• Jamian, Saifulnizan

Abstract

Functionally graded materials (FGMs) refer to the composite materials where thecompositions or the microstructures are locally varied so that a certain variation of thelocal material properties is achieved. Determination of compositional gradient and theprocess of making an FGM are dependent on its intended use. In this study, new possibleapplications of FGM and its production process were investigated. Three possibleapplication of FGM were proposed.First, the novel technique in producing ultra fine grain of difficult-to-work materialsby equal-channel angular pressing (ECAP) process at ambient temperature wasdeveloped by using FGM. For this study, Ti as the difficult-to-work material was tightlyencapsulated in a hollow host material made of Al-based FGM matrix. The Al-basedFGM as a host material assists the deformation of Ti. The ECAP process was simulatedby the finite element method (FEM) to determine the appropriate compositional gradientof Al-based FGM and the position to embed Ti wire. FEM was conducted with Tiembedded into a different host material type as well as different die channel geometry.The strain distribution of the specimen after a single ECAP pass was analyzed. From theobtained results, it is found that the strain distribution in Ti is strongly influenced by thehost material and the shape of the die channel. An experimental work was carried out toconfirm the ability of the proposed technique in producing ultra fine grain of Ti. The hostmaterial was prepared by embedding Al-Al3Ti alloy into Al. Three types of the Al-Al3Tialloys with different Al3Ti volume fractions were used to prepare the host materials.ECAP for specimens was carried out for up to eight passes by route A. Themicrostructure and hardness of ECAPed specimens were investigated. The changes inmicrostructure and the increase in the hardness value of Ti with increased number ofECAP passes are evidences showing that Ti is successfully deformed by this technique.Second, new types of FGM crash boxes with stepwise strength gradient in longitudinaldirections were proposed. The property of the proposed FGM crash boxes were analyzedusing FEM. Crash behavior of the crash box under axial quasi-static and dynamic impactloads were studied. The obtained load-displacement curves and the crash failure patternsthen were evaluated to assess the effect of the stepwise strength gradient of the crash-box. Moreover, four different shapes of cross-sectional i.e. square, circle, pentagon andhexagon were considered. The results show that the FGM crash box is superior to thanthe homogeneous crash box in overall crashworthiness. Although there were no triggermechanism introduced, the FGM crash boxes experience the progressive crushinginitiated at the impact side.Third, the FGMs were applied in pipe and pressure vessel field. A solution procedurefor finite element thermo-visco-plasticity and creep analysis in an FGM thick-walledpressure vessel subjected to thermal and internal pressure was presented. The thickwalledpressure vessel was replaced by a system of discrete rectangular cross-section ringelements interconnected along circumferential nodal circles. The property of FGM wasassumed to be continuous function of volume fraction of material composition. Thethermo-visco-plasticity and creep behavior of the structures were obtained by the use ofan incremental approach. The obtained results show that the material compositionsignificantly affects the stress as a function of time at the inside and outside surface ofthick-walled pressure vessel. The use of FGM can adjust the stress distribution in thestructure.Moreover, one of the FGM fabrication method, centrifugal casting, was investigated.Two types of centrifugal casting method namely, centrifugal solid-particle method(CSPM) and centrifugal mixed-powder method (CMPM), were used to fabricate Al/SiCFGM. Formations of graded distribution of SiC particles within molten Al by CSPM andCMPM under huge centrifugal force were examined and simulated. The movement ofSiC particles in viscous liquid under centrifugal force was explained theoretically basedon Stoke�s law. The effect of composition gradient of particles on viscosity was takeninto account. Also, the effect of temperature distribution on viscosity and density wereconsidered. A computer code to simulate the formation of compositional gradient in anAl/SiC FGM manufactured by CSPM and CMPM was developed. From the obtainedresults, it was found that the SiC particles can be graded from inner to outer surface ofAl/SiC FGM by CSPM. Meanwhile by CMPM, the SiC particles can be dispersed on thesurface of Al/SiC FGM. The graded distribution in Al/SiC FGM under huge centrifugalforce was significantly affected by the mold temperature but less affected by the initialtemperature of molten Al and casting atmosphere.

Topics

• density
• impedance spectroscopy
• surface
• polymer
• grain
• laser emission spectroscopy
• strength
• composite
• viscosity
• hardness
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• plasticity
• finite element analysis
• wire
• creep
• centrifugal casting