirectional composites had higher friction coefficients than those multidirectional composites. This was caused by the debonding between the carbon fiber and the PEEK sheet, which was proportional to the contact area between the sliding surface and the carbon fiber. The friction test results showed that there was no significant differences in relation to the fiber ply orientation. However, in a case where the speed was 2.5 m/s, the friction coefficient was relatively large for configuration I. The friction surface of the specimen was analyzed using an electron microscope. In all cases, the debonding of the fiber and PEEK could be confirmed.

• Journal of the Korean Society for Heat Treatment
• /
• v.18 no.2
• /
• pp.105-111
• /
• 2005

In sheet metal forming, drawbeads are often used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Appropriate setting and adjusting of the drawbead force is one of the most important parameters in sheet forming process control. Therefore in this study, drawbead test was performed at various bead surface treatment conditions to clarify the frictional characteristics between sheet and drawbead. Furthermore, the differences in drawing force between circular and rectangular shape beads have also been measured to estimate the effectiveness of bead shape on the material flow control. The results show that drawing and friction characteristic were strongly influenced by surface treatments of bead and bead shapes.