Jeong Hwan Jang

Selective laser melting of Fe-Ni-Cr layer on AISI H13 tool steel

Abstract An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N 2 or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.

Hydroforming of flanged tubular part

Tube hydroforming is the technology that utilizes hydraulic pressure to form a tube into desired shapes inside die cavities. Recently, tube hydroforming technology draws attentions of automotive industries due to its advantages such as weight reduction, increased strength, improved quality and reduced tooling cost. Hydroformed automotive parts used as structural components in vehicle body frame or subframe often have to be structurally joined at some point. Therefore it is useful if the hydroformed automotive parts can be given a localized attachment flange. In this study, a tube hydroformed product which has flange has been formed at various processing conditions. To accomplish successful flange hydroforming process, thorough investigation on proper combination of process parameters such as internal hydraulic pressure and tool geometry has been performed. For the process design FE analysis was performed with Dynaform 5.5. With optimized die parting angle and circumferential expansion ratio, hydroforming experiments to form flange were performed and forming characteristics at various process conditions were analyzed. The results show that flanged parts can be successfully produced by tube hydroforming process.

Characterization of the aluminium coating layer in the hot press forming of boron steel

Abstract Hot press forming allows geometrically complex parts to be easily formed from boron steel blanks. The rapid cooling after forming produces a product with extremely high strength. To prevent the blanks from oxidizing and decarburizing during heating, forming, and subsequent press cooling, the boron steel is supplied with an aluminium-based coating. This surface coating influences the formability of the component and the quality of the final product. The main purpose of the present research is to characterize the changes in the aluminium-coated layer on a boron steel during hot press forming. To characterize the evolution of the coated layer, experiments for hot press forming were conducted under various conditions that simulated a production process. Test specimens were heated to temperatures between 810 and 930 °C and were then press hardened. The aluminium-coated layer develops four distinct microstructural regions: (a) a diffusion zone; (b) an aluminium—iron (Al—Fe) zone I; (c) a low-aluminium zone (LAZ); and (d) an Al—Fe zone II. The band-like LAZ is clearly observable in the temperature range of 810 to 870 °C and becomes sparsely dispersed at temperatures above 900 °C. The microcracking behaviour of the aluminium-coated layer during forming was also analysed with both bending and deep-drawing tests. The strain concentration in the softer LAZ is found to be closely connected to both microcracking and exfoliation of the coated layer during the press forming.

Application of a direct laser melting process for fabricating a micropattern in bipolar plates used in direct methanol fuel cells

Direct laser melting (DLM) technology is applied to fabricate the micropattern of bipolar plates in a direct methanol fuel cell (DMFC). For the micropatterning, a DLM process with 316L stainless steel (SS316L) powder has been used. For the target point of 1 mm melt height, the DLM process conditions are optimized and an eight-layer structure is successfully formed. The performance of a bipolar plate made by the DLM micropatterning is compared with a numerical control (NC)-machined bipolar plate. For the DLM micropatterned bipolar plate, the kinetic energy in the fuel causes a more rapid chemical reaction due to the increased surface area of rough side walls. The smaller fluid rate loss enhances circulation of the fluid and, consequently, the efficiency of the micropatterned bipolar plate made by the DLM process is significantly improved. Furthermore, the use of SS316L powder material as a micropatterned material reduces the fabrication cost as well as size of the fuel cell stack.

Effect of Process Parameters on Forming Characteristics of Selective Laser Sintered Fe-Ni-Cr Powder

Selective laser sintering is a kind of rapid prototyping process whereby a three-dimensional part is built layer wise by laser scanning the powder. This process is highly influenced by powder and laser parameters such as laser power, scan rate, fill spacing and layer thickness. Therefore a study on fabricating Fe-Ni-Cr powder by selective laser sintering has been performed. In this study, fabrication was performed by experimental facilities consisting of a 200W fiber laser which can be focused to 0.08mm and atmospheric chamber which can control atmospheric pressure with argon. With power increase or energy density decrease, line width was decreased and line surface quality was improved with energy density increase. Surface quality of quadrangle structure was improved with fill spacing optimization.

Effect of Process Parameters on Surface Roughness and Porosity of Direct Laser Melted Bead

Direct laser melting(DLM) is promising as a joining method for producing parts for automobiles, aerospace, marine and medical applications. An advantageous characteristic of DLM is that it affects the parent metal very little. The mechanical properties of parts made by DLM are strongly affected by the porosity and surface roughness of the laser melted beads. This is a systematic study of the effects of the porosity and surface roughness of laser melted beads using various processing parameters, such as laser power, scan rate and overlapping ratio of the fill spacing. The specimens were fabricated with 316L and 304L austenitic stainless steel powder. Dense parts with low porosity were obtained at low laser scan speed, as it increased the aspect ratio of the parental material and the depth of penetration. The variations of surface roughness were examined at various processing parameters such as overlapping ratio and laser power.

Effect of Coating Layer Hardness on the Wear Characteristics of Diesel Engine Cylinder Liner-piston Ring

The wear between piston ring face and cylinder liner is an extremely unpredictable and hard-to-reproduce phenomenon that significantly decreases engine performance. This study will discuss characteristics of wear between hard and soft piston ring coatings with running surface of cylinder liner. Detailed tribological analysis by using Pin-on-Disk(POD) testing machine describes the lubricity mechanism between piston ring coatings and cylinder liner at different temperature with and without oil. The effect of surface roughness of the cylinder liner on the friction coefficient and wear amount of piston ring coatings will also be analyzed. To simulate scuffing mechanism between piston ring and cylinder liner, accelerated lab testing was performed. This study will provide the data from tribological testing of hard and soft piston ring coatings against cylinder liner. Furthermore, the microstructures and morphological features of the surface and the near-surface materials during wear will be investigated. From the scuffing test by using POD testing machine, scuffing mechanisms for the soft and hard coating will be analyzed and experimentally confirmed.

Characterization of Direct Laser Melting Technology for the Fabrication of Fuel Cell Bipolar Plate

Manufacturing of the bipolar plate of a direct methanol fuel cell (DMFC) by direct laser melting technology (DLM) was attempted. The DLM technology is highly influenced by process parameters such as laser power, scan rate and layering height. Therefore, an analysis of the DLM technology was performed under various conditions. The bipolar plates were fabricated using the DLM process with 316L stainless steel (STS 316L) plates and powder. Powder melting trials at various energy density were performed in order to select a feasible melting range for a given laser power. The melting line height increases and eventually saturates when the energy density increases, but decreases when the laser power increases at a given energy density. For the estimation of the potential performance of the bipolar plate, the surface roughness and contact resistance of the DLM layer were also analyzed. The changes of line height and thickness are useful information to report when manufacturing bipolar plate of fuel cell through the DLM process.

Analysis of the Stress Characteristics of Double Layered Tube at Elevated Temperature

Abstract Double layered tube that has been used for transportation and oil piping system is occasionally exposed to elevated temperature. The change in stress state at elevated temperature is important for the safe design of double layered tube. In this study, the variation of stress state for hydroformed double layered tube of which inner tube is stainless steel and outer tube is mild steel has been analytically analyzed. To characterize the thermal stress at elevated temperature, analytical model to provide thermal stresses between outer tube and inner tube was developed by using theories of elasticity and Lame equation. The feasibility of analytical model is verified by finite element analysis using ANSYS CLASSIC TM , commercially available code. The variation of thermal stress at various thickness combination of inner and outer tube has also been investigated by proposed analytical model. Key Words : Double Layered Tube, Tube Hydroforming, Thermal Stress, Coefficient of Linear Expansion

Shell Body Nose Forming by Rotary Swaging Process

Studies on the forming characteristics by a rotary swaging process using the sub-scale specimens have been carried out to obtain a shell body nose of desirable quality. To analyze the changes of the nose thickness and length at the respective reduction of inside diameter, the finite element simulations were carried out. As a result, the desired target dimension is satisfied with the diameter reduction of more than 64 % for the given preform. The thickness of nose area increased up to 56.1 % from initial thickness of 2.62 mm to 4.09 mm after swaging. The values of the hardness before and after swaging were 208 HV and 325 HV, respectively. To analyze the dimensional changes (length and thickness) of nose area with decreasing inside diameter, the rotary swaging test was carried out for two different diameter reductions such as 65 % and 67 %. The lengths of nose area for the diameter reductions are 11.79 mm in 65 % and 12.53 mm in 67 %, respectively. At the diameter reduction of more than 67%, the crack occurs when the localized strain hardening reduces ductility in internal area. Therefore, the nose area should be formed from 64% to 67% reduction in target inner diameter.