Thesis (M.A.)--Boston University

Este trabalho estuda o processo de trefilação combinada do aço SAE 1045, que consiste nas etapas de pré-endireitamento com múltiplos rolos, jateamento, trefilação, corte e endireitamento por rolos, tendo como produto final barras acabadas trefiladas. Este produto final é empregado na fabricação de hastes de amortecedores para indústria automotiva. O Objetivo do estudo é de entender como são geradas e como se comportam as principais heterogeneidades mecânicas do material processado e a influência dos parâmetros do processo e das características do material na formação e modificação dessas heterogeneidades, buscando como resultado parâmetros otimizados para o processo. Foram variados no processo três parâmetros e avaliadas a interação existente entre eles. Os parâmetros modificados foram ângulo da ferramenta de trefilação (fieira), o ângulo de endireitamento e polimento por rolos cruzados (PERC) e a colocação de revestimento de carbo-nitreto de titânio (TiCN) no núcleo da ferramenta de trefilação (fieira). Foi realizada caracterização do aço, como microestrutura, composição química e microdureza. As tensões residuais superficiais das amostras foram caracterizadas utilizando o método de difração de raios-X. Após avaliação das tensões residuais superficiais, foi analisada a variação das tensões com a profundidade através do método do furo cego (hole-drilling), A classificação das inclusões foi de acordo com o método microscópico A (“piores campos”) da norma ASTM E45-97, feita através da maior severidade encontrada analisando o material aleatoriamente através de varredura com microscópio em sua superfície. Os resultados obtidos permitiram conhecer a combinação de ângulo de fieira e de endireitamento por rolos cruzados (PERC) mais favoráveis para gerar níveis baixos de tensões residuais. Além do ângulo, foi possível verificar que a geometria das zonas da ferramenta de trefilação (fieira) influência os níveis de tensões residuais após a trefilação. O núcleo da fieira com revestimento em todos os ensaios realizados apresentou menores níveis de tensões residuais, mostrando ser uma condição importante para a otimização do processo principalmente para a redução das tensões trativas após a trefilação. / This paper studies the process of cold drawing combined SAE 1045 steel, which consists of the steps with multiple pre-straightening rolls, sandblasting, drawing, cutting and straightening by rollers, with the final product finished extruded bars. This final product is used in the manufacture of rods of shock absorbers for the automotive industry. The objective of the study is to understand how they are generated and how they behave the main mechanical heterogeneities of the processed material and the influence of process parameters and material characteristics of the formation and modification of these heterogeneities, seeking as a result the optimized parameters for the process. We varied three parameters in the process and evaluated the interaction between them. The parameters were modified angle of wire drawing tool (spinneret), the angle of straightening and polishing rolls (PERC) and the placement of coating titanium carbo-nitride (TiCN) in the core of the wire drawing tool (spinneret). We performed characterization of steel, such as microstructure, chemical composition and microhardness. The surface residual stresses of the samples were characterized using the method of X-ray diffraction. After evaluation of surface residual stresses, we analyzed the variation of stress with depth by the method of blind hole (hole-drilling), Classification of inclusions was according to the microscopic method ("worst fields") of ASTM E45 -97, made more severe by analyzing the material found randomly through scanning with microscope on its surface. The results obtained allow us to know the combination of angle stringer and straightening by rollers (PERC) more favorable to generate low levels of residual stresses. In addition to the angle, we found that the geometry of the areas of wire drawing tool (spinneret) influence the levels of residual stress after wire drawing. The core of the spinneret clad in all tests had lower levels of residual stress, proving to be an important condition for the optimization of the process mainly to the reduction of tensile stress after wire drawing.

This study examines the use of one finishing electrode to finish multiple dies without remachining the electrode. The multiple-use electrode finishing experiment in this study addresses technology in the die-forging industry. Methods of manufacturing spherical straight bevel forge gear dies have relied on die-sinking Electrical Discharge Machining (EDM) practices that showed great potential for advancement. The focus of this study is solely on the improvements of electrode use in EDM finishing-processes. The surface finish quality itself is not an area of concern other than maintaining that it does not diminish. The focused concern is maximizing the process by using one electrode unmodified for multiple-finishing operations. The objective for improvement is utilization of one finishing electrode used multiple times rather than only one finishing electrode per die. Utilizing a Coordinate Measuring Machine (CMM), the inspection of specific locations on the finishing electrode reveals the repeatability and accuracy of use for one finishing electrode for six gear forging-dies. Initial experimentation validates the capabilities to finish four dies accurately in two separate die configurations with one electrode. To accomplish finishing the four initial dies, a die-sinking EDM machine that possesses a large enough working envelope was included in the process. The transition of using graphite electrode materials in place of brass for finishing multiple dies aids in reducing what was a total eight-hour process time into a four-hour process time.A machine with a working envelope large enough for only setting up one die to be EDM machined generated the eight-hour process time. The researcher achieved the eight-hour process time by replacing brass electrodes in the roughing stages with graphite electrodes. The extent to which one finish electrode can finish a sample set of six complete dies with one electrode is studied. Data is extrapolated from the deviation of absolute locations on a three-dimensional solid model compared to the multiple-use finishing electrode. Specific locations inspected on the electrode conclude the study efforts with results revealing that the maximum repeated use of an electrode is seven uses.

Extrusion is a continuous solid state deformation process which is widely used in the aluminium industry. The demand for aluminium extrudates are growing and extruders are pressurized to extrude products as fast as possible without lowering the quality of the product. Important extrusion parameters and conditions are exit temperature, extrusion speed and alloy composition. It is widely accepted in extrusion industry that extrusion surface defects increase when the extrusion speed and exit temperature are increased for a constant alloy. One of the major surface defects is the so-called die pick-up and it is presently uncertain if increase with extrusion speed (from a low 25m/min) would result in an increase of the number of die pick-up defect.Die pick-up appears like a scratch mark or comet on the surface of the extrudate which damages the appearance. Previous research suggests that second phase particles, eutectic reactions (555°C - 600°C), extrusion process conditions and die conditions may influence the cause of die pick-up. However the influencing factors for die pick-up are not well established.The research started by determining the lowest melting temperature for AA6060 alloy as this temperature limit the highest temperature above which incipient melting starts. This temperature corresponds to the eutectic melting temperature for AA6060 alloy. Eutectic melting was only detected above 610°C and therefore the exit temperature could be increased to a maximum of 610°C. For an AA6xxx alloy system the lowest melting temperature is 555°C if Mg2Si and excess silicon were present. However as Mg2Si may have fully dissolved into the solid solution, no reaction can take place.A preliminary investigation was conducted to study the characteristics of the newly installed extrusion control and monitoring system. Through this study the relationship between the set extrusion speed and the actual extrusion speed was established. It was found that the actual extrusion speed was lower than the set extrusion speed and was further complicated by the capacity limit of the extrusion pressure. Exit temperature measurements were accurate, however it was measured about 1m away from the die exit. Experiments were carried out to estimate the exit temperature drop and hence the exit temperature measurements were corrected accordingly.Thus, the aim of the present research was to establish the relationship between die pick-up and extrusion conditions (extrusion speed, exit temperature and die condition) and to propose the likely formation mechanism for die pick-up.In this research AA6060 alloy was used and was extruded at 25m/min, 30m/min, 35m/min, 40m/min and 45m/min. The exit temperature was found to increases from 542°C to 567°C. Three types of die pick-up was identified which were named as normal pick-up, die line pick-up and lump pick-up. Normal pick-up occurred regardless of the extrusion speed and exit temperature; however the amount of normal pick-up did not increase when the extrusion speed was increased. Die line pick-up occurred when the extrusion speed was 45m/min and appeared only on the die lines. Lump pick-up is not significant since it was very rare.AA6060 (0.4%Mg and 0.5%Si) alloy has about 0.27% excess silicon and therefore at 555°C, Mg2Si particles react with aluminium and excess silicon to form liquid. However normal pick-up and die line pick-up still occurred at temperatures lower and higher than 555°C and therefore it confirms that eutectic reactions do not influence formation of pick-up. Therefore die pick-up is most likely to be caused due to a mechanical process rather than a metallurgical process.

In this paper, Finite Element method and full-scale experiments have been used to study a hot forging method for fabri-cation of a spindle using reduced initial stock size. The forging sequence is carried out in two stages. In the first stage, the hot rolled cylindrical billet is pre-formed and pierced in a closed die using a spherical nosed punch to within 20 mm of its base. This process of piercing or impact extrusion leads to high strains within the work piece but requires high press loads. In the second stage, the resulting cylinder is placed in a die with a flange chamber and upset forged to form a flange. The stock mass is optimized for complete die filling. Process parameters such as effective strain distribution, material flow and forging load in different stages of the process are analyzed. It is concluded from the simulations that minor modifications of piercing punch geometry to reduce contact between the punch and emerging vertical walls of the cylinder appreciably reduces the piercing load. In the flange chamber, a die surfaces angle of 52° instead of 45° is pro-posed to ensure effective material flow and exert sufficient tool pressure to achieve complete cavity filling. In order to achieve better compression, it is also proposed to shorten both the length of the inserted punch and the die “tongues” by a few mm.

As semiconductor technology advances into the nano-scale era and more functional blocks are added into systems on chip (SoC), the interface between circuit design and manufacturing is becoming blurred. An increasing number of features, traditionally ignored by designers are influencing both circuit performance and yield. As a result, design tools need to incorporate new factors. One important source of circuit performance degradation comes from deterministic within-die variation from lithography imperfections and Cu interconnect chemical mechanical polishing (CMP). To determine how these within-die variations impact circuit performance, a new analysis tool is required. Thus a methodology has been proposed to involve layout-dependent within-die variations in static timing analysis. The methodology combines a set of scripts and commercial tools to analyze a full chip. The tool has been applied to analyze delay of ISCAS85 benchmark circuits in the presence of imperfect lithography and CMP variation. Also, this thesis presents a methodology to generate test sets to diagnose the sources of within-die variation. Specifically, a delay fault diagnosis algorithm is developed to link failing signatures to physical mechanisms and to distinguish among different sources of within-die variation. The algorithm relies on layout-dependent timing analysis, path enumeration, test pattern generation, and correlation of pass/fail signatures to diagnose lithography-caused delay faults. The effectiveness in diagnosis is evaluated for ISCAS85 benchmark circuits.

Die surface shape may improve the flow of materials, reduce stress concentration of the products, and decrease the processing load to extend the life of die. The objective of this paper is to show that how to design the die surface shape of T-shape protrusion hydroforming with different diameters. A finite element code DEFORM 3D is used to simulate the process of THF, including adaptive simulation to predict the internal pressurization in the tube, and utilize flow net distribution to predict the axial feeding stroke and counter punch (CP) movement. After the amendment to the loading path, the flowability and appearance of the product quality will achieve the best results. Experiments of T-shape warm hydroforming of magnesium alloy AZ61 tubes are. The forming temperature is set as 250¢J. The simulated loading paths are used. From the comparisons of product shape, thickness distribution between analytical and experimental values, the validity of this analytical model is verified.. Keywords: Tube hydroforming, Finite element simulation, Die surface design.

The effect of laser power pattern on the temperature and thermal stress distributions in LED die bonding process is investigated in this work. The wavelength of 940nm diode laser source is used in this study. The laser light is focus on the back of an AlN substrate. The eutectic Au80Sn20 solder metallized between die and substrate is soldered by the heat conducted from the controlled laser power. The finite element package software-MSC. Marc is employed to simulate the laser soldering process. The thermal-elastic-plastic models of the solid elements are used. The temperature dependent material properties are applied to characterize the temperature variation effect during the die bonding. The measured temperature data have also been used to derive the absorption coefficient, conductivity, specific heat of AlN substrate and the convection coefficient in free convection via the inverse engineering process. A difference between the simulated and measured temperature can be kept in 10%. The temperature and thermal stress distributions during the die bonding process have been simulated and studied. The distributions of residual stress induced in this die bonding process have also been studied. The effects of different laser soldering parameters, e.g. focus shift, defocus, inclined angle, on the die bonding are also studied.

Extrusion is a metal forming process used extensively in industry to produce different structural, mechanical, electrical, architectural, automotive and aerospace application parts. Currently after extrusion, the rod is subjected to environmental wear due to long storage time and hence requires an additional cleaning process before further use. This cleaning process can be eliminated by extruding a polymer coated metal rod workpiece such that the polymer coating is sustained on the final product after the extrusion process. In the present research study a new upper bound analytical model is developed to predict the forces required to conduct extrusion of a polymer coated metal rod successfully. The search for the lower upper bound power functional is modeled as a non linear optimization problem. Optimizing the functional also determines the set of constraints defining the shape of rigid plastic deformation boundaries and the final coating thickness. Also an upper bound analytical model was developed to predict forces for failure of the polymer coating during the extrusion. Both the analytical models for successful and failed extrusion are compared to obtain critical die angle which can provide tooling and process design guidelines. Finite element analysis simulations were modeled using commercially available software package, ABAQUS. Predictions of FEA simulations were in good agreement with published results and with the predictions of analytical model developed in this study.

Univ., Diss.-2006--Osnabrück, 2005. / Literaturverz. S. 443 - 469.