Papers by Author: Zainuddin Sajuri

Abstract: Cold-work tool steel is considered to be a non-weldable metal due to its high percentage content of carbon and alloying elements. To address this problem the application of a new process of semisolid joining using a direct partial remelting method was developedto achieve a spherical join structure between two parts of AISI D2 cold-work tool steel. Since the surface oxidation of this metalis very high, the control of the atmosphere during joining had to be considered. Samples were heated in an argon atmosphere at two different temperatures of 1250°C and 1275°C for 10 minutes. Metallographic analyses along the joint interface showed that an increase in temperature promoted the final joining properties and also that at a liquid fraction of 15% joining was not fully practicable. However, a20% liquid fraction can produce a very good joint and microstructure as compared to the other experimental liquid fraction. Metallographic analyses along the joint interface showed a smooth transition from one to the other and neither oxides nor microcracking was observed. The current work confirmed that avoidance of a dendritic microstructure in the semisolid joined zone and high bonding quality components can be achieved without the need for force or complex equipment when compared to conventional welding processes.

Abstract: Steel is a mostly challenging metal to semisolid process because of the high temperatures implicated and the prospective for surface oxidation. Slurry processing experiment was performed with AISI D2 cold work tool steel to identify the evolution of globular microstructures via Direct Partial Re-Melting Method (DPRM). Samples were heated in an argon atmosphere up to 1330°C which corresponded to about 38% of liquid fraction and held for 5 minutes. The typical microstructure after DPRM consists of globular grains (average grain size about 50μm) while the remaining interspaces were filled by precipitated eutectic carbides on the grain boundaries and lamellar network. Based on the requirements of thixoformability, the current work confirms the suitability of the AISI D2 cold work tool steel as a candidate material for semi-solid forming.

Abstract: This paper discusses the effect of different loading analysis on crack initiation life of wheel/rail in the contact region. A simulated three dimensional (3D) elastoplastic model of a wheel/rail contact is modelled using the fine mesh technique in the contact region by using Finite Element Method FEM code ANSYS 11.0 software. Different loads of approximately 70, 80, 90, 100, 110, 120, 130 and 140 KN were applied to the wheel tread during the running surface of the railhead to simulate stress distribution (Von Mises) and a life prediction of the crack initiation. Stress analysis is performed and the fatigue damage to the railhead surface is calculated numerically by using a multi-axial fatigue life of crack initiation model. Results obtained from previous researches are compared with this research.

Abstract: The effect of heat treatment on tensile and creep properties of nickel-base superalloy, Inconel 718 in room and at high temperature was investigated. Solution treatment was applied on the as-received material at 980oC for 1 hour before water quenched followed by double aging treatments at 720oC and 621oC for 8 hours, respectively and then cooled in air. The tensile strength at elevated temperatures of 550oC and 650oC were slightly deteriorated for heat treated and as-received materials. Beside strength, significant difference was observed in the elongation. The elongation of heat treated samples drastically reduced to 4 to 5% only compared to that of the as received materials which exhibited more than 30% elongation. The significant increased in tensile strength is suspected due to the present of γ’, γ” and δ precipitates which pinned the movement of grain boundary and sliding. However, the present of these precipitates caused the material to become harder and brittle. Moreover, the increase in load from 70% to 90% UTS and in temperature significantly accelerated the creep rate.

Abstract: This paper presents the identification of damages and stress analysis of rail/wheel rolling contact region. The railhead surface of used rail track was investigated to identify damages and the hardness of the rail/wheel contact area was measured. Finite element method FEM code, ANSYS was used to determine the stress distribution at vicinity of rail/wheel contact area. The results showed that the hardness increased on the contact area between rail and wheel due to repeated rolling contact of rail and wheel surface. Severe damages and cracks were observed on the railhead surface and in the cross section of the rail at the contact region. The FEM simulation showed that the highest stress distribution regions were matched with the area of severely damage and high hardness obtained from the observation and experimental results.

Abstract: A stress intensity factor K was used as a fracture parameter to determine the plane strain fracture toughness KIC of AZ61 magnesium alloy using a single edge notch bend (SENB) specimen in accordance to ASTM E399 testing method. Five different specimen thicknesses of 2 to 10 mm were used in the test. A sharp fatigue pre-crack was initiated and propagated to half of specimen width at a constant crack propagation rate of about 1 x 10-8 m/cycle before the specimen was loaded in tension until the fracture stress is reached and then rapid fracture occurred. The fracture toughness KC values obtained for different thicknesses showed that KC value decreased with increasing specimen thickness. The highest KC value obtained was 16.5 MPa√m for 2 mm thickness specimen. The value of KC became relatively constant at about 13 MPa√m when the specimen thickness exceeds 8 mm. This value was then considered as the plane strain fracture toughness KIC of AZ61 magnesium alloy. Calculation of the minimum thickness requirement for plane strain condition and the size of the shear lips of the fracture surface validate the obtained KIC value.

Abstract: The high cycle fatigue characteristics of magnesium alloys under low humidity, high humidity (80% RH) and sprayed 5%NaCl solution environments have been introduced. Fatigue limit of bulk magnesium alloy was significantly reduced even under high humidity condition, while other structural materials such as steel and aluminum alloy showed no influence of humidity on fatigue limit. The reduction of fatigue limit under 5% NaCl environments was much larger than that under high humidity environment. The remarkable reduction of fatigue limit under corrosive environments was attributed to the formation of corrosion pit, which was induced by simultaneous action of mechanical loading and corrosive environment. To improve the reduced fatigue strength under corrosive environment, coating used to apply on the surface. Non-chromium chemical conversion coating showed superior effect on the improvement of fatigue strength under corrosive environment compared to anodized coating. Fatigue strengths of the coated and painted AZ61 alloy under high humidity and 5%NaCl environments showed almost the same fatigue strength as bulk material under low humidity.