Demir, Ali GokhanKim, JinwooCaltanissetta, FabioHart, A. John...
12页
查看更多>>摘要：The layer-by-layer building in the laser powder bed fusion (LPBF) process can be exploited to achieve graded alloy compositions along the build direction. The local control over alloying element composition would allow for tailored material properties. This work demonstrates the use of LPBF to achieve gradient structures by mixing two austenitic steels, namely AISI 316L and Fe35Mn, by varying the relative deposition amounts of two alloy feedstocks. For this, a custom-built LPBF system equipped with a double-hopper and a mixing chamber was used. The system allowed the powders to be mixed on demand before deposition of each layer. The process parameters were studied to produce graded specimens starting from AISI 316L, and gradually changing to Fe35Mn, along the build direction. Characterization of the elemental composition verified good mixing of the elements, both due to the preparation of the mixed powder within the machine, and homogeneous melting of each new layer into the underlying layer of the build. Element-wise chemical composition control was therefore achieved by gradually substituting Ni, Cr, and Mo (in AISI 316L) with Mn (in Fe35Mn). The specimens were characterized for their mechanical properties at different chemical compositions along the build direction. The microhardness and the ultimate tensile strength could be varied from 240 HV to 150 HV and from 750 MPa to 600 MPa over 6 mm distance along the build direction. The results confirmed the functional gradient and the possibility to use this technique to design complex components with locally specified mechanical properties and geometry.
查看更多>>摘要：Mold electromagnetic stirring (M-EMS) has been introduced into the continuous casting of steel billets to promote the formation of a central equiaxed zone; however, the formation mechanism of the equiaxed crystals and the effect of M-EMS on crystal transport are not fully understood. Currently, a three-phase volume average model was used to study the solidification in a billet continuous casting (195 mm x 195 mm). The modeling results showed that the main function of M-EMS in this type of billet casting is to promote superheat dissipation in the mold region, leaving the liquid core out of the mold region undercooled. Although both, heterogeneous nucleation and crystal fragmentation, are considered to be the origins of equiaxed crystals, M-EMS appeared to impact crystal fragmentation more effectively. A small portion of equiaxed crystals could be brought by the M-EMS induced swirling flow into the superheated zone (upper mold region) and remelted; most equiaxed crystals settled in the lower undercooled zone, where they continued to grow and form a central equiaxed zone. These simultaneous phenomena represent an important species/energy transport mechanism, influencing the as-cast structure and macrosegregation. Negative segregation occurred in the central equiaxed zone, positive segregation occurred at the border of the columnar zone, and a trail of negative segregation occurred in the subsurface region of the billet. Finally, parameter studies were performed, and it was found that the shielding effect of the copper mold, electrical isolation at the strand-mold interface, and relatively high electrical conductivity of the strand shell affect the M-EMS efficiency.
查看更多>>摘要：Defect free machining of Fibre Reinforced Polymer (FRP) Composites, crossing the challenges posed by anisotropic non-homogenous fibre-matrix system, is one of the important material processing requirements with a wide scope in industrial applications. Eccentric Sleeve Grinding (ESG) projected in this paper is a unique strategy with progressive-intermittent cutting scheme for achieving minimal damage machined surfaces on FRPs. Progressively varying depth of engagement for active abrasive grains in the cutting zone with an intermittent and periodically repeating cutting pattern achieved through precisely controlled eccentric rotation of grinding wheel is the key highlight of ESG. Through this step-by-step cutting methodology, significant reduction in grinding force has been achieved on Carbon Fibre Reinforced Polymer (CFRP) composites under varying cutting conditions. Coordination of work feed with grinding wheel rotation is an important aspect to be considered for the minimization of scallops on FRP surfaces during ESG. A detailed theoretical model to guide the selection of work feed for a scallop free surface with reduced waviness, roughness and defects, with a demonstration of the associated graphical user interface, has been introduced in this paper. Experiments reported to demonstrate the methodology and capability of intermittent-progressive cutting scheme may provide useful guidance for the adaptation of ESG in similar FRP machining applications. Studies organized in this paper provides an in-depth practical insight towards a futuristic minimal damage grinding strategy for fibre reinforced polymer composites in critical functional applications.
查看更多>>摘要：As important structural freeform panels (FFPs) for medical implants and aerospace system, lightweight thin wall panels with lower ductility are conventionally produced by hot forming process by utilizing the die on the press, which is not efficient or cost-effective for small batch or customized manufacturing. The dieless incremental sheet forming (ISF) provides a promising solution with increased formability, but making sheet metal panels with low-ductility at room temperature is still a challenging work. Targeting to solve this puzzle, a novel ISF without edge constraint on the target blank between the two edge-fixed supporting layers is developed to get flexible and free plastic deformation for the target blank. To reveal the deformation mechanism of this new method, an analytical prediction model of sheet thickness in ISF process is developed to calculate the deformed thickness in severe thinning region, which indicates that the undesired severe thinning region of conventional ISF process is caused by different stress states near the constrained edge, and can be completely eliminated by this proposed method to obtain uniform plastic deformation and improved formability. Different FFPs of magnesium alloy AZ31 and titanium alloy TC4 are fabricated by the novel flexible free ISF (FFISF) to validate the improved formability, and compared with conventional ISF and three-sheet incremental forming. Experimental studies demonstrate that the proposed method provides potentials for flexible fabrication of FFPs with low ductility at room temperature. More importantly, the promising geometric accuracy without trimming can be obtained by this new method, and multiple axisymmetric parts can also be formed within a cycle time.
查看更多>>摘要：Three-dimensional integration using through-silicon via (TSV) can significantly improve the performance and power consumption of microelectronic devices. In order to connect more chips vertically using TSV, the silicon wafer should be as thin as possible. As the most widely used method for wafer thinning, constant-feed grinding inevitably introduces serious mechanical damage and stress within a wafer, while common stress-relief processes such as chemical mechanical polishing (CMP) and etching cannot effectively balance the material removal rate and surface quality. To address these issues, in the present research, a novel thinning method was proposed that sequentially used constant-pressure diamond grinding and fixed-abrasive CMP to thin a wafer. It was found that by selecting the proper abrasive size and pressure, the constant-pressure diamond grinding can achieve both high-efficiency thinning and low-damage ductile removal of silicon. However, this method always generated processing stresses of 500-2000 MPa on the surface and produced a stress propagation layer with a depth of several tens of microns. The fixed-abrasive CMP utilized the tribochemical reaction between the CeO2 abrasive and silicon under friction to achieve effective material removal. The results demonstrated that this approach could provide an ultra-smooth surface with a roughness less than Ra 2 nm while the processing stress was no more than 150 MPa, making it ideal for the removal of grinding damage layers. By conducting sequential thinning with constant-pressure diamond grinding and fixed-abrasive CMP, this method proved to be an effective technique for the preparation of ultra-thin silicon wafers, exhibiting superior application prospects.
NSTL
Elsevier