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RISS 인기검색어
- 검색키워드
- 저자 : Hamidreza Najafi (검색결과 4 건)
- 무료
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Behdad Nayebi,Hamidreza Najafi,Amirreza Farnia 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.10
AISI 410S ferritic stainless steel was surface melted via a continuous fiber laser at different scan rates. The microstructural characteristics, thickness, hardness, and tribological behavior of the laser melted (LSMed) surface were studied after different scan rates. The hardened layer thickness was inversely related to the scan rate. The thickness reduced from 146 μm at the scan rate of 15 mm.s -1 to 110 μm at the scan rate of 24 mm.s -1 . The microstructural studies revealed that the microstructure of the hardened layers mostly consisted of martensite. At the lowest scan rate, the martensite was in two forms of fine and coarse packets. The surface hardness enhanced with decreasing the scan rate. The maximum average hardness (330 HV) was obtained from the scan speed of 15 mm.s -1 . However, the most surface hardness scattering was observed in the sample treated by the lowest scan speed. The sample treated using the moderate scan rate (20 mm.s -1 ) demonstrated superior wear resistance due to its lowest coefficient of friction (COF) and weight loss. The reason for this behavior could be related to the appropriate melted thickness. Microstructural studies revealed that abrasive wear and adhesive wear were the dominant mechanisms in the worn surfaces.
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Creep Behavior of ZK60 Alloy and ZK60/SiCw Composite After Extrusion and Precipitation Hardening
Morteza Tayebi,Hamidreza Najafi,Said Nategh,Alireza Khodabandeh 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.10
Creep properties of ZK60 alloy and ZK60/SiCw composite have been investigated after extrusion and precipitation hardeningby accelerated creep test. Creep tests were conducted at 150 °C in the stress range of 10–120 MPa. At low stresses, the stressexponents of 1.93 and 1.75 were obtained for the unreinforced alloy and the composite, respectively. Stress exponents of theunreinforced alloy and composite sample were 5.82 and 7.07, respectively, at high stresses. The creep mechanism changedby increasing the stress from grain boundary sliding (GBS) to dislocation creep due to the fact that the average true creepactivation energy changed from 55 to 95.06 kJ/mol. Based on the microstructural observations, at low stresses, the grainrefinement induced by twinning caused the GBS mechanism. However, at high stresses, slip changed from basal planes tothe pyramidal secondary slip system which was associated with increase in twin density. Examination of the fracture surfacesrevealed that cavity nucleation in the grain corners and around the precipitates was the main reason for creep failure.
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Morteza Kalantar,Hamidreza Najafi,M. Reza Afshar 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.1
In this study, a comparison between V- and Nb-containing low-carbon microalloyed cast steels has been made in terms ofstrength and impact energy after intercritical heat treatment (IHT). After austenitizing at 950 °C and quenching in water,the alloys were intercritically heated at 760, 800 or 840 °C followed by quenching and tempering at 620 °C. In comparisonwith conventional heat treatment, IHT increased the impact energies at the expense of some reduction in the strength dueto the dual ferritic/martensitic microstructure. However, the impact energy of the alloy containing only V, especially afterintercritical heating at 760 and 800 °C, was higher than that of the Nb-containing samples. The improvement in the impactenergy required an optimum amount of coarse carbonitride precipitates. While, the impact fracture surface of the alloy V wascomposed of dimples indicating ductile fracture, cleavage facets in the fracture surface of Nb-containing samples indicatedbrittle fracture.
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Moslem Paidar,Alireza Khodabandeh,Hamidreza Najafi,Alireza Sabour Rouh-aghdam 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.12
In this work, friction stir spot welding with 1.6 mm thickness of the 2024-T3 aluminum alloy is carried out. The effects of the tool rotationalspeed and shoulder penetration depth on surface appearance, macrostructure, temperature profile, maximum failure load and failuremodes are investigated. Results show that, the effect of the tool rotational speed on maximum tensile shear load is similar to the effect ofthe shoulder penetration depth, increasing tool rotational speed and shoulder penetration depth resulted in the increase of the tensile shearload. Maximum load of about 8282 N is obtained by using 1000 rpm rotational speed and 0.7 mm shoulder penetration depth. Observationof the failed specimens indicates two types of failure modes under tensile shear loading, the shear fracture that occurs in low shoulderpenetration depths and tensile shear fracture that occurs in high shoulder penetration depths.
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