Jyhwen Wang

Gaussian process method for form error assessment using coordinate measurements

A Gaussian process method for modeling and assessing form errors is presented. The Gaussian process method decomposes a geometric feature into three components: designed geometric form, systematic manufacturing errors and random manufacturing errors. It models the systematic manufacturing errors as a spatial model using a Gaussian correlation function and models the random manufacturing errors as independent identically distributed noises. Based on a handful of coordinate measurements, the Gaussian process model reconstructs the part surface and assesses the form error better than traditional methods. The Gaussian process method also provides an empirical distribution of the form error, allowing engineers to quantify the decision risk on part acceptance. This method works for generic geometric features. The method is implemented on two common features: a straight and a round feature. Simulated datasets as well as actual coordinate measuring machine data are used to demonstrate the improvement achieved by the proposed method over the traditional approaches.

Tube bending under axial force and internal pressure

Tube bending is a widely used manufacturing process in the aerospace, automotive, and various other industries. During tube bending, considerable in-plane distortion and thickness variation occurs. Additional loadings such as axial force and internal pressure can be used to achieve better shape control. Based on plasticity theories, analytical models are developed to predict cross-sectional distortion and thickness change of tubes under various loading conditions. The model predictions are in good agreement with finite element simulations and published experimental results. The models can be used to evaluate tooling and process design in tube bending.

Finite element analysis of dual hydroforming processes

Abstract The tube hydroforming process has gained increasing attention in recent years. With the hydroforming process, manufacturers can realize substantial cost savings as it provides reduced part count, increased part strength and stiffness, and reduced weight. Coordination of the pressurization and feeding curves is critical to generate successful parts without fracture or wrinkling failure. Herein a new process parameter, counter pressure, is introduced to achieve favorable tri-axial stress state during deformation process. This paper will establish the merits of applying external counter pressure in tube hydroforming. It is observed that the counter pressure will provide back support to the tube material. Excessive thinning and premature wrinkling could be prevented. Thus, larger tube expansion could be achieved. The process will be referred as dual hydroforming.

Lead-free Solder Joint Reliability – State of the Art and Perspectives

There is an increasing demand in replacing tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry due to health and environmental concern. The European Union recently passed a law to ban the use of lead in electronic products. The ban will go into effect in July of 2006. The Japanese electronics industry has worked to eliminate lead from consumer electronic products for several years. Although currently there are no specific regulations banning lead in electronics devices in the United States, many companies and consortiums are working on lead-free solder initiatives including Intel, Motorola, Agilent Technologies, General Electric, Boeing, NEMI and many others to avoid a commercial disadvantage. The solder joints reliability not only depends on the solder joint alloys, but also on the component metallization and PCB metallization. Reflow profile has significant impact on lead-free solder joint performance also because it influences wetting and microstructure of the solder joint. Majority researchers use temperature cycling for accelerated reliability testing since the solder joint failure mainly comes from thermal stress due to CTE mismatch. A solder joint failure could be caused by crack initiation and growth or by macroscopic solder facture. There are conflicting views of the reliability comparison between lead-free solders and tin-lead solders. This paper first reviews lead-free solder alloys, lead-free component finishes, and lead-free PCB surface finishes. Tin whisker issue is also discussed. Then the lead-free solder joint testing methods are presented; finite element modeling of lead-free solder joint reliability is reviewed; and experimental data comparing lead-free and tin-lead solder joint reliability are summarized. Finally the paper gives perspectives of transitions to a totally leadfree manufacturing.

Modeling Springback of Metal-Polymer-Metal Laminates

Metal-polymer-metal laminate is an emerging material that has many potential applications. The laminated structure consists of two outer layers of sheet metal and a polymeric center core. The material offers excellent sound deadening properties and is being introduced to applications where noise reduction is desired. Part manufacturing for laminates involves converting a flat sheet into a deformed body. Springback has been a major concern in shape control. While bending of a single layered sheet metal does not exhibit significant sidewall curl, the problem is pronounced in bending laminates. This paper presents an analytical approach to predict springback and sidewall curl of laminates due to wiper die bending. Based on the integration of a straight beam and a curved beam models, the springback factor K s is calculated. It is shown that the prediction is in good agreement with the published experimental data. Application of the integrated model to minimize the springback and side wall curl is demonstrated. The analytical model leads to a simple expression that predicts the springback factor. The ability to predict the shape analytically is significant, since other methods require extensive finite element simulation of the deformation process.

Analysis of draw–redraw processes

Abstract An analytical model is developed to analyze the draw–redraw processes. Thickening of the flange is modeled using a complete pure radial drawing analysis which takes into account interfacial friction and radial thickness variation. A plastic bending analysis is used to calculate thinning in all forming radii. Material properties, tooling geometry, and process parameters are included in the model. The derivation is then developed into computer subroutines. By arranging the subroutines into the deforming sequence, draw–redraw process can be simulated. The predicted wall thickness profiles agree very well with the experimental measurements. Characterization of wall thickness profile is also discussed.

Influence of Process Parameters on the Ironing of Deep-Drawn Cups

An analytical model for the ironing of deep-drawn cups is developed using the slab method. Inhomogeneous deformation of the drawn cup is taken into consideration. Elastic recovery after a cup being ironed is also modeled. Predictions on punch load and frictional force at the punch-cup interface show excellent agreements with experimental measurements. Influence of die semi-angle, ironing reduction ratio and friction on the process is investigated. With this model, new process parameters for ironing can be designed and analyzed before actual die tryout.

Thermal Effects on Polymer Laminated Steel Formability in Ironing

In the current manufacturing processes for can making, a time consuming and therefore expensive process involves spraying a food-contact safe polymer coating onto the can interior before filling. This process can be eliminated by using a prelaminated metal workpiece as long as the polymer will survive the manufacturing operations involved in can making. The most demanding operation in can making is ironing because of the high pressures involved as well as the necessary generation of new surface Previous research [5] has demonstrated the feasibility of using a polymer coated steel sheet stock for can making. However, ironing is commonly performed with elevated tooling temperatures which result from friction and plastic deformation in the workpiece. As such, it is possible that the polymer could significantly soften or melt during the ironing process when tooling/workpiece contact is most intimate. In this paper, the thermal effects of hot tooling on polymer coated steel formability are explored through both experiments and mathematical models.

Microstructural Refinement of Niobium for Superconducting RF Cavities

The mechanical properties of commercial polycrystalline pure niobium sheet used for superconducting radiofrequency cavities are known to provide inconsistent yield, springback and surface smoothness characteristics when plastically formed into a radiofrequency cavity. These inconsistent properties lead to significant variations in cavity geometry and thus superconducting cavity performance. One approach to reduce these problems is to refine the microstructure so that its properties are more uniform. Microstructural refinement of Nb sheet for RF cavities using multi-axis severe plastic deformation via equal channel angular extrusion (ECAE) was examined. ECAE was performed on 25 mm square cross-section bars of Reactor Grade Nb in a right angle die at room temperature following different extrusion routes to true strains above nine. This heavily worked material was rolled to 4 mm thick sheet and recrystallized. Measurements of hardness, springback, texture, and microstructural uniformity are reported and compared to those of commercial RRR Grade Nb sheet. Preliminary results show noteworthy promise for bulk Nb processed via severe plastic deformation prior to sheet rolling.

Design optimization of rigid metal containers

Evaluating the structural capability of a food container has been viewed as a challenging task. With more powerful computer hardware and software, finite element methods are now in place for efficient and accurate assessment of axial load and paneling performances of cans. Thus, the metal container optimization problem is tackled with FEA tools. In this paper, the structure tests and numerical simulation methods are presented. The effects of design parameters on the structure performance of containers are discussed. A design optimization method is then developed based on the solution of point inclusion problem in computational geometry. The iterative planar subdivision algorithm generates the optimal design that meets the structure requirements with least material consumption. The proposed method can be used to optimize the existing and new metal containers.