Neil Krishnan

Microforming: Experimental Investigation of the Extrusion Process for Micropins and its Numerical Simulation Using RKEM

Microforming using a small machine (or so-called desktop machine) is an alternative new approach to those using full-size heavy equipment for manufacturing microparts. Microparts are commonly defined as parts or structures with at least two dimensions in the submillimeter range, which are used extensively in electronics and micromechanical products. However when scaling down a conventional forming process to microscale, the influence of the so-called size effect needs to be considered. The individual microstructure (size, shape, and orientation of grains) and the interfacial conditions show a significant effect on the process characteristics. In this paper, the process of extrusion is investigated to establish it as a viable process for microforming. A forming assembly is fabricated and used in conjunction with a loading substage to extrude micropins with a final diameter of I mm. The effect of grain size is investigated by using workpieces heat treated to produce grain sizes varying from 32 μm up to 211 μm. Two extrusion dies with different roughness are used to study the effect of surface finish. While experiments lead to interesting questions and new discoveries, theoretical or numerical solutions are necessary tools for process optimization. Here, knowing the limits of the current widely used numerical simulation tools [i.e., the Finite Element Method (FEM)], a new method, the Reproducing Kernel Element Method (RKEM), has recently been developed to address the limitations of the FEM (for example, remeshing issue), while maintaining FEMs advantages, e.g., the polynomial reproducing property and function interpolation property. The new RKEM method is used to simulate the microextrusion problem. Its results are compared with that obtained from the FEM and the experiment result. Satisfactory results were obtained. Future directions on the experimental and simulation work are addressed.

Investigation of deformation size effects during microextrusion

Microextrusion has recently emerged as a feasible manufacturing process to fabricate metallic micropins having characteristic dimensions on the order of less < 1 mm. At this length scale, the deformation of the workpiece is dominated by the so-called size effects, e.g., material property and frictional behavior variations at small length scales. In extrusion experiments performed to produce submillimeter-sized pins having a base diameter of 0.76 mm and an extruded diameter of 0.57 mm, the extruded pins exhibited a curving tendency when a workpiece with a relatively coarse grain size of 211 μm was used. This phenomenon was not observed when workpieces with a finer grain size of 32 μm were used. In this paper, results from microhardness tests and microstructure analyses for both grain sizes are presented to investigate this phenomenon and to characterize the deformation during microextrusion. The results obtained from this analysis show that as the grain size approaches the specimen feature size, the deformation characteristics of the extruded pins are dominated by the size and location of specific grains, leading to a nonuniform distribution of plastic strain and measured hardness and, thus, the curving tendency. Microhardness tests of the initial billet material and tensile test specimens are also presented as supplementary analyses.

An approach in modeling the temperature effect in thermo-stamping of woven composites

Made with high-strength continuous fibers, textile composites are of increasing interest in automotive and aerospace industries due to their high-strength/weight performance as compared to sheet metals. Nevertheless, significant reduction in manufacturing cost is required to use textile composites for mass production applications. Highly efficient thermo-stamping operations possess the potential to substantially reduce fabrication time and cost compared to the much slower autoclave forming process. In this paper, thermo-forming of woven fabric-reinforced thermo-plastic composites is simulated using a non-orthogonal material model. The temperature effect is taken into account by modifying the equivalent material properties for the composite sheet based on the contact status between the tooling and the blank. The approach is exemplified on the hemispherical thermo-stamping of a plain weave composite sheet.

Estimation of Optimal Blank Holder Force Trajectories in Segmented Binders Using an ARMA Model

Sheet metal forming is one of the most important and frequently used manufacturing processes in industry today. One of the key parameters affecting the forming process is the blank holder force (BHF). In the past, researchers have demonstrated the advantages of varying the blank holder force during the forming process, that is, the two primary modes of failure in sheet metal forming (wrinkling and tearing) are avoided. This gives rise to improved formability, higher accuracy and better part consistency. In recent years, researchers have also shown increasing interest in forming processes where the blank holder force is varied spatially with the help of segmented binders or flexible binders. In this paper, we have combined the above two aspects and used a robust method to deter, mine the blank holder force trajectories for a non-circular part using segmented binders. The proposed strategy is verified by implementing it into a finite element simulation. Binder force is treated as a system input. The displacement of the binder is used as a measure of the tendency to wrinkle, and is therefore treated as a system output. The parameters of the system are continuously identified and updated using a deterministic Auto-Regressive Moving-Average model (ARMA). The model is then used to control the binder displacement to a prescribed value by adjusting the system input, i.e., the binder force. In this manner, individual binder force profiles for each of the segmented binders are obtained. Due to the generic nature of the ARMA model, the strategy proposed in this paper can be applied to a variety of forming problems, making it a robust approach.

Study of the Size Effects and Friction Conditions in Microextrusion—Part II: Size Effect in Dynamic Friction for Brass-Steel Pairs

In this paper, the results of experiments conducted to investigate the friction coefficient existing at a brass-steel interface are presented. The research discussed here is the second of a two-part study on the size effects in friction conditions that exist during microextrusion. In the regime of dimensions of the order of a few hundred microns, these size effects tend to play a significant role in affecting the characteristics of microforming processes. Experimental results presented in the previous companion paper have already shown that the friction conditions obtained from comparisons of experimental results and numerical models show a size effect related to the overall dimensions of the extruded part, assuming material response is homogeneous. Another interesting observation was made when extrusion experiments were performed to produce submillimeter sized pins. It was noted that pins fabricated from large grain-size material 211 m showed a tendency to curve, whereas those fabricated from billets having a small grain size 32 m, did not show this tendency. In order to further investigate these phenomena, it was necessary to segregate the individual influences of material response and interfacial behavior on the microextrusion process, and therefore, a series of frictional experiments was conducted using a stored-energy Kolsky bar. The advantage of the Kolsky bar method is that it provides a direct measurement of the existing interfacial conditions and does not depend on material deformation behavior like other methods to measure friction. The method also provides both static and dynamic coefficients of friction, and these values could prove relevant for microextrusion tests performed at high strain rates. Tests were conducted using brass samples of a small grain size 32 m and a large grain size 211 m at low contact pressure 22 MPa and high contact pressure 250 MPa to see whether there was any change in the friction conditions due to these parameters. Another parameter that was varied was the area of contact. Static and dynamic coefficients of friction are reported for all the cases. The main conclusion of these experiments was that the friction coefficient did not show any significant dependence on the material grain size, interface pressure, or area of contact. DOI: 10.1115/1.2738131

Microforming: Study of Friction Conditions and the Impact of Low Friction/High-Strength Die Coatings on the Extrusion of Micropins

Microforming is a relatively new realm of manufacturing technology that addresses the issues involved in the fabrication of metallic microparts, i.e., metallic parts that have at least two characteristic dimensions in the sub-millimeter range. The recent trend towards miniaturization of products and technology has produced a strong demand for such metallic microparts with extremely small geometric features and high tolerances. Conventional forming technologies, such as extrusion, have encountered new challenges at the micro-scale due to the influence of ‘size effects’ that tend to be predominant at this length scale. One of the factors that shows a strong influence is friction. This paper focuses on the frictional behavior observed at various sample sizes during micro-extrusion. A novel experimental setup consisting of forming assembly and a loading stage has been developed to obtain the force-displacement response for the extrusion of pins made of brass (Cu/Zn: 70/30). This experimental setup is used to extrude pins with a circular cross-section that have a final extruded diameter ranging from 1.33 mm down to 570 microns. The experimental results are then compared to finite-element simulations and analytical models to quantify the frictional behavior. It was found that the friction condition was non-uniform and showed a dependence on the dimensions (or size) of the micropin. The paper also investigates the validity of using high-strength/ low friction die coatings to improve the tribological characteristics observed in micro-extrusion. Three different extrusion dies coated with diamond-like carbon with silicon (DLC-Si), chromium nitride (CrN) and titanium nitride (TiN) were used in the micro-extrusion experiments. All the coatings worked satisfactorily in reducing the friction and correspondingly, the extrusion force with the DLC-Si coating producing the best results.Copyright

Investigation of deformation characteristics of micropins fabricated using microextrusion

Microextrusion has recently emerged as a feasible manufacturing process to fabricate metallic micropins having characteristic dimensions of the order of less than 1 mm. At this length scale the deformation of the workpiece is dominated by the so-called ‘size effects’, e.g. material properties and frictional behavior vary at small length scales. In recent extrusion experiments performed to produce sub-millimeter sized pins having a base diameter of 0.76 mm and an extruded diameter of 0.57 mm, certain interesting deformation characteristics were observed. When a workpiece with a relatively large grain size of 211 μm was used, the billet tended to deform inhomogenously, and the extruded pins showed a tendency to curve. This phenomenon was not seen when workpieces with a smaller grain size of 32 μm were used. It is believed that the relative size and orientation of the large grains in the 211 μm grain size sample are responsible for this behavior and the aim of this paper is to investigate this phenomenon. Microindentation tests were performed on micropins extruded from workpieces of both grain sizes to obtain a measure of the distribution of induced strain. The results obtained from this analysis show that the deformation characteristics of the extruded pins are dominated by the size and location of specific grains leading to a non-uniform distribution of plastic strain and measured hardness.Copyright

Comparison of Analytical Model to Experimental and Numerical Simulations Results for Tailor Welded Blank Forming

Tailor welded blanks (TWBs) offer several notable benefits including decreased part weight, reduced manufacturing costs, and improved dimensional consistency. However the reduced formability and other characteristics of the forming process associated with TWBs has hindered the industrial utilization of this blank type for all possible applications. One concern with TWB forming is that weld line movement occurs, which alters the final location of the various materials in the TWB combination. In this technical brief, an analytical model to predict the initial weld line placement necessary to satisfy the desired, final weld line location and strain at the weld line is used. Results from this model are compared to an experimental, symmetric steel TWB case and a 3D numerical simulation, nonsymmetric aluminum TWB case. This analytical model is an extension of one previously presented, but eliminates a plane strain assumption that is unrealistic for most sheet metal forming applications. Good agreement between the analytical model, experimental, and numerical simulation results with respect to initial weld line location was obtained for both cases. Results for the model with a plane strain assumption are also provided, demonstrating the importance of eliminating this assumption.

Using FEA Simulations to Determine Advanced Process Parameters for Tailor Welded Blank Forming

In order to realize the potential of sheet metal forming and take advantage of new process control capabilities, innovative modifications to the traditional sheet metal forming process must be developed. These modifications are particularly important with respect to Tailor Welded Blank (TWB) forming, which offers an excellent opportunity to reduce manufacturing costs, decrease part weight, and improve the quality of sheet metal stampings. However, tearing near the weld seam and wrinkling in the formed wall area and die addendum of the part often occurs when a traditional forming process is used to form a TWB. Research and industrial experience has shown that these forming concerns can be alleviated through advanced forming techniques, for example using a segmented die process or a non-uniform binder force. The difficulty then becomes determining the key process parameters associated with these forming methods. In this paper, a methodology is presented to effectively and easily determine both the location of a segmented die and a non-uniform binder force by evaluating nodal reaction forces provided from FEA simulations. Also, using FEA simulations to determine the process parameters for another advanced forming process, strain path control tooling, is discussed. The advanced forming processes presented in this paper and the use of FEA to determine key process parameters are critical components to the continued evolution of sheet metal forming processes.Copyright

Characterization and Investigation of Deformation During Microextrusion Using X-Ray Texture Analyses

In microforming scaling down the size of the process while the grain size is kept relatively constant usually results in inhomogeneous deformation. In most works, the inhomogeneous deformation of miniaturized samples is presented and evaluated by microstructure analyses of the deformed grains. However, in certain microforming processes, such as microextrusion, where the final texture of the conventional macro size samples is well known, texture analyses can provide useful information about the deformation. In our past research, extrusion experiments were performed to produce sub-millimeter sized pins having a base diameter of 0.76 mm and an extruded diameter of 0.57 mm. Curvature of differing degrees and directions was observed in workpieces with a coarse grain size of 211 μm. However, a similar effect did not occur in workpieces with a fine grain size of 32 μm. Microstructure analyses showed that when the sample size approaches the grain size, the deformation becomes inhomogeneous and the properties of individual grains can dominate the overall deformation of their cross-sections. Moreover, microhardness measurements revealed that deformation size effects are present and as a result the coarse grained pins strain hardened more than the fine grained pins during microextrusion. This result along with microstructure analyses suggested that the coarse grains in the central region possibly undergo more shear deformation. In this paper, X-ray texture analyses of the pins were performed to validate that there is penetration of shear deformation into the central regions of the coarse grained pins. Also, the texture analyses point to the possibility that the deformation in the curved region of the coarse grained pins is not axially symmetric which causes the curvature observed.© 2007 ASME