Variable-structured robust controller by fuzzy logic forservomotors

A variable-structure robust controller whose structure is continuously changed by fuzzy logic so that the system responds quickly if the error or its rate is large and vice versa is proposed. Such a controller is insensitive to both the plant noise and the observation noise. It is applied to speed control for an induction servomotor. Experiments show that the controller is superior to both a sliding-mode controller and a proportional integral-derivative (PID) controller. The paper includes the stability analysis of the overall system and the design procedure by using Lyapunov's method     

Microstructural characterization,defect and hardness of titanium femoral knee joint produced using vertical centrifugal investment casting

Centrifugal investment casting is an attractive method of producing orthopedic implants. This study investigated the effects of low rotational speed of vertical centrifugal investment casting on the microstructural characteristics, porosities and hardness of a titanium femoral knee. The centrifugal investment casting was conducted at 45-65 rpm. The microstructure, defects and microhardness of titanium resulting from centrifugal investment casting were discussed. Experimental results showed that centrifugal casting at a rotational speed of 45-65 rpm could obtain the casting without damaging the surface. The largest inner defect was typically the shrinkage because of the solidification process beneath the smallest parts. The microstructure transformed from a fine grain on the subsurface to a slightly coarse one in the inner region. Microhardness profiles on the cross section presented a decreasing tendency from the surface to the inner region because of the shift of the microstructure.     

Hot tearing criteria evaluation for direct-chill casting of an Al-4.5 pct Cu alloy

Predicting the occurrence of hot tears in the direct-chill (DC) casting of aluminum alloys by numerical simulation is a crucial step for avoiding such defects. In this study, eight hot tearing criteria proposed in the literature have been implemented in a finite-element method simulation of the DC casting process and have been evaluated. These criteria were based on limitations of feeding, mechanical ductility, or both. It is concluded that six criteria give a higher cracking sensitivity for a higher casting velocity and that five criteria give a higher cracking sensitivity for the center location of the billet. This is considered in qualitative accordance with casting practice. Seven criteria indicate that use of a ramping procedure (lower casting speed during start-up phase) does not make a significant difference. However, in industrial practice, this is a common procedure, needed for avoiding hot cracking. Only one criterion is in qualitative accordance with casting practice, but it fails to quantitatively predict the hot tearing occurrence during DC casting.     

Effect of surface mechanical attrition treatment (SMAT) on microhardness,surface roughness and wettability of AISI 316L

Surface roughness and wettability are among the surface properties which determine the service lifetime of materials. Mechanical treatments subjected to the surface layer of materials are often performed to obtain the desired surface properties and to enhance the mechanical strength of materials. In this paper, the surface microhardness, roughness and wettability of AISI 316L stainless steel resulting from surface mechanical attrition treatment (SMAT) are discussed. The SMAT was conducted with various processing parameters, including the duration of treatment, the number and diameter of milling ball, and the motor speed of the SMAT machine. The result indicates an increasing surface microhardness due to the SMAT. A harder surface is yielded by the SMAT with a longer duration, a bigger and a larger number of milling balls, and a higher vibration frequency. The SMAT also creates craters on the steel surfaces which correspond to the increasing roughness from 0.046 μm to the values in ranging from 0.681 to 0.909 μm. The change on the surface roughness by the SMAT does not only depend on the duration of treatment, but also the other processing parameters. In addition, the wettability of AISI 316L surface slightly increases by the SMAT as seen on the decreasing droplet contact angle from 88.6° to the values ranging from 74.4° to 87.0°. Such a droplet contact angle reduction is related to the increasing surface roughness after the SMAT. In conclusion, this study reveals the possibility of the SMAT to be used for surface properties optimization in addition to the strength enhancement of stainless steel.     

Contraction of aluminum alloys during and after solidification

A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.     

Finite element method simulation of mushy zone behavior during direct-chill casting of an Al-4.5 pct Cu alloy

In this article, the stresses, strains, sump depth, mushy zone length, and temperature fields are calculated through the simulation of the direct-chill (DC) casting process for a round billet by using a finite-element method (FEM). Focus is put on the mushy zone and solid region close to it. In the center of the billet, circumferential stresses and strains (which play a main role in hot cracking) are tensile close to the solidus temperature, whereas they are compressive near the surface of the billet. The stresses, strains, depth of sump, and length of mushy zone increase with increasing casting speed. They are maximum in the start-up phase and are reduced by applying a ramping procedure in the start-up phase. Stresses, strains, depth of sump, and length of mushy zone are highest in the center of the billet for all casting conditions considered.     

Effects of alloy composition and casting speed on structure formation and hot tearing during direct-chill casting of Al-Cu alloys

Effects of casting speed and alloy composition on structure formation and hot tearing during direct-chill (DC) casting of 200-mm round billets from binary Al-Cu alloys are studied. It is experimentally shown that the grain structure, including the occurrence of coarse grains in the central part of the billet, is strongly affected by the casting speed and alloy composition, while the dendritic arm spacing is mostly dependent on the casting speed. The hot cracking pattern reveals the maximum hot-tearing susceptibility in the range of low-copper alloys (1 to 1.5 pct) and at high casting speeds (180 to 200 mm/min). The clear correlation between the amount of nonequilibrium eutectics (representing the reserve of liquid phase in the last stage of solidification) and hot tearing is demonstrated. A casting speed-copper concentration-hot-tearing susceptibility chart is constructed experimentally for real-scale DC casting. Computed dimensions of the solidification region in the billet are used to explain the experimentally observed structure patterns and hot cracking. Thermomechanical finite-element simulation of the solidifying billet was used as a tool for testing the applicability to DC casting of several hot-tearing criteria based on different principles. The results are compared to the experimentally observed hot tearing. It is noted that hot-tearing criteria that account for the dynamics of the process, e.g., strain rate, actual stress-strain situation, feeding rate, and melt flow, can be successfully used for the qualitative prediction of hot tearing.