外文翻译--快速成型与虚拟成型在产品设计和制造中的应用 英文版

桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 1 页 共 29 页 Rapid Prototyping Versus Virtual Prototyping in Product Design and Manufacturing C. K. Chua1, S. H. Teh1 and R. K. L. Gay2 School of Mechanical & Production Engineering； and 2Gintic Institute of Manufacturing Technology, Nanyang Technological University, Singapore Abstract Rapid prototyping (RP) is the production of a physical model from a computer model without the need for any jig or fixture or numerically controlled (NC) programming. This technology has also been referred to as layer manufacturing, material deposit manufacturing, material addition manufacturing, solid freeform manufacturing and three-dimensional printing. In the last decade, a number of RP techniques has been developed. These techniques use different approaches or materials in producing prototypes and they give varying shrinkage, surface finish and accuracy. Virtual prototyping (VP) is the analysis and simulation carried out on a fully developed computer model, therefore performing the same tests as those on the physical prototypes. It is also sometimes referred to as computer-aided engineering (CAE) or engineering analysis simulation. This paper describes a comparative study of the two prototyping technologies with respect to their relevance in product design and manufacture. The study investigates the suitability and effectiveness of both technologies in the various aspects of prototyping, which is part and parcel of an overall design and manufacturing cycle. Keywords: Product design； Rapid prototyping； Virtual prototyping 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 2 页 共 29 页 1. Introduction Rapid prototyping (RP) is emerging as a key prototyping technology with its ability to produce even complicated parts virtually overnight. It enables product designers to shorten the product design and development process. The coming-of-age of this technology is clearly reflected in the inclusion of a stereolithography (STL) file generator in most, if not all, CAD. systems today. The STL file is the de facto standard used by RP systems in the representation of the solid 3D CAD models. While RP is a relatively young technology, virtual prototyping (VP) has been in steady development since the 1970s in many guises. Virtual prototyping is taken to mean the testing and analysis of 3D solid models on computing platforms. Today, VP is often tightly integrated with CAD/CAM software and sometimes referred to as CAE packages. It provides the ability to test part behaviour in a simulated context without the need to manufacture the part first 1. 2. Definitions of RP and VP Rapid prototyping (RP) is a widely used term in engineering, particularly in the computer software industry where it was first coined to describe rapid software development. This term has also been adopted by the manufacturing industry to characterise the construction of physical prototypes from a solid, powder, or liquid in a short period of time when compared to “traditional” subtractive machining methods. This technology has also been variously referred to as layer manufacturing, material deposit manufacturing, material addition manufacturing, solid freeform manufacturing and threedimensional printing 2. Virtual prototyping (VP) refers to the creation of a model in the computer, often referred to as CAD/CAM/CAE. Virtual or computational prototyping is generally understood to be the construction models of products for the purpose of realistic graphical simulation 1. In this paper, VP will refer to thesimulation, virtual reality and manufacturing process design domains 3. Nevertheless, there are many areas where the distinction between RP and VP is blurred. As RP systems rely on CAD systems to generate the files needed to produce the prototype, it would seem that RP is a downstream process from VP in the product or part development cycle. Indeed, Pratts definition of VP reveals the fact that VP is a term which is loosely used in the prototyping community. As such, it would be 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 3 页 共 29 页 appropriate to clearly define both RP and VP. Rapid prototyping will be taken to mean, as above, the production of a physical model from a computer model without the need of any jig or fixture or NC programming. This also includes other related processes and applications which use RP-produced objects, such as rapid tooling. Similarly, VP is defined as the subsequent manipulation of a solid CAD model as a substitute for a physical prototype for the purposes of simulation and analysis, and is not inclusive of the construction of the solid 3D model. VP includes the following functions: 1. Finite element analysis. 2. Mechanical form, fit and interference checking. 3. Mechanical simulation. 4. Virtual reality applications. 5. Cosmetic modelling. 6. Assemblability. The relationships between RP and VP are shown in Fig. 1. Fig. 1. Classification of RP and VP 3. Prototyping in Singapore Two selected multi-national companies (one American and one French) based in Singapore with significant product development activities showed differing approaches to both RP and VP. Both use RP in their prototyping activities. The first company, B, placed more emphasis on virtual prototyping. It 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 4 页 共 29 页 manufactures telecommunications equipment such as pagers and handphones. It is moving all prototyping applications upstream, which is to move prototyping from RP to VP. At present, their RP models are used only for proof of concept and marketing purposes. Other prototyping activities are being carried out with VP. The second company, C, manufactures consumer electronics products such as television sets, video cassette recorders and telephones. It uses VP only as a tool to create a solid 3D model. From the solid 3D model, C generates the STL file needed to produce the RP prototype. Company C then uses the RP part as a master for silicone rubber moulds to produce a limited number of physical ABS (polyacrylonitrite butadienestyrene) prototypes for the various prototyping tests and simulation. Company B intends to move more prototyping to VP, rather than using physical models. Virtual prototyping allows for improvements in reliability and quality as well as reducing costs. Manipulation of virtual prototypes makes it easier for B to implement design improvements compared to an iterative cycle using physical prototypes. Company B drafts the CAD models in Pro/ENGINEER, then uses Patran to pre-process the models. Static finite-element analysis (FEA) is carried out with ABAQUS Standard whereas dynamic scenarios are analysed with ABAQUS Explicit. ALIAS/Wavefront is used for cosmetic modelling when presenting different conceptual and actual designs. The bulk of the VP carried out by B uses FEA, which typically takes 46 weeks for a pager design. Of all the FEA carried out, the majority are concentrated on structural strength (static) analysis and drop test (dynamic) analysis. Vibration tests are occasionally carried out. Some cosmetic modelling is carried out, but usually only for presentation purposes. Finite-element analysis is used to investigate the following problems: Relative comparison of different design options； to see how one design compares to another. Possible failure modes are: 1. To evaluate a design change or design correction. 2. To assess the possibility of failure, based on past experience. 3. To make some educated-guess correlation with physical testing. 4. To try to identify what initiated a failure. According to B, the drawback of VP is that it cannot simulate process problems efficiently and effectively. The accuracy of FEA is also limited because of the 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 5 页 共 29 页 inconsistent behaviour of material. The amount of computing power also determines the accuracy of FEA. The application of RP is rather limited in B. The in-house laminated object manufacturing (LOM) RP system is used to produce design prototypes for proof of concept only, and not geometrical prototypes. Company C uses RP heavily, but has very little VP. The parts produced using RP range from audio products to 29-in. television casings. Typically, it takes 1 year from the conception of the product to the sale of the product. Company C aims to prototype all (mostly plastic) parts by RP. A comparison between numerically controlled (NC) machining of prototypes from ABS against RP is shown in Table 1. Company C projected 50% savings using an in-house RP system versus an NC machining system. CAD models are created using I-DEAS. The .STL format is then created for production of the RP part. The main purpose of the RP parts is to verify the design. Rapid prototyping parts are used for the following functions: 1. Form fitting. 2. Ergonomics check. 3. Proof of concept (to confirm design with industrial designers). 4. Manufacturability (design for tooling, design for assemblability). 5. Reliability check (whether part dislodges or breaks when force applied, especially snap-on covers). 6. Kinematic check. Company C offers some insight into the limitations of VP, in that VP is unable to model: 1. Tactile feeling (for buttons) not quantified； may be able to VP if able to quantify “pressing” force. 2. Assemblability (e.g. PCBs inserted at an angle, difficult to visualise). 4. Case Study 1: Prototyping of a Telephone Handset This case study investigates the design verification, assembly, interference check and form fitting aspects of both the RP and VP model. The production ABS, RP and VP parts or models were evaluated in the above aspects. The RP system used here is the stereolithography apparatus (SLA). Both the ABS and RP parts are shown in Fig. 2. Inspection of the RP parts reveal that: 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 6 页 共 29 页 1. The surface finish was much poorer than in the ABS part. 2. Warpage was clearly evident (see Fig. 3). 4.1 Design Verification As a true dimensional physical part, the RP model is able to give the designer a sense of size estimation. The judgement of a VP part can be erroneous because parts are often automatically sized to fit the viewing window. Another advantage of a physical part is that it allows for ergonomic checks, ranging from the fit of a telecommunications device in a users palm to the inspection of potentially dangerous corners and edges. Also, it offers tactile inspection which is crucial in products for which ergonomics is important, such as touch buttons on audio or video products, which is not possible on VP systems. Rounded edges which appear innocuous on a VP model may prove to be unsafe upon scrutiny of the RP part. Above all, most RP parts are produced for aesthetic evaluation purposes. Aesthetic evaluation is also possible on VP models. All CAD software allows the model to be viewed in any spatial orientation, along with at least rudimentary rendering capabilities. It is then possible to view the part under the desired simulated lighting conditions with millions of shading and colour combinations. RP parts cannot be coloured, thus surface preparation and painting introduce additional finishing processes. Any visibly apparent design discrepancies could be immediately rectified without having to invest in a physical part. It also allows designers to evaluate the aesthetics of the design and make corrections, if necessary. In the case of most multi-national companies, the design and manufacturing facilities are often a considerable distance apart and in different countries and continents. The ease with which CAD files can be sent and received via electronic means greatly helps the design process, be it iterative or concurrent. With identical or compatible CAD software, the prototyping process can be swift and cheap. Any design change of the virtual prototype can bemade almost instantly available to all parties involved in the design process. 4.2 Assembly Assembly of RP parts must be carried out quickly, as warpage and shrinkage increases with time. Warpage is a function of both part geometry design and shrinkage. All but the bestdesigned parts suffer from varying degrees of warpage and shrinkage. Some RP material such as the SLA inherently shrinks and the part is 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 7 页 共 29 页 actually built slightly larger to allow it to shrink to its proper dimensions. With such arrangements, assembly is possible but is often hampered by warpage and/or shrinkage. Some parts can be mated only with the application of some force. Assembly of RP parts allows the user not only to attempt different assembly sequences, but also if a part cannot be positioned in a linear movement, to insert the part, say, at an angle before being set into its proper location. The drawback in assembling RP parts is that for some RP parts such as SLA, the material is weak and brittle, and fails when attached using fasteners or under low to moderate loading (see Figs 4 and 5). CAD software allows for the assembly of parts and subassemblies in the form of 3D solid or surface models. Assembly in the virtual realm is very often used to check for interference and form fitting which will be discussed later. The ability of CAD software to assemble parts and/or subassemblies allows a product designer to quickly check to see if he or she has designed the part or parts correctly, i.e. whether a boss is tall enough to accept a screw inserted through another part or if two slots are aligned to form a larger slot. The advantage of assembling in a virtual environment is that no physical parts need be produced and thus this reduces cost. The absence of physical parts also means that tooling time is eliminated. The assembly in a virtual environment can be done in a matter of minutes or up to a few days, but is much faster than producing the physical parts and then assembling them. The user can also build or change a part, or modify its attributes when all instances of the part will be changed accordingly. Assembly relationships can be written in engineering parameters, part dimensions and orientation dimensions. The equations are solved variationally to allow for flexibility while working with the assembly. Evaluation of the tolerance specifications of the design to optimise the engineering performance at the lowest possible cost can be carried out. This allows the user to measure the sensitivity of a critical dimension in an assembly to changes in individual constraints. Manufacturing cost can then be reduced by tightening the tolerances which contribute most to the overall variation of a critical dimension, and loosening tolerances that have little impact. 4.3 Interference Check and Form Fitting Again, interference checking and form fitting is hampered by warpage and shrinkage of the RP part. Therefore, the problem of parts which interfere or fit poorly may be due to one or more of: warpage； shrinkage； or design error. Even when RP 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 8 页 共 29 页 parts fit well, there is no assurance that the parts are dimensionally correct, as shrinkage of two or more parts in the same direction or directions could still produce a good fit. When such situations arise, CAD models are often used to determine whether the interference or poor fit is due to design flaws. The ability to check for interference as well as form fitting is very widely used in CAD systems. It gives the user the ability to fit two parts together and check for interference without having to produce a part or parts which are potentially dimensionally incorrect, thereby increasing cost. The interactive nature of the process in a CAD system also frees the user or designer from the need to manually interpret engineering drawings to detect interference. This process also allows the user to establish tolerances which are crucial in the manufacturing process. The advantage of interference checking on a CAD system is not evident when an assembly consists of a small number of parts. For complex assemblies with a large number of parts, there are often many features on a particular part that must be mated or aligned with features on one or more other parts. CAD systems allow not only the detection of any misalignment or interference but also immediate rectification of the problem. Interference checking is performed by the CAD system on an assembly when required by the user, and is relatively faster and more accurate and precise than other methods. The CAD system would also identify and list the features which interfere. The user can then view the entities to rectify the situation. 5. Case Study 2: Prototyping of a Knee Prosthesis 5.1 Background Rapid prototyping has applications in the field of medicine. However, in this application the STL file is no longer obtainable from a CAD model. There is a need to generate the necessary STL files from data acquired by medical equipment. Swaelens and Kruth 4 proposed three approaches to producing an RP part from computer assisted tomography (CT) scanner data (see Fig. 6). In most cases, STL-interfacing was used. In STL-interfacing, a CT scanner maps the contour of a 3D surface. This data is then converted into triangular file format which is then converted into the STL format required by RP machines. There is a direct conversion of data from the CT scanner to the RP machines. In effect, the scanned surface is faithfully reproduced by the RP machine. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 9 页 共 29 页 When used in this fashion, VP plays an almost negligible role, in RP-assisted surgery prototyping, as a viewer to verify the contour of the surface. Jacob et al. 5 constructed 3D models from CT scanner data using CTrans from Proform. They reported that the decisive advantage lies in the clearness and manual “getting in touch” as the surgery proper is elaborate manual craftsmanship. The model can be viewed and palpated from any angle and could even be operated upon. In that way, surgeons could literally grasp the problem. This study shows VP as a viewer for a 3D model. While the study did not state whether the 3D model was a solid model, it opened the possibility of integrating CAD software into the process, data exchange problems notwithstanding. This contrasts with the CAD system route shown above. Researchers in the University of Leuven, Belgium identified contours from CT scanner data and introduced them into CAD software to generate surface models. The physical model of a hip was produced with much effort, and the whole procedure took several working weeks. The procedure of converting CT scanner data to a solid 3D model is tedious and prone to error. Given the triangulation points from the CT scanner, they must be joined to the appropriate adjacent points to form curves. Confusion sometimes occurs when a surface folds back； while a point “below”is the nearest point, it may not be an adjacent point. These curves must then be individually and manually selected to define surfaces. Again, care must be taken to ensure that the appropriate surfaces which approximate the original surfaces are formed. After the surfaces are formed, they are connected to form patches or quilts. These quilts are then combined to form a surface model. If the surface model is fully enclosed, the CAD system may then convert it into a shell or solid 3D model. The complexity and shape of the human body also presents problems. Most of the extracted outlines are represented as complicated Bezier curves. A mapping algorithm sometimes fails to combine these Bezier outlines to form 3D data. So, it is necessary that this process be supported by hand 6. Human supervision is also required where software is unable to recognise features such as joints where bone structures abut. The data must be separated into individual components (disarticulation) in order to evaluate a function (e.g. a jaw joint). Direct interfacing has two major problems. The data from the CT scanner are in 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 10 页 共 29 页 the form of shaded images and are automatically segmented. While it is possible to calculate triangles from the images, they do not contain enough surface information. Therefore, it is difficult for the RP system which requires supports to construct the appropriate support structures. Secondly, the interpolation from successive contours obtained from the segmentation is not evident. The CT scanner has a threshold filter to isolate regions within the desired density range. When tissue density ranges across this threshold setting, there are problems in identifying the tissue. For example, for cortical bone (high density) with a structure larger than the voxel dimension, surfaces are well defined and the transitions are easy to recognise. When lowerdensity structures are scanned (cancerous bone) or the structure is so thin that it only partially fills the voxel, the density measured at the surfaces may not surpass the threshold. Consequently, a fixed threshold filter will result in shrinking the structure dimension or creating a void 7, so most research is focused on the STL interface. Virtual prototyping has more applications in biomechanics. CAD systems are used to design prostheses and the simulation and analysis modules are used to help refine the design of the part. Finite-element analysis is a useful tool in the design of load-bearing prostheses such as knees and hips. Kinematics simulation and analysis is applied where the range of movement of the limbs linked by the prostheses is specified. Thermal simulation is not usually carried out as the service condition for Rapid prototyping parts produced for the prostheses are for the proof of the concept as well as for size estimation. Formfitting or assembly can be done in some cases but is not possible for others such as a ball-socket joint found in a hip prosthesis. For prostheses with moving parts, a rough kinematics check can be performed. 5.2 Finite-Element Modelling Investigation This case study explores the basic finite-element modeling (FEM) capabilities of VP packages and how corresponding RP parts compare to them. The basis for this study is a knee prosthesis designed by Chow 8. The prosthesis was designed on Mechanica. The files were exported into IGES format. When retrieved using Pro/ENGINEER Release 15, the surface model was discontinuous and in certain cases, incomplete. (See Figs 7 and 8.) The analysis software used is Ansys version 5.4 by Ansys Inc. The parts were constructed in Pro/ENGINEER Release 15 by Parametric 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 11 页 共 29 页 Technology Corporation. Pro/ENGINEER does not have a finite-element modelling module. The RP parts for this study were built on an SLA system. However, Pro/ENGINEER has a FEM post-processor that allows the user to: 1. Add or modify finite-element analysis loads or boundary conditions on the model. 2. Specify maximum and minimum element sizes for both local and global elements. 3. Specify the number of points for the mesh on an edge. 4. Set material properties for the model. Pro/ENGINEER can pre-process the part by creating the mesh. The part created in Pro/ENGINEER was then exported to ANSYS using the IGES standard. As the tibial assembly is symmetrical, only half was built and meshed, as shown in Fig. 9. The ability to use a finite-element modelling module or package is highly dependent on the users skill and knowledge. The user must be familiar with the concepts and terms used in finite-element modelling. Not all CAD software has an integrated finite-element solver. In these cases, the finite-element package may or may not be able to accept that particular softwares CAD file format. Then, a data exchange format is required such as IGES, DXF or VDA. Data exchange is not the only barrier to the transfer of part data to a finite-element software. Each CAD software system represents the solid models differently. In the construction of the tibia, two geometrically identical parts were produced using different feature-creation techniques. One part could be meshed by Pro/ENGINEER but not by ANSYS； the other could be meshed by both. Again, the users judgement is required to avoid such problems. A users judgement is also crucial in deciding what features of a part can be safely suppressed to facilitate analysis, but at the same time retain the integrity of the analysis results. Certain geometries and features, especially the intersection of a few edges, can create degeneracies. The solver is unable to create elements or nodes at these degeneracies. Therefore, these degeneracies must be removed. Some finite-element packages allow the editing of the part but some allow only limited editing. The changes then would have to be made in the CAD software and then re-exported to the finite-element software. It would take an experienced user to foresee these problems or to identify the problems correctly, and then correct them. The RP model is more useful as a visualising tool. An actual part always gives a 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 12 页 共 29 页 better perception of size and shape than an image on a screen. In the case of the knee prosthesis, a rough assembly could be made to see how the femur and the tibial assembly fitted together. In fact, an RP assembly helps in determining the placement of parts in a VP assembly. A rough kinematics check could also be done and the designer is able to assess the part intuitively when simulating the femur sliding against the tibial assembly. The designer can get a “feeling” of whether rocking motion along the axis perpendicular to the sliding motion is possible. While moments can be obtained from a virtual prototype, it does not always show visual clues such as whether a design is ungainly and cumbersome which only a physical prototype can properly exhibit. 6. Conclusion Rapid prototyping is preferred to VP for kinematic simulation, assembly, fit and interference checking. As a physical part, RP allows the user to gauge the size of the prototype. It is also used for ergonomic and tactile evaluations. Rapid prototyping parts are also used for manufacturing input, usually for a cross-functional team where representatives from all disciplines evaluate the prototype from their own specialist requirements. Most RP parts suffer from mechanical property drawbacks. SLA components are brittle and prone to warpage. The need to build supports in some RP systems also creates problems. In addition, very thin parts cannot be built by some RP systems. Virtual prototyping provides a quick iterative design process, where problems can be rectified immediately whenever indicated from analysis. Solving the problems in the VP domain helps reduce physical prototyping costs and time. Virtual prototyping has high initial investment costs in hardware and software and demands skilled and experienced operators to extract the full benefit from the software. Transfer of data between differing VP systems is poor and vendors often recommend total reconstruction of parts. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 13 页 共 29 页 prolonging lifetime service life of die based on DEFORM Abstract This paper describes the estimation method of die service life based on wear and the plastic deformation of dies in hot forging processes. Die service life is considerably shortened due to the thermal softening of surface layer, caused by the high thermal load and long contact time between the dies and the deforming material. Also, the die service life depended on wear and the plastic deformation of dies can be to a large extent determined by finite element (FE) analysis, wear and thermal softening tests. These are some of the major limiting factors affects die accuracy and die service life, and forming velocity and initial die temperatures influence greatly wear and the plastic deformation of hot forging dies. In this study, two methods are suggested for estimating the service life of hot forging dies by plastic deformation and abrasive wear, and these applied to predict the product quantity according to two main process variables, forming velocity and initial die temperature for a spindle component. Through the applications of the suggested methods, the thermal softening of dies due to the local temperature rise led to the reduction of the service life of hot forging dies by plastic deformation more than by abrasive wear. 2004 Elsevier B.V. All rights reserved. Keywords: Hot forging； Die service life； Wear； Plastic deformation； Thermal softening； Tempering parameter 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 14 页 共 29 页 1. Introduction Hot forging is one of the most conventional metal-forming processes used in the production of critical parts in various industries . Actually, it is widely used in the manufacturing of automobiles and industrial machine components. In particular, this process can be effectively used to form materials with the high flow stress. Die service life greatly influences manufacturing costs, productivity and product quality. During hot forging process, die service life is dramatically shortened by thermal cycle, excessive metal flow and a decrease in die hardness. Nowadays, manufacturing costs depend on how die service life can be extended for sound products without any kinds of internal and external defects during hot forging process. Subcontractors and suppliers are increasingly under pressure with regard to cost reduction and responsibility for the development of new components. These requirements are more critical in the automotive industry. Therefore, it is important to improve the technical skills in the areas of material science and metallurgy as well as in the area of tool design. The knowledge of computer aided design (CAD) and numerical simulation also becomes very helpful. In the forging industry, tooling costs can reach up to about 50% of a component cost. Therefore, it is obvious that the reduction of component costs requires an optimization of tools, in particular, an improvement in performance and service life. During hot forging process, forging tools are not only subjected to mechanical stresses, but also to thermo mechanical stresses induced by the thermal cycling and successive forging operations. Proper selection of the die material and of the die manufacturing technique determines, to a large extent, the useful life of forming dies. Dies may have to be replaced for a number of reasons, such as changes in dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage. Many researchers have been investigated the influences of process conditions on die service life during metal forming process. The surface hardness of a die decreases owing to the thermal softening of hot forging dies. This thermal softening effect accelerates tool failures. The limiting factors of die service life can occur simultaneously or separately during hot forging process. Due to the different characteristics of processes or products, die service life can be decreased by wear or by the plastic deformation. This study developed two methods to estimate die service life in hot forging processes. One is a method that can predict the plastic deformation of a die and the other is to calculate the amount of die wear. These methods have been applied to evaluating the service life of a finisher die for the hot forging process of an automobile part, and the possible maximum production quantity which describes die service life will be evaluated according to the variations of initial die temperature and forming velocity. 2. Methods for estimating die service life This study developed two methods for estimating the service life of dies in hot 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 15 页 共 29 页 forging process. One is a method that can predict the plastic deformation of the die； the other is for calculating abrasive tool wear. 2.1. Die service life based on plastic deformation During the hot forging process, the temperature of a die increases due to the contact between the dies and the hot deforming material. The rate of temperature rise can be attributed to several factors, such as the initial temperature of dies and billet, the contact time and pressure, the die material and surface treatment conditions. The thermal softening induced by this temperature rise gradually reduces die hardness, and finally leads to the plastic deformation of a die. The longer contact time at the elevated temperature gives rise to a decrease of the surface hardness of a die. In order to consider the thermal softening effect in estimating die service life against plastic deformation, it is required to introduce the tempering parameter, M, as shown in Eq. (1), which represents the effect of die hardness change on the contact temperature and time successive forging cycles : 310 10)l o g( tCTM Eq.(1) where T is the tempering temperature (K), C is the material constant which has about 20 for carbon steel, t is the tempering time. Also, from starting to deform until ejecting the forged part, the temperatures of die surface change during one forging cycle, so the introduction of equivalent temperature is required. The equivalent temperature, eqT, can be approximately expressed as shown in Eq. (2): 32 m i nm a x TTTeq Eq. (2) Where maxT, and minT are the highest and lowest temperatures during one forging cycle, respectively. To estimate die service life for the plastic deformation of a die induced by thermal softening, the tempering time, t, at Eq. (1) is replaced with hardness holding time th, where th is the time which takes until initial die hardness gradually reduces to reach the critical hardness by thermal softening, as shown in Eq. (3): )1000e xp( CTMteqy i e l dh Eq. (3) whereyieldM is the M value when initial die hardness is equals to the corresponding hardness of the yield strength of the die. When the material is a perfect plastic, the hardness (HrC) of material is about three 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 16 页 共 29 页 times of the yield strength of material. The main tempering curves of this hot work die material, H13, obtained from thermal softening experiments is shown in Fig 1.1.An actual working finishing die was quenched at 1030 C, and then it had the first tempering for 3 h at 550 C and the second tempering for 3.5 h at 600 C. Die surface was treated as ion-nitriding process for 14 h at 520 C. Fig 1.1 Main tempering curves of H13. Therefore, for hardness holding time for estimating the die service life considers the first and second tempering time, which can be derived as follows: 321 10)lo g ( tttCTM heqy i e ld 21)10 00e xp( ttCTMteqy ie l dh Where, CtCTTt heq )lo g(ex p1011 CtCTTt heq )lo g(ex p1021 where T1, T2 are the first and second tempering temperatures, t1, t2 are the hardness holding times at the first and the second Myield values for Teq, respectively. In order to calculate the hardness holding time, effective stresses and equivalent temperatures can be obtained from rigid-plastic finite element analysis. Myield value can be determined from the main tempering curve. t1 and t2 are substituted into Eq. (4) to obtain the hardness holding time. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 17 页 共 29 页 Finally, the die service life of the finishing die is calculated by dividing the hardness holding time by one forging cycle time, and the die service life is expressed as the possible maximum production quantity. The outline of a method for estimating die service life affected by plastic deformation is shown in Fig1.2 Fig1.2 Flow chart for plastic deformation analysis. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 18 页 共 29 页 Fig 1.3 Flow chart for abrasive wear analysis. 2.2. Die service life based on abrasive wear Abrasive wear is defined as the intentional removal of materials from a surface, as in grinding and polishing of engineering components, and the unwanted loss of material that occurs when machine components are in relative motion . In hot forming, the die steel should have a high hot hardness and should retain this hardness over extended periods of exposure to elevated temperatures. The factors affecting abrasive wear during metal contacts are temperature the roughness of contacting surfaces, the hardness of die material, the normal pressure on die surface, the sliding distance between contacting metals, and lubrication conditions, etc. The abrasive wear of dies influences dimensional accuracy and the surface finish of products during hot forging processes. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 19 页 共 29 页 Fig2.1 Shape and dimensions of a product and finishing die. Fig2.2 Process design of a spindle product. In this study, in order to predict the wear profile of a die in metal forming processes, Archard wear model is applied as shown in Eq. (5): hkPlV3 Eq. (5) where V is the wear depth, k is the wear coefficient, P is the normal pressure on die surface, l is the sliding distance and h is the surface hardness of the die. To estimate the die service life based on abrasive wear, it is needed to consider the hardness change at high temperature of a die and the wear amount increase of surface layer with regard to the contact time and temperature. A numerical model of abrasive wear as shown in Eq. (6), is developed by considering the hardness change of a die toward the direction of wear depth. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 20 页 共 29 页 Nisn tw ea r d e p t hMhkW1)(),(3 Eq. (6) Table 1 Process conditions of FE analysis Billet Material AISI 1045 Thermal conductivity (N/s C) 74.93 Emissivity 0.3 Heat capacity ( N/mm C) 3.602 Die Material H 13 Thermal conductivity (N/s C) 28.6 Emissivity 0.3 Heat capacity (N/mm C) 3.574 Surface treatment Ion-nitride Forging conditions Friction factor (m) 0.3 Heat transfer coefficient (N/smmC) 11.3 Convection coefficient (N/smmC) 0.02 Initial Billet/die temperature ( C) 1200/200 Forging velocity (mm/s) 250 Table 2 Variation conditions of process variables Process variables Initial die temperature (C) 200 300 400 Forging velocity (mm/s) 200 250 300 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 21 页 共 29 页 The normal pressure ( n), the sliding velocity (vs), and the temperature distributions on die surface are calculated from the rigid-plastic FE analysis, and the permitted amount of abrasive wear and the critical value of surface hardness were obtained from wear test and thermal softening experiments. The amount of abrasive wear at each point on the die surface for one forging cycle was calculated through the wear analysis of Eq. (6), and then compared with the permitted value. Also, the hardness at the worn surface that resulted from this amount of abrasive wear was compared with the critical value. If the amount of abrasive wear is smaller than the permitted value, and the hardness at worn die surface is still greater than the critical value, then abrasive wear analysis will repeat until the integrated amount of abrasive wear reaches the permitted value. Finally, the production quantity which expresses die service life was determined from the total number of wear analysis. The flowchart of a method for estimating the die service life based on abrasive wear is shown in Fig. 3. 3. Analyses and result Fig. 3.1shows a hot forging product to be analyzed based on plastic deformation and abrasive wear. One of automobile components, spindle part, is manufactured in three stages composed of upsetting and two forward/backward hot-forging operations. Fig. 5 shows the process design result for the hot forming of spindle part. Fig3.1 Damage factor of a final product. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 22 页 共 29 页 Fig3.2 Temperature distributions for the initial die temperature. 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 23 页 共 29 页 This product has the height of 320 mm, maximum diameter of 131mm and a long extruded part. This discrete part requires a minimum machining and high dimensional accuracy. Unfortunately, abrasive wear or plastic deformation of the die occurred at the stepped corners as shown as point 1, 2 in Fig. 3.1, the die service life of this part depends on the change of the initial shape and dimension of these stepped corners during hot forging. The forming analysis conditions and the variations of process variables for estimating die service life are listed in Tables 1 and 2, respectively. The distributions of damage value at final stage obtained from the FE analysis is shown in Fig. 3.3, these values appeared highly at two stepped corners. The damage factor can be used to predict fracture in forming operations. Fig. 3.3 Nodal force and velocity distributions for the initial die temperature. Therefore, the damage degree of these corners may directly relate to die service life. When the initial die temperature is low, it may influence product quality. When the initial die temperature is high, die hardness decreases. When the forming velocity 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 24 页 共 29 页 becomes faster, the contact time between the hot deforming material and the dies is shortened and the equivalent temperatures become low. The initial die temperature control and selection of deformation velocity are very important to the die life. Fig.3.4 Effective stress and wear depth for initial die temperature. 3.1. Influence of the initial die temperature In metal forming process, both plastic deformation and friction contribute to the heat generation. The temperatures developed in the process influence lubrication conditions, tool life, the properties of the final product, and the rate of production 4. Above all, when the initial die temperature is high, the temperature difference between inside and outside of a billet becomes small, and this small temperature 桂林电子科技大学 毕业设计（论文） 外文翻译（原文） 第 25 页 共 29 页 difference assists the sound metal flow. On the other hand, a high surface temperature may reduce die service life. But the low temperature of die surface can disturb metal flow and cause the surface defects. As can be seen in Fig. 7, the temperature on die surface at two stepped corners (point 1, 2) increase differently, due to initial die temperature effect, for the same forging process. For the initial die temperature 400 C at point 1, the die temperature is initially higher, but the maximum temperature is lower than for either 200 or 300 C. Also, these results clearly indicate that the temperature gradient for the initial die temperature 400 C is very large at point 2. The distributions of nodal force and velocity are shown in Fig. 8. It can be seen that nodal force acting on die surface decreases as the initial die temperature increases, whereas velocity of the workpiece at the vicinity of the die/material interface increases as the initial die temperature increases. The reason for this is that the metal flow increase with increasing temperature. The results of abrasive wear and stress analysis of finisher die are shown in Fig. 9, when initial die temperature is 400 C, the w