Ik-Min Park

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al2O3. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.

Effect of solution treatment and artificial aging on microstructure and mechanical properties of Al−Cu alloy

Abstract In order to achieve good mechanical properties of Al-Cu alloys such as high strength and good toughness, precipitation hardening and artificial aging treatment were applied. As defined by the T6 heat treatment, the standard artificial aging treatment for Al-Cu alloy followed heat treatments of solution treatment at 510–530 °C for 2 h, quenching in water at 60 °C and then artificial aging at 160–190 °C for 2–8 h. The effects of solution treatment and artificial aging on the microstructure and mechanical properties of Al-Cu alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and tensile test. The results of solution treatment indicate that the mechanical properties of Al-Cu alloy increase and then decrease with the increase of solution temperature. This is because the residual phases dissolve gradually into the matrix, and the fraction of the precipitation and the size of the re-crystallized grain increased. Compared to the solution temperature, the solution holding time has less effect on the microstructure and the mechanical properties of Al-Cu alloy. The artificial aging treatments were conducted at 160–180 °C for 2–8 h. The results show that the ultimate tensile strength can be obtained at 180 °C for 8 h. Ultimate tensile strength increased with increasing time or temperature. Yield strength was found as the same as the ultimate tensile strength result.

Characterization of protective oxide layers formed on molten AZ91 alloy containing Ca and Be

AZ91 alloy in its molten state oxidizes rapidly in atmospheric conditions and begin to burn from the melt surface. In order to prevent the molten AZ91 alloy from burning, it is usually protected by a cover gas such as sulfur hexafluoride (SF6). However, the use of SF6 gas poses a serious threat to the environment as it is widely acknowledged to contribute to the global warming effect. Thus, several types of studies aimed at controlling the oxidation behavior of molten magnesium alloys have been carried out. This paper describes the characteristics of the oxide layers formed on molten AZ91 alloys containing Ca and Be. The surface analysis technique of GDS (Glow Discharge Spectroscopy) was used to investigate the oxide layers of AZ91 alloys containing Ca and Be. The GDS results showed that the oxide layer consists of CaO, MgO, BeO and Al2O3.

Fabrication and characterization of AI/TINI shape memory composites

Metal matrix composites reinforced with shape memory alloys induce compressive residual stress in the matrices resulting in improved tensile properties in the composite. In the present study, Al/TiNi shape memory composites were fabricated by squeeze casting and powder metallurgy the so-called Plasma Activated Sintering (PAS). Fairly good matrix/reinforcement bonding was observed in the Al/TiNi composites. The effects of the residual stress caused by the shape recovery force were compared as a function of reinforcement volume fraction, prestrain, matrix strength and reinforcement shape. Strengthening effect due to TiNi reinforcement as well as the compressive residual stress were investigated. It was found that the yield strength of the composites increased the volume fraction of TiNi reinforcement increased and the amount of prestrain applied to the composite. Moreover, the percentage of strength increment of the pure continuous fiber Al/TiNi (Al/TiNicf) composite caused by prestrain was higher than that of the AC4A Al/TiNicfcomposite. The percentage of strength increment of the Al/TiNicf composite was higher than that of the particulate Al/TiNi (Al/TiNip) composite.

A study on die wear model of warm and hot forgings

Factors influencing service lives of tools in warm and hot forging processes are wear, mechanical fatigue, plastic deformation and thermal fatigue, etc. Wear is the predominant factor for tool failure among these. To predict tool life by wear, Archard’s model where hardness is considered as constant or function of temperature is generally applied. Usually hardness of die is a function of not only temperature but operating time of die. To consider softening of die by repeated operation it is necessary to express hardness of die by a function of temperature and time. In this study wear coefficients were measured for various temperatures and heat treatment for H13 tool steel. Also by experiment of reheating of die, die softening curves were obtained. From experimental results, relationships between tempering parameters and hardness were established to investigate effects of hardness decrease by the effect of temperatures and time. Finally modified Archard’s wear model in which hardness is considered to be a function of main tempering curve was proposed. And finite element analyses were conducted by adopting suggested wear model. By comparisons of simulations and real profiles of worn die, proposed wear model was verified.

Effects of Palm Kernel Expellers on Growth Performance, Nutrient Digestibility, and Blood Profiles of Weaned Pigs

This experiment was conducted to investigate the effects of palm kernel expellers on growth performance, nutrient digestibility, and blood profiles of weaned pigs. A total of 88 weaned pigs (6.94±0.76 kg body weight [BW]; 28 d old) were randomly allotted to 2 dietary treatments (4 pigs/pen; 11 replicates/treatment) in a randomized complete block design (sex as a block). The dietary treatments were a typical nursery diet based on corn and soybean meal (CON) and CON added with 20% of palm kernel expellers (PKE). Pigs were fed for 6 wk using a 3-phase feeding program with declining diet complexity and with phases of 1, 2, and 3 wk, respectively. Blood was collected from randomly selected 2 pigs in each pen before weaning and on d 7 after weaning. Pigs were fed respective dietary treatments containing 0.2% chromic oxide from d 29 to 35 after weaning. Fecal samples were collected from randomly selected 2 pigs in each pen daily for the last 3 days after the 4-d adjustment period. Measurements were growth performances, digestibility of dry matter, nitrogen and energy, white and red blood cell counts, packed cell volume, and incidence of diarrhea. The PKE increased average daily gain (ADG) (246 vs 215 g/d; p = 0.06) and average daily feed intake (ADFI) (470 vs 343 g/d; p<0.05) and decreased gain-to-feed ratio (G:F) (0.522 vs 0.628 g/g; p<0.05) during phase 2 compared with CON, but did not affect growth performance during phase 1 and 3. During overall experimental period, PKE increased ADG (383 vs 362 g/d; p = 0.05) and ADFI (549 vs 496 g/d; p<0.05) compared with CON, but did not affect G:F. However, no differences were found on digestibility of dry matter, nitrogen, and energy between CON and PKE. The PKE reduced frequency of diarrhea (15% vs 25%; p = 0.08) for the first 2 wk after weaning compared with CON. Similarly, PKE decreased white blood cells (8.19 vs 9.56×103/μL; p = 0.07), red blood cells (2.92 vs 3.25×106/μL; p = 0.09), and packed cell volume (11.1% vs 12.6%; p = 0.06) on d 7 after weaning compared with CON. In conclusion, addition of 20% palm kernel expellers to nursery diet based on corn and soybean meal had no negative effects on growth performance, nutrient digestibility, and blood profiles of weaned pigs.

Microstructure and Mechanical Properties of Reaction Squeeze Cast Hybrid Al Matrix Composites

Hybrid Al matrix composites reinforced with Kaowool ceramic fiber (Al2O3 · SiO2) and reaction formed intermetallic compound particles, starting compositions of (10%Al2O3 · SiO2 + 5%Ni)/Al and (10%Al2O3 · SiO2 + 5%TiO2)/Al were fabricated employing squeeze casting technique. Microstructures of the Al composites were examined, and the reactivity of Ni and TiO2 powders added to Kaowool short fiber preform with infiltrating molten Al, resulting products of Al3Ni and Al3Ti, during squeeze casting was analyzed. Hardness, strength (at 25 and 300°C) and wear resistance of the composites were characterized. To investigate the fracture behavior of the composites, in-situ fracture test was performed within SEM. The hybrid Al composites revealed the microstructure of uniformly distributed reinforcements (ceramic short fiber and reaction formed intermetallic compounds). Ni powder added hybrid Al composite achieved the highest hardness, strength and wear resistance among the composites. It was found that the strength drop at elevated temperature (300°C) is effectively reduced by hybridization with ceramic fiber and intermetallic compounds reinforced in Al. In-situ fracture observation demonstrated the fracture behavior of Ni powder added hybrid Al composite as microcrack initiated mainly by short fiber/ matrix interfacial debonding and the crack propagated through the cluster of intermetallic compound particles as loading is raised.

Microstructure and mechanical properties of Ti-B-C-N-Si nanocomposite films deposited by unbalanced magnetron sputtering

Quinary Ti–B–C–N–Si nanocomposite thin films were deposited on AISI 304 stainless steel substrates by d.c. unbalanced magnetron sputtering from a TiB2–TiC compound target and a pure Si target. The relationship between microstructure and mechanical properties of the films was investigated in terms of the nanosized crystallites/amorphous system. The synthesized Ti–B–C–N–Si films were characterized using x-ray diffraction, x-ray photoelectron spectroscopy, atomic force microscopy, and high resolution transmission electron microscopy. The results showed that the Ti–B–C–N–Si films were nanocomposites composed of nanosized TiB2, TiC, and TiSi2 crystallites (2-3 nm in size) embedded in an amorphous matrix. The addition of Si to the Ti–B–C–N film led to precipitation of nanosized crystalline TiSi2 and percolation of amorphous SiC phases. The Ti–B–C–N–Si films with up to 7 at. % Si content presented high hardness (≥35 GPa), H/E (≥0.0095), and We (>50%) with compressive residual stress (∼0.5 GPa). A systematic inve...

국부가압 다이캐스팅 공정에서 3차원 유동 및 응고해석을 통한 자동차 변속기 Gear Housing의 주조방안 설계 최적화

【In the partial squeeze casting process, the filling behavior of liquid metal and solidification pattern in thick area have significant influence on the quality of casting products and die life. For the optimal casting design of automobile transmission gear housing, various analyses were performed in this study by using computer simulation code, MAGMAsoft and the simulation results were compared and analyzed with experimental results. By air pressure criteria, internal porosities caused by air entrap during the mold filling were predicted and reduced remarkably by modification of gating system. Also, optimal squeeze-time lag to apply partial squeeze pin in thick area was calculated and the castings was free from shrinkage defects with the result of solidification analysis. Consequently, casting design for automobile transmission gear housing was optimized and approved by Computer Tomography.】

Effect of Sub-T g Annealing on the Corrosion Resistance of the Cu-Zr Amorphous Alloys

Despite the economy of material cost and excellent toughness of Cu-based amorphous alloys, especially Cu50Zr50, their poor corrosion resistance to a chloride medium limits their widespread applications. In this study, corrosion tests were performed on the Cu50Zr50 amorphous alloy with different degrees of short-range order, which were prepared by annealing below the glass transition temperature (Tg). It was found that the corrosion resistance of amorphous alloys is improved to a significant level when the alloys were heated below Tg. Calorimetric studies showed that thermally activated relaxation process of created disorder, which occurs during sub-Tg annealing, is responsible for the improvement in the corrosion resistance. Molecular dynamics simulations performed on the Cu–Zr amorphous alloys demonstrated that the relaxation process of the alloys is associated with the formation of energetically stable icosahedra and icosahedron-like structures. Our study highlights the effects of sub-Tg annealing on the improvement in the corrosion resistance of the amorphous alloys from the viewpoint the relaxation process of the short-range orders.