Mohd Zaidi Omar

An Overview of Semisolid Processing of Aluminium Alloys

Semisolid metal processing (SSM) or thixoforming is a new technology that offers several advantages over liquid processing and solid processing. This process utilizes semisolid behavior as well as reduces macrosegregation, porosity, and forming forces during shaping process. A lot of research work has been carried out by various researchers in order to exploit the potential of this process to produce different products especially for automotive industry. This paper will summarise the rheological behavior of aluminium alloys in semisolid slurries, thixoformability of modified aluminium alloys, the effect of feedstock production method on mechanical properties, and the importance of developing low-cost raw materials for semisolid processing.

Semisolid Metal Processing Techniques for Nondendritic Feedstock Production

Semisolid metal (SSM) processing or thixoforming is widely known as a technology that involves the formation of metal alloys between solidus and liquidus temperatures. For the procedure to operate successfully, the microstructure of the starting material must consist of solid near-globular grains surrounded by a liquid matrix and a wide solidus-to-liquidus transition area. Currently, this process is industrially successful, generating a variety of products with high quality parts in various industrial sectors. Throughout the years since its inception, a number of technologies to produce the appropriate globular microstructure have been developed and applied worldwide. The main aim of this paper is to classify the presently available SSM technologies and present a comprehensive review of the potential mechanisms that lead to microstructural alterations during the preparation of feedstock materials for SSM processing.

Employability skills for an entry-level engineer as seen by Malaysian employers

In an earlier publication, the authors have proposed a model on practical framework of engineering employability skills, i.e. the Malaysian Engineering Employability Skills (MEES). The framework discusses on the possible employability skills as required for an entry level engineer in Malaysia based on many other earlier studies. A set of questionnaires was later developed to gauge this model from the perspective of Malaysian employers from engineering sectors. Based on the results obtained, this paper will discuss, among others, the preferred entry-level employability skills as valued by employers when hiring fresh engineering graduates. The data was collected from September to December 2009 through face-to-face and telephone interviews with employers from Kelang Valley area, the main industrial area in Malaysia. Five hundreds employers were approached, and of this, three hundreds were successfully interviewed. The interviews were only carried out with high ranking officers, in this case; Head of Division or higher to assure accurate results. The questionnaires comprise ten (10) main employability skills as described in MEES. A “multi-attribute value technique” and statistical methods were used for data analysis. The findings suggested that the employers perceived employability skills as “required” with an average rating score of 4.06 out of 5.00. The most required skill is “communication skills” while the least required is “engineering system approach”. However, there are differences in the priorities of employability skills by the four different categories of industries considered in this study. The framework and the findings presented perhaps can be used as guidance for the employers in their recruitment exercise. The findings could also be helpful in guiding the education providers, trainers, career advisors and the likes in increasing the employability skills of engineering fresh graduates.

Microstructural evolution and mechanical properties of thixoformed A319 alloys containing variable amounts of magnesium

The effects of Mg content on the microstructure and tensile properties of thixoformed A319 alloys were studied. The samples were thixoformed at 50% liquid content and some of the thixoformed samples were subjected to the T6 heat treatment. The samples were then examined by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis as well as tensile tests. The results showed that magnesium was able to refine the eutectic silicon in the samples. It was also observed that a compact Al9FeMg3Si5 phase was formed when the magnesium content was 1.0% and 1.5%. The results also revealed that as the magnesium content in the alloy increases, the tensile strengths of the thixoformed alloys also increase. The ultimate tensile strength, yield strength and elongation to fracture of the thixoformed A319 heat treated alloy were 298 MPa, 201 MPa and 4.5%, respectively, whereas the values of the thixoformed heat treated alloy with 1.5% Mg content were 325 MPa, 251 MPa and 1.4%, respectively. Thixoformed A319 alloy showed a dimple fracture behaviour, while thixoformed A319 alloys with 1.5% Mg showed a mixed mode fracture behaviour, where dimple and cleavage ruptures were seen on the fracture surface of the samples.

Microstructural evolution during DPRM process of semisolid ledeburitic D2 tool steel

Semisolid metal processing is a relatively new technology that offers several advantages over liquid processing and solid processing because of the unique behaviour and characteristic microstructure of metals in this state. With the aim of finding a minimum process chain for the manufacture of high-quality production at minimal cost for forming, the microstructural evolution of the ledeburitic AISI D2 tool steel in the semisolid state was studied experimentally. The potential of the direct partial remelting (DPRM) process for the production of AISI D2 with a uniform globular microstructure was revealed. The liquid fraction was determined using differential scanning calorimetry. The microstructures of the samples were investigated using an optical microscope and a scanning electron microscope equipped with an energy dispersive spectroscopy analyser, while X-ray phase analysis was performed to identify the phase evolution and the type of carbides. Mechanical characterisation was completed by hardness measurements. The typical microstructure after DPRM consists of metastable austenite which was located particularly in the globular grains (average grain size about 50 μm), while the remaining interspaces were filled by precipitated eutectic carbides on the grain boundaries and lamellar network.

Viscosity - shear rate relationship during the thixoforming of HP9/4/30 steel

Thixoforming involves shaping metal components in the semi-solid state. Work on the thixoforming of high temperature materials, such as steel, is still at its initial stage; this is mainly due to the high processing temperatures involved and the potential for oxidation. For thixoforming to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must ideally consist of solid metal spheroids in a liquid matrix. This paper discusses the thixoforming load versus displacement curves of HP9/4/30 steel semi-solid slugs under compression. The data from the corresponding load-displacement curves is converted into viscosity against shear rate adopting Stefan’s equation for flow between two parallel planes. The viscosity at processing conditions, i.e. at processing temperatures in the range of 1470 to 1480°C and zero to two minutes soak times, showed a rapid decrease initially, which corresponds to a rapid breakdown in the structure, followed by a steady decrease to a near constant value with increasing rate. The work shows pseudoplasticity (or shear-thinning) behaviour of the HP9/4/30 semi-solid slurries. This data would be required for modeling the die fill with these slurries.

Influence of Cu content on microstructure and mechanical properties of thixoformed Al-Si-Cu-Mg alloys

Abstract The effects of Cu content on the microstructure and mechanical properties of thixoformed Al–6Si– x Cu–0.3Mg ( x = 3, 4, 5 and 6, mass fraction, %) alloys were studied. The samples were thixoformed at 50% liquid content and several of the samples were treated with the T6 heat treatment. The samples were then examined by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) analysis, as well as hardness and tensile tests. The results show that the cooling slope casting and thixoforming process promote the formation of very fine and well distributed intermetallic compounds in the aluminium matrix and the mechanical properties of the alloys increase considerably compared with the permanent mould casting. The results also reveal that as the Cu content in the alloy increases, the hardness and tensile strength of the thixoformed alloys also increase. The ultimate tensile strength, yield strength and elongation to fracture of the thixoformed heat-treated Al–6Si–3Cu–0.3Mg alloy are 298 MPa, 201 MPa and 4.5%, respectively, whereas the values of the thixoformed heat-treated alloy with high Cu content (6%) are 361 MPa, 274 MPa and 1.1%, respectively. The fracture of the thixoformed Al–6Si–3Cu–0.3Mg alloy shows a dimple rupture, whereas in the alloy that contains the highest Cu content (6%), a cleavage fracture is observed.

BALLISTIC LIMIT OF HIGH-STRENGTH STEEL AND AL7075-T6 MULTI-LAYERED PLATES UNDER 7.62-MM ARMOUR PIERCING PROJECTILE IMPACT

THIS PAPER PRESENTS THE COMPUTATIONAL-BASED BALLISTIC LIMIT OF LAMINATED METAL PANELS COMPRISED OF HIGH STRENGTH STEEL AND ALUMINIUM ALLOY AL7075-T6 PLATE AT DIFFERENT THICKNESS COMBINATIONS TO NECESSITATE THE WEIGHT REDUCTION OF EXISTING ARMOUR STEEL PLATE. THE NUMERICAL MODELS OF MONOLITHIC CONFIGURATION, DOUBLE-LAYERED CONFIGURATION AND TRIPLE-LAYERED CONFIGURATION WERE DEVELOPED USING A COMMERCIAL EXPLICIT FINITE ELEMENT CODE AND WERE IMPACTED BY 7.62 MM ARMOUR PIERCING PROJECTILE AT VELOCITY RANGE OF 900 TO 950 M/S. THE BALLISTIC PERFORMANCE OF EACH CONFIGURATION PLATE IN TERMS OF BALLISTIC LIMIT VELOCITY, PENETRATION PROCESS AND PERMANENT DEFORMATION WAS QUANTIFIED AND CONSIDERED. IT WAS FOUND THAT THE MONOLITHIC PANEL OF HIGH-STRENGTH STEEL HAS THE BEST BALLISTIC PERFORMANCE AMONG ALL PANELS, YET IT HAS NOT CAUSED ANY WEIGHT REDUCTION IN EXISTING ARMOUR PLATE. AS THE WEIGHT REDUCTION WAS INCREASED FROM 20-30%, THE DOUBLE-LAYERED CONFIGURATION PANELS BECAME LESS RESISTANCE TO BALLISTIC IMPACT WHERE ONLY AT 20% AND 23.2% OF WEIGHT REDUCTION PANEL COULD STOP THE 950M/S PROJECTILE. THE TRIPLE-LAYERED CONFIGURATION PANELS WITH SIMILAR AREAL DENSITY PERFORMED MUCH BETTER WHERE ALL PANELS SUBJECTED TO 20-30% WEIGHT REDUCTIONS SUCCESSFULLY STOPPED THE 950 M/S PROJECTILE. THUS, TRIPLE-LAYERED CONFIGURATIONS ARE INTERESTING OPTION IN DESIGNING A PROTECTIVE STRUCTURE WITHOUT SACRIFICING THE PERFORMANCE IN ACHIEVING WEIGHT REDUCTION.

Explicit Nonlinear Finite Element Geometric Analysis of Parabolic Leaf Springs under Various Loads

This study describes the effects of bounce, brake, and roll behavior of a bus toward its leaf spring suspension systems. Parabolic leaf springs are designed based on vertical deflection and stress; however, loads are practically derived from various modes especially under harsh road drives or emergency braking. Parabolic leaf springs must sustain these loads without failing to ensure bus and passenger safety. In this study, the explicit nonlinear dynamic finite element (FE) method is implemented because of the complexity of experimental testing A series of load cases; namely, vertical push, wind-up, and suspension roll are introduced for the simulations. The vertical stiffness of the parabolic leaf springs is related to the vehicle load-carrying capability, whereas the wind-up stiffness is associated with vehicle braking. The roll stiffness of the parabolic leaf springs is correlated with the vehicle roll stability. To obtain a better bus performance, two new parabolic leaf spring designs are proposed and simulated. The stress level during the loadings is observed and compared with its design limit. Results indicate that the newly designed high vertical stiffness parabolic spring provides the bus a greater roll stability and a lower stress value compared with the original design. Bus safety and stability is promoted, as well as the load carrying capability.

Evolution of Globular Microstructures during Direct Partial Re-Melting Experiment of AISI D2 Tool Steel

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.