Advanced Friction Model Research Articles

Friction and lubrication modelling in sheet metal forming: Influence of local tool roughness on product quality

To improve the accuracy of forming simulations, advanced friction models are increasingly used in the industry. These models account for the physical properties of the sheet, tool and lubrication and describe the tribological conditions during the forming operation. One of the main influencing factors on the tribology system, and therefore the friction coefficient, is the surface roughness of the tools. Until now, it is often assumed all tools have the same uniform surface roughness. In reality however, the tool might have different surface conditions dependent on the type and location of the tool. That is, the blank holder might be differently polished than the punch, and sharp radii might have a different tool roughness compared to flat areas. This paper investigates a significant number of tool measurements from different tool sets from Volvo Cars, and quantifies the effect of local surface conditions on product quality.

On the Use of Advanced Friction Models for the Simulation of an Industrial Stamping Process including the Analysis of Material and Lubricant Fluctuations

The use of numerical simulations for tool tryout and process control is becoming increasingly prevalent. In this work, the deep drawing process of a car inner door panel of DC06 mild steel is numerically analyzed and compared with industrial process results. Five batches of DC06 material were analyzed mechanically and tribologically. Diverse tribological models were developed based on experimental strip drawing tests, where a Coefficient of Friction (CoF) was obtained as a function of contact pressure, sliding velocity, and amount of lubricant. A topography analysis was defined to compare material batches and to replicate industrial tool conditions. The simulation was fed with three tribological models: constant (CoF 0.15), Filzek pressure and velocity dependent, and TriboForm with lubrication zones. Thinning, Forming Limit Diagram (FLD) and draw-in were used as indicators for the comparison. Using the industrial tool, both FLD and draw-in were measured and compared with the numerical models. The constant model predicted the most conservative strain state and also differed most from the experimental results. The P-v-dependent and TriboForm models more accurately predicted the experimental results. This work highlights the importance of considering more complex tribological models to feed numerical simulations to yield results closer to real process conditions.

Influence of alloy composition and lubrication on the formability of Al-Mg-Si alloy blanks

The formability of 1.0 mm-thick Al-Mg-Si alloy blanks with different Si and Mg contents was numerically and experimentally investigated using two deep drawing tools: (i) a cross-shaped tool with open die and (ii) a complex-shaped panel tool with closed die. For the cross-shaped tool, forming velocity and blankholder force were constant and the formability was characterized based on the maximum drawing depth at crack initiation. For the complex-shaped panel tool the forming velocity and the drawing depth was constant and the formability was characterized based on the maximum blankholder force at crack initiation. Two types of commercial lubricants were applied on the blank surfaces. In order to investigate critical forming conditions, numerical models of both deep drawing processes were built using the AutoForm finite element (FE) software. Parameters required as input for these models were experimentally determined. The flow curve and the anisotropy of each of the aluminum alloys were determined using uniaxial tensile tests. Based on the results of pin-on-plate tests an advanced friction model which considers different contact pressures and sliding velocities was created using the TriboForm software. Good agreement between deep drawing simulations and experiments was achieved in terms of forming force, local thinning and strains. Experiments and simulations confirmed that the maximum drawing depth or blankholder force, respectively, tended to increase with increasing Si and Mg contents. However, the influence of lubrication on the formability of the Al-Mg-Si alloy blanks was much more significant than the influence of the alloy composition.

An industrial application case to predict galling in hot stamping processes

Severe tool wear is an unwanted phenomenon which occurs widely during hot stamping processes due to extreme process conditions like high temperatures and the absence of lubricant. Galling is a wear mechanism in the form of adhesive wear in which some material from the sheet transfers to the tooling. In a longer term, the build-up of material on the tool can damage the sheet in the form of scratches and can negatively affect the heat transfer between sheet and tool. Therefore, it is important to develop advanced models to predict and control tool wear and galling during hot stamping processes. More recently an advanced friction model for hot stamping processes has been introduced to accurately describe frictional behavior of 22MnB5-AlSi. This study aims to further extend the advanced friction model of 22MnB5-AlSi into a galling prediction tool by first evaluating possible galling initiation models and next assessing growth models. These models are calibrated using experimental data from hot strip draw tests performed at Tata Steel. This results in a multi-dimensional galling model as a function of temperature, pressure, strain and also on the relative sliding distance in contact between the tool and the sheet. Finally, the predicted galling distribution on the tooling surfaces for two industrial parts from Volvo Cars are verified. The galling locations are accurately found on the parts by the applied galling model.

Numerical study of advanced friction modelling for sheet metal forming: Influence of the die local roughness

Numerical simulation of sheet metal forming processes has become indispensable in the last decades. Although the complexity of the frictional behaviour is identified as a key factor for the prediction accuracy, the industry commonly considers a constant friction coefficient for the whole tool. Furthermore, the influence of roughness distribution of the tool in the friction behaviour has not yet been addressed. In this study the influence of the die local roughness in an advanced friction model has been evaluated in three industrial automotive components. The newly implemented advanced friction model (TriboZone) is suggested for advance or mature process verifications.

Enhancement of springback prediction of AHSS parts by advanced friction modelling

The complex springback behaviour of advanced high strength steel parts makes a robust manufacturing process challenging, mainly in terms of part dimensional accuracy. Therefore, an accurate springback prediction using sheet metal forming simulation is of a great importance to realize reliable forming processes and to achieve the target geometry. An accurate springback prediction requires an exact computation of the elastic stresses from the plastic deformations, which can be influenced among others by the retention of the sheet metal and the frictional properties of the parts. However, the frictional behaviour in the forming process has been often simplified by using a constant friction coefficient in the simulation, which could lead to a significant deviation in the springback prediction. To address this limitation, TriboForm software provides an advanced friction model in which friction is a function of several process setting parameters, namely pressure, strain, temperature and velocity. In this contribution, the advanced friction models for DH800 and CP800 materials are generated. Compared to a constant friction coefficient of 0.15, it is found that the prediction of flange angle of deep drawn hat profiles, produced by several blank holder forces, can increase up to 70% and of the sheet’s draw-in up to 10%.

Application of an Advanced Friction Model in Hot Stamping Simulations: A Numerical and Experimental Investigation of an A-Pillar Reinforcement Panel from Volvo Cars

Although hot stamping is now an established method, numerical analysis of the process is still a challenge. In industrial applications, process engineers use sophisticated models which consider complex physical effects including phase transformation, phase dependent flow curves, temperature dependent r-values and temperature dependent FLC’s. However, in most of the industrial applications the tribological system is still oversimplified by using a constant Coulomb friction coefficient. Currently, there is enough experimental evidence in literature showing that friction coefficients evolve during the process, depending on contact pressure, temperature and sliding velocity. This work focuses on accurate modelling of friction in hot stamping processes. The advanced modelling technique presented in this paper tracks each material point and calculates the corresponding coefficient of friction depending on the current process conditions. This technique was used on simple part geometries in order to generate fundamental knowledge. In this analysis, part geometry, process conditions and tool kinematics are varied. In the next step, an A-Pillar reinforcement panel from Volvo Cars was analyzed experimentally and numerically. It was seen that accurate friction modelling enhances the quality of the simulation results and evolving friction conditions have a direct effect on the feasibility analysis of industrial parts.

Modelling real contact areas caused by material straining effects in sheet metal forming simulation

Shortened product development processes in automotive industry combined with the upcoming lack of experts do challenge sheet metal part production fundamentally. Tryout time and manufacturing costs of large forming dies today are significantly influenced by their digitally supported engineering. The forming process by such tools is beside other influences is affected by elastic deformations of forming dies and press structure as well as contact areas between die and sheet metal part. In deep drawing such contact areas are influenced by the blank properties and the flange behavior in terms of thickening and thinning. Recent developments in sheet metal forming simulation do consider advanced friction models and structural modeling of die and press components improving simulation accuracy. Nevertheless thinning or thickening of sheet metal results into localized surface pressure distribution during deep drawing. For this reason, it is not sufficient to use the currently common practice of homogeneous surface pressure distribution in sheet metal forming simulation. In this respect, this paper presents a numerical approach for consideration of straining effects in the sheet metal part during forming operation. For this purpose, a systematic process improvement was developed in this paper to identify contact areas via a numeric simulation parameter. Validating the numerical investigation, a rectangle cup die is used, considering major strain. The main results of this contribution for that reason show how simulated contact areas can be estimated by reverse engineering of real forming parts. Hereby straining based contact areas lead to a novel contact area design in process planning, resulting in efficient die tryout.

Development and validation of a new friction model for cutting processes

The friction model in the tool-chip interface has significant influences on predicting chip forms, cutting forces, and cutting tool temperatures, when simulating the chip formation using the finite element method. In this paper, the friction behavior in the tool-chip interface was investigated experimentally and numerically under cutting conditions. The friction tests were performed with different workpiece materials (AISI 1045 and Direct Aged Inconel 718) in combination with uncoated and coated cemented carbide cutting tools. Various process normal forces were applied to achieve different contact pressures. The experimental results showed strong influences of relative speeds, contact pressures, and contact temperatures on apparent friction coefficients. In addition, an advanced friction model was proposed and validated with 3D FE-simulations of the friction test.

A comprehensive experimental setup for identification of friction model parameters

Parameter identification for advanced friction models is the most demanding task and involves a wide range of experimental dynamic analysis. The identification results directly affect the quality of the friction model and significantly depend on the setup. While many setups have been proposed to investigate the various characteristics of friction, rather less attention has been paid to designing one that is practical for parameter identification dedicated to certain common models. An experimental setup featuring simplicity, flexibility and good versatility is proposed here. The setup consists of a mass block connected to a motor-powered ball screw mechanism. Such a block slides on a metal base and the friction at the contact junction is varied accordingly. The materials and lubricants in the contact surface can be easily altered if needed. A capacitance displacement sensor is designed and installed at where the precise presliding profiles can be captured. The experimental setup, the calibration procedure for the displacement sensor and the experimental results are presented. It is shown that the experimental data produced by our setup leads to the success in parameter identification procedures of the selected friction models. Some limitations of the friction models, from the perspective of parameter identification, are discussed.

Effects of friction model on forging process of Ti-6Al-4V turbine blade considering the influence of sliding velocity

This work is motivated by the fact that one key parameter of sliding velocity at die-workpiece interface in numerical analysis of blade forging process is usually ignored, thereby resulting in poor prediction accuracy and limited application of friction model. A realistic description of the friction boundary conditions shows great practical importance for the usability of simulation results. In this paper, an advanced friction model named IFUM considering the influence of sliding velocity is introduced in the simulation of Ti-6Al-4V turbine blade forging operation, and its effects on metal flow and forging forces are compared and evaluated with two traditional friction models. The results indicate that the Institute of Metal Forming and Metal-Forming Machines (IFUM) friction model gives the better match with the experimental results than the traditional ones, in which the material flow velocity distribution and forging forces characterized by the sliding velocity are found to be more uneven and smaller, respectively. This research improves the accuracy of the simulation results and can be served as a basis for design and optimization of blade forging process.

Constitutive and friction modeling for accurate springback analysis of advanced high strength steel sheets

The present work aims to analyze the influences of the material and friction models and to suggest the optimum selection of the models for springback simulations. Firstly, a transformation-induced plasticity (TRIP) steel was characterized using the conventional and advanced models for the springback prediction in U-draw/bending. A special attention was paid to the friction model, which was derived based on the macroscopically observed frictional behavior and interpreted with the microscopic contact mechanism. The combination of advanced constitutive and friction models could predict springback and punch load quite well. It was found that the hardening law and elastic unloading modulus had the major influences, while the friction model also had a major influence in the case of high blank holding force. A specific value of constant friction coefficient could provide similar predictions as the variable friction coefficient, but the former could not describe a non-uniform distribution of friction coefficient. Next, the analysis was extended to the sensitivity studies that considered wider ranges of material and frictional behaviors. These studies suggested that (1) the plastic anisotropy has to be modeled using an anisotropic yield function if the yield stresses or r-values affecting the relevant stress state are far from the isotropic assumption; (2) the reverse loading behavior has to be characterized using a hardening model that can precisely describe the transient hardening and permanent softening; (3) the elastic modulus should be defined using an advanced model if the material shows a visible reduction of elastic modulus; and (4) the friction coefficient should be carefully defined using an advanced friction model if the blank holding force is high.

An Experimentally Evaluated Friction Model Adapted to FEM Extrusion Processes

During the extrusion process complex frictional effects occur between the bearing of the extrusion die and the extruded material. The recently developed Tribo-Torsion-Test is introduced and experimental results are summarized, briefly. Based on these results a friction model is introduced considering the effects of temperature, velocity and pressure. The model is adapted to a FE-tool and validation results are discussed. The introduced advanced friction model shows qualitative correct results for a friction sensitive extrusion process whereas the standard shear friction model shows qualitative wrong results.

Multi-scale friction modeling for sheet metal forming: The mixed lubrication regime

A mixed lubrication friction model is presented to accurately account for friction in sheet metal forming FE simulations. The advanced friction model comprises a coupling between a hydrodynamic friction model and a boundary lubrication friction model, based on the lubricant film thickness. Mixed lubrication interface elements are introduced to solve the governing differential equations. The interface elements have been implemented in FE forming software. Two deep-drawing applications are discussed to demonstrate the performance of the friction model. Results show friction coefficients that vary in space and time, and depend on external process settings like the amount and type of lubricant. A comparison with experimentally obtained punch-force displacement diagrams is made to prove the enhanced predictive capabilities of FE forming simulations.

Multi-scale friction modeling for sheet metal forming: The boundary lubrication regime

A physical based friction model is presented to describe friction in full-scale forming simulations. The advanced friction model accounts for the change in surface topography and the evolution of friction in the boundary lubrication regime. The implementation of the friction model in FE software codes is discussed. Results show that friction coefficients vary in space and time, and depend on local process conditions such as the nominal contact pressure and the plastic strain in the sheet material. The advanced friction model is validated by two small-scale forming processes, proving the enhanced predictive capabilities of FE simulations. The moderate increase in FE computation time, compared to using a Coulomb based friction model, demonstrates the efficiency of the proposed friction model.

Prediction of factor of safety of a slope with an advanced friction model

Landslides and other mass movements are quite prevalent phenomena in hilly areas, and sometimes cause huge losses of both life and property [1]. To mitigate these natural calamities, a precise scientific study that can predict the slope failures is required. Prediction of the mechanical stability of slopes both man made as well as natural is a tedious task after considering the complexity involved in the physical behaviour of the slope forming material as well as their intricate failure mechanisms [1–4]. Traditional approaches to predict stability of a slope are generally based on evaluation of a factor of safety through limit equilibrium analysis (LEA) [1–4]. The factor of safety (FOS) of a slope is defined as the ratio of shear strength to driving stress generally due to gravitational force along the failure plane [1–3]. Physically, FOSo1 signifies the slope to be unstable; however it is stable if FOS41 [1]. The limitation of the LEA is that the analysis is carried out with a single fixed value of shear strength of solids such as rocks and soils, while ignoring the sliding rate dependent failure strength. Experiments on rocks have shown that failure strength (also known as frictional strength) does depend on sliding velocity and time of stationary contact (also known as waiting time) of the sliding interface [5–7]. Moreover, based on these experimental observations, an advanced friction model has been proposed which is known as the rate and state dependent friction (RSF) model. The RSF model is basically a modified form of the Mohr–Coulomb failure criterion [13]. It is to be noted that the RSF model has been extremely successful in recent times for predicting the frictional behaviour of hard and rough solids such as rocks, soils, hard polymers etc. [8–11]. As a result, the RSF model has found widespread applications in the study of mechanics of earthquakes and faulting [10].

Frictional behaviour of CaCO3 powder compacts

During powder compaction processes friction has an influence on the final shape and properties of components. It is therefore important to understand how the friction influences the compaction process. Since detailed friction measurements of the powder compact–tool interface are rare, simulation models of the powder compaction process often involve a more accurate description of the frictional behaviour. This limits the accuracy of the simulated results. More accurate numerical models can give improved results, especially in regards to the evolution of density and its distribution within the powder compacts. This study is as a step towards more advanced friction models for powder compaction simulations. A universal tribometer has been used to investigate the frictional behaviour of contact interfaces between a carbide counter surface and CaCO 3 powder compacts with different densities. Both static and dynamic frictional properties were measured in a variety of conditions to build a fundamental foundation for friction modelling in powder compaction simulations. The results show that increasing the powder compact density decreases the dynamic friction coefficient but that the static friction coefficient remains fairly constant. The measured friction coefficient can be used to improve the simulation of a powder compact process. Also investigated is the change in friction coefficient that occurs in the compaction process when the surfaces are worn such that loose powder appears in the interface between the tool and the powder compact. This behaviour is important to take into account to accurately describe the compaction process. During powder compaction processes friction has an influence on the components final shape and properties. A universal tribometer was used to investigate the frictional behaviour of contact interfaces between a carbide counter surface and CaCO 3 powder compacts. The results show that increasing the powder compact density decreases the dynamic friction coefficient but that the static friction coefficient remains constant. ► A study of frictional behaviour of CaCO 3 powder mix. ► Dynamic and powder sprinkled samples (static) investigated. ► The dynamic friction coefficient decreases with increasing density. ► Static friction is independent of the powder compact's density. ► A density dependent mathematical function for the dynamic friction coefficient has been found.

Sensitivity analysis of nanoparticles manipulation based on different friction models

The advent of miniaturization and transition from the macro world to the micro/nano world has brought into existence materials with more ratio of area to volume. In transition from the macro world to the micro/nano world, surface forces such as friction and adhesion that were formerly less significant or even negligible are under solemn scrutiny. In this respect, various friction models have been developed by numerous researchers in an effort to model the governing conditions in the micro/nano manipulation. In this paper, nanoparticle displacement dynamic modeling and simulation in AFM-based manipulation is addressed using advanced friction models in the nano scale including HK and LuGre which are both on the basis of true contact surface. Moreover, the effect of accurate modeling on the process of manipulation is compared with the results of the previous model. In addition, critical force and time variations by using different friction models are analyzed under the influence of model parameters. Previous research entailed a simplification of nanoparticles manipulation modeling using modified Coulomb friction model and assuming apparent contact surface while in reality, true contact surface is much less than apparent contact surface due to the raggedness of surfaces. This causes a significant decrease in friction force and hence manipulation critical parameters. Implementing an accurate friction model upgrades the precision of the prior less complex models. Simulation results show that the predicted critical force necessary for the initiation of particle movement by HK and LuGre models are reduced by 15.87% and 22.22% respectively and the critical time by 50% and 75% respectively compared to Coulomb model.

Advanced friction modeling for sheet metal forming

AbstractThe Coulomb friction model is frequently used for sheet metal forming simulations. This model incorporates a constant coefficient of friction and does not take the influence of important parameters such as contact pressure or deformation of the sheet material into account. This article presents a more advanced friction model for large-scale forming simulations based on the surface changes on the micro-scale. When two surfaces are in contact, the surface texture of a material changes due to the combination of normal loading and stretching. Consequently, shear stresses between contacting surfaces, caused by the adhesion and ploughing effect between contacting asperities, will change when the surface texture changes. A friction model has been developed which accounts for the change of the surface texture on the micro-scale and its influence on the friction behavior on the macro-scale. This friction model has been implemented in a finite element code and applied to a full-scale sheet metal forming simulation. Results showed a realistic distribution of the coefficient of friction depending on the local process conditions.

Advanced Friction Modeling in Sheet Metal Forming

The Coulomb friction model is frequently used for sheet metal forming simulations. This model incorporates a constant coefficient of friction and does not take the influence of important parameters such as contact pressure or deformation of the sheet material into account. This article presents a more advanced friction model for large-scale forming simulations based on the surface changes on the micro-scale. When two surfaces are in contact, the surface texture of a material changes due to the combination of normal loading and stretching. Consequently, shear stresses between contacting surfaces, caused by the adhesion and ploughing effect between contacting asperities, will change when the surface texture changes. A friction model has been developed which accounts for these microscopic dependencies and its influence on the friction behavior on the macro-scale. The friction model has been validated by means of finite element simulations on the micro-scale and has been implemented in a finite element code to run large scale sheet metal forming simulations. Results showed a realistic distribution of the coefficient of friction depending on the local process conditions.