A novel approach towards a lubricant-free deep drawing process via macro-structured tools

In today’s industry, the sustainable use of raw materials and the development of new green technology in mass production, with the aim of saving resources, energy and production costs, is a significant challenge. Deep drawing as a widely used industrial sheet metal forming process for the production of automotive parts belongs to one of the most energy-efficient production techniques. However, one disadvantage of deep drawing regarding the realisation of green technology is the use of lubricants in this process. Therefore, a novel approach for modifying the conventional deep drawing process to achieve a lubricant-free deep drawing process is introduced within this thesis. In order to decrease the amount of frictional force for a given friction coefficient, the integral of the contact pressure over the contact area has to be reduced. To achieve that, the flange area of the tool is macro-structured, which has only line contacts. To avoid the wrinkling, the geometrical moment of inertia of the sheet should be increased by the alternating bending mechanism of the material in the flange area through immersing the blankholder slightly into the drawing die

자동차용 판재성형 해석 적용을 위한 다중 스케일 마찰 모델 개발 및 평가

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2022.2. 이명규.Sheet metal forming of advanced high strength steels (AHSS) has drawn significant attentions in automotive industry for their improved fuel efficiency by lightweightness and passenger safety by higher strength. However, the manufacturing of automotive parts with the AHSS accompanies inferior springback and formability compared to the conventional lower strength steels, which results in more time consuming trial and error in the tool design stage. To overcome this challenges in applying the AHSS to the automotive parts, finite element simulations have been commonly used as a numerical tool for predicting springback and formability of sheet metal parts prior to real try-out. Accurate modeling of finite element simulation in sheet metal forming process requires reliable numerical techniques, constitutive models, realistic boundary conditions, etc. Among these, the friction is one of important factors to determine the accuracy of the simulation, but it has been overlooked in most simulations. The frictional behavior in sheet metal forming is known to be very complex and depend on various parameters such as surface roughness, contact pressure, sliding velocity, lubrication condition, etc. However, it is a common practice to use the simplest Coulomb friction law in the finite element modeling. In the present study, a microscale asperity based friction model is further modified by imposing new model parameters for satisfying force equilibrium between contact surfaces. In addition, a geometrical shape model of the tool surface is newly proposed to determine the plowing effect of the friction. The tool geometry is modeled based on primary summits in tool height distribution determined by the measured wavelength, rather than the summits dependent on the resolution of surface measurement instrument. The friction models are required not only in the preceding boundary lubrication condition, but also in the mixed-boundary lubrication condition where sufficient lubrication exists in non-contacting surface valleys. The hydrodynamic friction model uses a load-sharing concept that considers the lubrication area and metal-to-metal contact separately. In this study, the hydrodynamic friction model is combined with the boundary lubrication friction model to account for the friction in the mixed lubrication domain. The lubricant film thickness, calculated as the volume of non-contacting surface valleys, is used to realize the coupling. The film lubrication behavior is implemented by the finite element coding of the Reynolds equation, which enables the calculation of the hydrodynamic pressure. To validate the boundary lubrication friction model, the calculated friction coefficient and the measured friction coefficient are compared according to the contact pressure under boundary lubrication conditions. Also, the boundary lubrication friction model is verified by the finite element simulation that is applied to the U-draw/bending process. Finally, the boundary lubrication friction model and the mixed boundary lubrication friction model are applied to the finite element simulation of the newly developed press-forming process, which represents the influence of various variables such as contact pressure, sliding speed and lubrication. The results of the validations show that the developed multi-scale friction models and their implementation can be efficiently used to the sheet metal forming simulations where the frictional behavior is critical for the quality of the automotive parts.AHSS(고장력강판)의 판금 성형은 경량화에 의한 연비 향상과 고강도화에 의한 승객 안전으로 자동차 산업에서 큰 주목을 받고 있습니다. 그러나 AHSS를 이용한 자동차 부품 제조는 기존의 저강도 강재에 비해 스프링백 및 성형성이 좋지않기에 툴 설계 단계에서 시행착오가 더 많이 발생하게 됩니다. 자동차 부품에 AHSS를 적용할 때 이러한 문제를 극복하기 위해 유한 요소 시뮬레이션은 실제 시험 전에 판재 성형 부품의 스프링백 및 성형성을 예측하기 위한 수치해석적 도구로 일반적으로 사용되었습니다. 판재 성형 공정에서 유한 요소 시뮬레이션의 정확한 모델링은 신뢰할 수 있는 수치해석적 기술, 구성 방정식, 정확한 경계 조건 등이 필요합니다. 이 중 마찰은 시뮬레이션의 정확도를 결정하는 중요한 요소 중 하나이지만 대부분의 시뮬레이션에서 간과되어 왔습니다. 판재 성형에서 마찰 거동은 매우 복잡하고 표면 거칠기, 접촉 압력, 미끄럼 속도, 윤활 조건 등과 같은 다양한 매개변수에 따라 달라지는 것으로 알려져 있습니다. 그러나, 대부분의 유한 요소 해석에서 가장 간단한 쿨롱 마찰 법칙을 사용하는 것이 일반적입니다. 본 연구에서는 접촉면 사이의 힘 평형을 만족시키기 위해 새로운 모델 매개변수를 부과하여 마이크로 스케일 돌기 기반 마찰 모델을 추가로 수정했습니다. 또한 마찰의 쟁기질 효과를 결정하기 위해 툴 표면의 기하학적 형상 모델이 새로 제안되었습니다. 툴 형상은 표면 측정 장비의 분해능에 의존하는 정점이 아니라 측정된 파장에 의해 결정되는 툴표면 높이 조도의 서밋을 기반으로 모델링됩니다. 마찰모델은 경계윤활조건뿐만 아니라 충분한 윤활이 존재하는 혼합경계윤활조건에서도 필요하다. 유체역학적 마찰 모델은 윤활 영역과 금속 대 금속 접촉을 별도로 고려하는 하중 공유 개념을 사용합니다. 본 연구에서는 유체역학적 마찰 모델을 경계 윤활 마찰 모델과 결합하여 혼합 윤활 영역의 마찰을 설명합니다. 비접촉 표면 밸리의 부피로 계산된 윤활유 필름 두께는 커플링을 구현하는 데 사용됩니다. 필름 윤활 거동은 유체역학적 압력의 계산을 가능하게 하는 Reynolds 방정식의 유한 요소 방법을 사용해 구현됩니다. 경계 윤활 마찰 모델을 검증하기 위해 경계 윤활 조건에서 접촉 압력에 따라 계산된 마찰 계수와 측정된 마찰 계수를 비교합니다. 또한 경계 윤활 마찰 모델은 U-draw/bending 과정에 적용된 유한 요소 시뮬레이션을 통해 검증되었습니다. 마지막으로 경계 윤활 마찰 모델과 혼합 경계 윤활 마찰 모델을 새로 개발된 프레스 성형 공정의 유한 요소 시뮬레이션에 적용했는데, 이는 접촉 압력, 미끄럼 속도 및 윤활과 같은 다양한 변수의 영향을 나타냅니다. 검증 결과는 개발된 다중 스케일 마찰 모델과 그 구현이 마찰 거동이 자동차 부품 품질에 중요한 판재 성형 시뮬레이션에 효율적으로 사용될 수 있음을 보여줍니다.1. Introduction 1 1.1. Sheet metal forming and deep drawing process 1 1.2. Motivation and objective 2 1.3. Literature review 5 1.3.1. Friction modeling on the boundary lubrication condition 6 1.3.2. Friction modeling on the mixed-boundary lubrication condition 22 2. Friction model in boundary lubrication 35 2.1. Framework of friction model in boundary lubrication 35 2.2. Statistical contact model for describing surface deformation 38 2.2.1. Assumptions for modeling 39 2.2.2. Flattening of workpiece asperity due to normal load 41 2.2.3. Flattening of workpiece asperity due to normal load and sliding 48 2.2.4. Flattening of workpiece asperity due to normal load and bulk strain 50 2.3. Friction model through a new approach 53 2.3.1. An elliptical paraboloid asperity model 53 2.3.2. A tool geometry model 56 3. Friction model in mixed-boundary lubrication 65 3.1. Overview of the mixed-boundary friction model (Hol [106]) 67 3.2. Finite element modeling for film fluid behavior 71 3.3. Verification of the developed finite element modeling 75 4. Application of boundary lubrication and mixed-boundary lubrication friction model to sheet metal forming process 82 4.1. Friction model parameters 82 4.1.1. Material properties 82 4.1.2. Surface data 83 4.1.3. Friction experiments 86 4.2. Application to sheet metal forming processes under non-lubrication conditions 91 4.2.1. Application to U-draw/bending simulation 94 4.2.2. Application to prototype press-forming process without lubricant 105 4.3. Application to sheet metal forming processes under lubrication conditions 116 5. Conclusions 129 Reference 134박

Further development of wear calculation and wear reduction in cold forging processes

Tools are of strategic importance for industrial manufacturing processes. Their behaviour has a great influence on the productivity of the process and the quality of the product. A material saving and efficient technique for processing metallic workpieces is cold forging. One major challenge of this production method is the handling of high contact normal stresses in the tool contact, which can lead to severe tool wear. To investigate promising approaches for understanding wear modelling and wear reduction a demonstrator process based on the first stage of a total five-staged cold forging process for the manufacturing of a bolt anchor is considered in the scope of this research. This work aims at the further development of a numerical wear calculation based on an adapted Archard model in order to be able to realistically predict the tool wear in cold forging processes. Therefore, the material characterization of the used workpiece material as well as an investigation of the worn tool dies takes place to validate a numerical wear calculation model. Furthermore, this research addresses a reduction in wear by identifying critical areas and changing the inlet geometry of the investigated demonstrator tool die. This way, conclusions can be drawn about the wear sensitivity during numerical process design, and particularly critical areas can be geometrically modified in terms of the design. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Recent developments and trends in the friction testing for conventional sheet metal forming and incremental sheet forming

Friction is the main phenomenon that has a huge influence on the flow behavior of deformed material in sheet metal forming operations. Sheet metal forming methods are one of the most popular processes of obtaining finished products, especially in aerospace, automobile, and defense industries. Methods of sheet forming are carried out at different temperatures. So, it requires tribological tests that suitably represent the contact phenomena related to the temperature. The knowledge of the friction properties of the sheet is required for the proper design of the conditions of manufacturing processes and tools. This paper summarizes the methods used to describe friction conditions in conventional sheet metal forming and incremental sheet forming that have been developed over a period of time. The following databases have been searched: WebofKowledge, Scopus, Baztool, Bielefield Academic Search Engine, DOAJ Directory of Open Access Journals, eLibrary.ru, FreeFullPdf, GoogleScholar, INGENTA, Polish Scientific Journals Database, ScienceDirect, Springer, WorldCat, WorldWideScience. The English language is selected as the main source of review. However, in a limited scope, databases in Polish and Russian languages are also used. Many methods of friction testing for tribological studies are selected and presented. Some of the methods are observed to have a huge potential in characterizing frictional resistance. The application of these methods and main results have also been provided. Parameters affecting the frictional phenomena and the role of friction have also been explained. The main disadvantages and limitations of the methods of modeling the friction phenomena in specific areas of material to be formed have been discussed. The main findings are as follows—The tribological tests can be classified into direct and indirect measurement tests of the coefficient of friction (COF). In indirect methods of determination, the COF is determined based on measuring other physical quantities. The disadvantage of this type of methods is that they allow the determination of the average COF values, but they do not allow measuring and determining the real friction resistance. In metal forming operations, there exist high local pressures that intensify the effects of adhesion and plowing in the friction resistance. In such conditions, due to the plastic deformation of the material tested, the usage of the formula for the determination of the COF based on the Coulomb friction model is limited. The applicability of the Coulomb friction model to determine the COF is also very limited in the description of contact phenomena in hot SMF due to the high shear of adhesion in total contact resistance.publishedVersio

Galling wear detection and measurement in sheet metal forming

Galling wear of sheet metal stamping tooling is an expensive issue for sheet metal forming industries. Forming of high strength steels, particularly in the automotive industry, has led to accelerated tool wear rates. These wear rates lead to product quality and die maintenance issues, making galling wear an expensive issue for automotive manufacturers and the sheet metal forming industries in general. Process monitoring allows for the continuous monitoring of tooling condition so that wear development can be detected. The aim of this investigation was to develop an in-depth understanding of the relationship between punch force variation and wear for implementation in future process monitoring regimes. To achieve this aim, the effect of wear and other friction influencing factors on punch force signatures were investigated. This required the development of an accurate method for quantifying galling wear severity so that the relationship between galling wear progression and punch force signature variation could be quantified. Finally, the specific effects of wear and friction conditions on the punch force signatures were examined. An initial investigation using a statistical pattern recognition technique was conducted on stamping force data to determine if the presence of galling wear on press tooling effected punch force variation. Galling wear on tooling, changes in lubrication type, and changes in blank holder pressure were all found to effect variation in punch force signatures shape. A new galling wear severity measurement methodology was developed based on wavelet analysis of 2D surface roughness profiles that accurately provided an indication of the location and severity of galling wear damage. Using the new method for quantifying galling wear severity in the relationship between punch force variation and galling wear progression was investigated, and a strong linear relationship was found. Finally, two prominent vii forms of punch force signature shape variation were linked to friction conditions driven by wear, lubrication, and blank holder pressure. This work describes and quantifies the relationship between galling wear and punch force signature variation. A new methodology for accurate measurement of galling wear severity is presented. Finally, specific forms of punch force signature variation are linked to different friction conditions. These results are critical for future implementation of punch force based galling wear process monitoring and a significant reduction in costs for the metal forming industries

Accelerated Life Testing to Predict Service Life and Reliability for an Appliance Door Hinge

Appliance manufacturers have traditionally performed physical testing using prototypes to assess reliability and service integrity of new product designs. However, for white goods where service lives are measured in years or decades, the use of endurance testing to analyze long time reliability is uneconomical. As accelerated life testing (ALT) is more efficient and less costly than traditional reliability testing, the methodology is finding increased usage by appliance manufacturers. In the present study, a simulation-based ALT approach was used to predict the service life of a polyacetal hinge cam from a consumer refrigerator. A predictive life stress model based on cumulative surface wear under accelerated stress conditions was developed and used to predict time to failure under consumer use. Results show that the life stress model demonstrated good agreement with performance testing data and reasonably predicts hinge life

New proposal to calculate the friction in sheet metal forming through bending under tension test

There are 6 different types of equations that have been formulated to measure friction through the bending under tension test, however, there is no work to show whether these calculations actually represent what is happening in a sheet metal stamping process. This paper aims to make a direct comparison between the bending under tension test and the sheet metal forming of a test piece to see if the friction coefficient reported by the test is able to predict friction in a real part. Several sources of information were used such as computer simulations, bending under tension test with different sensors and sheet metal forming tests of a cylindrical geometry to evaluate the friction. The results indicate that the equations already developed are not able to accurately predict the friction at the sheet interface and, therefore, a new equation was developed for this that is simpler to measure and presented satisfactory results

Numerical investigation of key stamping process parameters influencing tool life and wear

The influence of various combinations of punch and die materials, such as different carbide grades, and also the cutting radius on tool wear is the aim of this investigation. The numerical analysis results are supported by the relevant experimental evidence to validate the main model assumptions such as assumed material flow stress curve and the damage criteria. Taguchi method is utilised to effectively model and analyse relationship between process parameters. Roll-over and burr formation for a given punch-die clearance and cutting radius have been discussed and analysed in terms of tool wear reduction for different materials