The Computer Simulation of Tribological Influence on Strain Path and Forming Limit in Punch Stretching of Sheet Metal

The purpose of this paper is to describe the principles and results of some numerical simulations of strain path and forming limit analysis in punch stretching operations which include the tribological influence by using a realistic friction model. Three tribological variables (i.e. the mean lubricant film thickness, tooling roughness, and workpiece roughness) are required for the simulation. The calculation of these variables using lubrication theory and related semi-empirical equations are described. The active lubrication regime and suitable friction model can be determined from the current local values of these tribological variables. Friction stress can then be computed from these variables combined with more traditional parameters such as pressure and sliding speed. The limiting dome height and variation of strain path are then predicted by using the coupled FEM and lubrication/friction model. The comparison between the calculated and measured results shows that the present scheme is efficient in computation and will provide a useful tool for industrial applications.     

Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Friction in aluminium extrusion—Part 1: A review of friction testing techniques for aluminium extrusion

In the aluminium extrusion process, the friction at the workpiece/tooling interface is a highly complex phenomenon, affected by local temperature, relative velocity, contact pressure, geometry and tooling surface roughness, etc. Despite of this understanding, general rules for the selection of friction testing techniques have not been established yet. This paper summarises the recent development of the friction testing techniques for aluminium extrusion processes and detailed comparisons of these techniques are presented. Of the existing friction testing techniques, the combination of extrusion friction tests and short sliding distance ball-on-disc tests is recommended.     

Die-Workpiece Interfacial Behaviors in Axisymmetric Forging Processes with Flat Dies

A new piezo-viscous, average flow model combined with an advanced slab method is used to investigate the lubrication in axisymmetric forging processes with flat dies. Interactions between lubricant transport and surface roughness are studied. Influence of friction on the outward motion of the workpiece is also considered. It is shown that the lubricant is transported outward in a speed slower than half the surface speed, and the film thickness is smaller than the case of the smooth surface if asperity contact is not severe. However, when asperity contact is pronounced, the lubricant pressure distribution is quite different from the prior research. Mixed lubrication is observed in most of the region where asperity contact occurs. Boundary lubrication can only be found in a narrow area near the workpiece edge.     

Hydrodynamic analysis of partial film lubrication in the cold rolling process

Lubrication in cold rolling plays an important part for process feasibility and process quality. The hydrodynamic process of lubrication is very complicated and affected by many material and process parameters. This paper examined partial lubrication in the cold rolling process. The average flow Reynolds equation for rolling lubrication was 2set, which considered the pressure?Cviscosity and average flow effects. Lubricating factors such as sidling, surface waviness, lubricant viscosity, surface roughness, and reduction ratio were investigated. The results of the lubrication equation show that sliding, lubricant viscosity, and surfaces roughness affect the values of rolling friction. Surface waviness and reduction ratio also influence both rolling pressure and rolling friction.     

A Refined Friction Modeling for Lubricated Metal Forming Process

In spite of the existence of a well-developed realistic friction model, it has only been applied to the simple geometric problems, such as the axisymmetric and plane strain conditions, where the formulations of lubricant transport can be decomposed into two characteristic equations. Accordingly, a unified procedure of combining the current lubrication/friction model and finite element code of metal forming has been developed in this article for either steady or unsteady three-dimensional process including both axisymmetric and plane strain cases. In the part of the lubrication analysis, a finite element method is derived for the average Reynolds equation that is appropriated for 3-D metal forming process, and regardless of whether the tooling/workpiece surfaces are in contact or not. With regard to the theory of asperity contact, in addition to the well-known smoothing and roughening effects, significant deformation of asperities could be caused by the elastic microwedges on the tool surface. The availability of the new friction model was proved by a published problem and an axisymmetric stretch forming process was therefore adopted as a benchmark. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Analysis of rough line contacts operating under mixed elastohydrodynamic lubrication conditions

Heavily loaded machine elements, such as gears, usually operate in the mixed lubrication regime. Surface roughness has a significant effect on the pressure distribution, the subsurface stress field, and the friction coefficient. Based on the superposition of a dry rough and a fully flooded smooth contact, a mixed lubrication model has been developed. The roughness profile is assumed to be known. Surface deformation is calculated by taking into account the pressure distribution that is built up by asperity contacts, asperity interactions, and lubricant flow. Thermal and sliding effects are incorporated into the analysis. Non‐Newtonian lubricant behaviour is considered by using a power‐law rheological model. The pressure distribution, subsurface stress field, and friction coefficient were calculated from the model at several points along the contact path for an FZG type C gear pair. It was shown that a significant part of the load is carried by the contacting asperities. The position of the maximum shear stress is very close to the surface.     

Evaluation of interfacial friction condition by boss and rib test based on backward extrusion

Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in metal forming, since interfacial friction condition plays an important role in metal forming by influencing the metal flow, forming load, die wear, etc. In order to quantitatively estimate such friction condition, a new friction testing method “Boss and Rib Test” based on the backward extrusion process is proposed in this work. In boss and rib test, a key design is to use a tube-shaped punch so that the boss and rib at the deforming workpiece along the inner and outer surfaces of the punch are formed during backward extrusion. It was experimentally and numerically revealed that the heights of the boss and rib vary according to the friction condition applied. It was also found that the height of the boss is higher than that of the rib when the friction condition at the contact interface is severe. From this finding, the shear friction factor can be evaluated according to lubricant characteristics assigned. In addition, simulation results revealed that calibration curve demonstrating deformation pattern of the workpiece is affected by strain-hardening exponent of the workpiece.     

The lubrication mechanism in low speed plastic deformation with a synthetic lubricant — infiltration of lubricant into the boundary contact region

To investigate and evaluate the micro-pool mechanism in a mixed lubrication regime using polybutene lubricants, experiments were performed using 1 mm brass sheet, for various surface rough nesses of a work piece and die, lubricant viscosities and extrusion speeds. Systematic study showed that (1) the lower viscosity lubricant is easily squeezed out from the micro-pools into the flat are a through the troughs between the tool and work piece caused by plastic deformation of work piece, and (2) with decrease in lubricant viscosity and speed, or the larger the tool surface roughness becomes, the more lubricant infiltrates into the tool-work piece interface, which reduces the actual friction coefficient.     

冷轧铝工作区的混合润滑特性研究

为研究冷轧铝工作区的混合润滑特性,基于平均流量理论建立考虑表面粗糙度的冷轧铝工作区混合润滑模型,并通过相关文献的数据验证模型的正确性.在不同轧制速度、润滑油黏度以及前后张应力条件下对整个工作区内的润滑特性进行分析,研究轧制工艺参数对油膜厚度、接触面积比以及应力分布的影响.仿真结果表明:随着轧制速度的提高,轧制压力有一定...     

On the hydrodynamic liquid lubrication analysis of slider/disk interface

A numerical solution for ultrathin hydrodynamic liquid lubrication of slider/disk interface is introduced. Both surface roughness effects and non-Newtonian behavior of the liquid lubricant are incorporated into the hydrodynamic lubrication analysis. A non-Newtonian liquid is used as the lubricant, and its behavior is described by a power-law rheological model. The contact pressure is calculated for a Gaussian surface roughness. The hydrodynamic load capacity is calculated by using an averaged form for the Reynolds equation. The finite difference scheme, with Gauss–Seidel iterative-relaxation method, is applied to solve the average Reynolds equation. The effects of surface roughness parameter, surface pattern parameter, and the power-law exponent on hydrodynamic pressure distribution, hydrodynamic load capacity are studied and discussed.     

冷挤压塑性流体动力润滑分析

应用流体动力润滑理论和塑性成形原理，分析了冷挤压润滑过程。在挤压的起始和终止阶段为非稳态流体动力润滑，而中间阶段可近似为稳态流体动力润滑。考虑冷挤压在高压及大剪切应变率工况下润滑剂的非牛顿特性，运用Ostwald非牛顿体模型，分别建立了冷挤压非稳态和稳态的塑性流体动力润滑（PHD）模型。采用Monte Carlo法得到了冷挤压润滑过程的油膜厚度、油膜压力以及摩擦力的分布规律。     

Friction at high normal pressures

Friction conditions between tool and workpiece in metal working are of the greatest importance to a number of factors such as force and mode of deformation, properties of the finished specimen and resulting surface roughness.It is shown, theoretically and experimentally, that the Amonton friction law expressed by τ = μq does not apply when normal pressure is higher than approximately the yield stress of the specimen; in this case it is necessary to consider the frictional stress as a function of normal pressure, surface topography, length of sliding, viscosity, and compressibility of the lubricant.The theoretical work was carried out by means of upper bound and slipline field analysis based on experiments with model surfaces in wax and metal. The theoretical model applied is one of multihole extrusion, the material beneath the valleys of the workpiece surface being extruded up towards the tool when the real area of contact exceeds a certain value. The effect of the trapped lubricant is to build up a back-pressure on the extrusion process.The experimental work was carried out with newly developed equipment enabling direct determination of the abovementioned function; construction and calibration of the equipment are described. The equipment allows determination of frictional stress on a surface with well-defined values of normal pressure, sliding length, and sliding velocity. The normal pressure can attain about 8 times the yield stress for commercially pure aluminium.The results obtained show reasonably good agreement between theory and experiment, and a dependence of the frictional stress on the sliding length, this dependence being a function of normal pressure.     

On the theory of time-dependent micro-TEHL for a non-Newtonian lubricant in line contacts

This paper presents a relatively complete numerical solution to time-dependent micro-thermoelastohydrodynamic lubrication in line contacts subjected to constant load and entraining velocity. Sinusoidal functions are employed to model the traverse roughness on contact surfaces. The Eyring model is used to describe the non-Newtonian flow of the lubricant. With respect to time, the problem is treated as a periodic one so that most variables, such as pressure, film thickness and temperature, can be considered as periodic functions. An efficient algorithm is developed, and a number of cases are solved. The results indicate that the lubricant squeeze induced by the motion and interaction of rough surfaces significantly affects the solution of micro-thermoelastohydrodynamic lubrication.     

Hydrodynamic lubrication of rough slider bearings with couple stress fluids

The combined effects of couple stresses and surface roughness on the performance characteristics of hydrodynamic lubrication of slider bearings with various film shapes, such as plane slider, exponential, secant and hyperbolic, are studied. A stochastic random variable with non-zero mean, variance and skewness is used to mathematically model the surface roughness of the slider bearing’s. The Stokes couple stress fluid model is used to characterize the rheological behavior of the lubricant with polymer additives. The modified expressions for the bearing characteristics, namely pressure, load carrying capacity, center of pressure, frictional force are obtained for the general lubrication film shape on the basis of Stokes microcontinuum theory for couple stress fluids. Results are computed numerically for various film shapes under consideration. It is observed that, for all the lubricant film shapes under consideration, the negatively skewed surface roughness increases the load carrying capacity, frictional force and temperature rise, while it reduces the coefficient of friction. On the contrary, the reverse trend is observed for positively skewed surface roughness. Further, these effects are more pronounced for the couple stress fluids.     

Modeling Sliding Contact of Rough Surfaces with Molecularly Thin Lubricants

The sliding contact between two rough surfaces in the presence of a molecularly thin lubricant layer is investigated. Under very high shear rates, the lubricant is treated as a semi-solid layer with normal and lateral shear-dependent stiffness components obtained from experimental data. The adhesive force in the presence of lubricant is also adapted from the Sub-boundary lubrication model and improved to account for variation in surface energy with penetration into the lubricant layer. A model is then proposed, based on the Improved sub-boundary lubrication model, which accounts for lubricant contact and adhesion and its validity is discussed. The model is in good agreement with published experimental measurements of friction in the presence of molecularly thin lubricant layers and suggests that a molecularly thin lubricant bearing could be successfully used to reduce solid substrate damage at the interface.     

Friction Reduction in Mixed Lubrication

Minimization of frictional losses in the drivetrain of heavy-duty vehicles is important from both consumer satisfaction and environmental perspectives. Approaches to friction reduction in these components can be evaluated using simulation-based investigations. However, nearly all drivetrain components operate in the mixed lubrication regime which is difficult to model because both hydrodynamic lubrication and surface contact are significant and therefore, the total friction consists of hydrodynamic friction due to lubricant shearing and boundary film friction at asperity contact locations. Recent advances in simulation methods for mixed elastohydrodynamic lubrication (EHL) have enabled improved virtual design tools, such as those developed by Zhu and Hu and further improved by Liu et al. Here, these simulation tools are used to evaluate friction reduction and predict the effects on a mixed EHL interface under severe operating conditions. Three practical means of friction reduction are discussed based on the experimentally validated mixed lubrication friction model and its predictions made for representative, sample cases.     

A Universal Model for Both Flooded and Starved Lubrication Regimes and Its Application in Ring–Liner System

This article presents a new flow continuity model by modifying the traditional Elrod-Adams model. Both fully flooded lubrication and starved lubrication can be predicted using the universal model. In particular, for the starved lubrication problem, the inlet and outlet boundaries of oil film can be automatically determined. The discontinuity of the convection flow (or lubricant transport velocity) at the interface between the full lubricant film region and the partial lubricant film region is overcome by introducing a transition region. In addition, the two-dimensional version of the presented model is deduced. Furthermore, using the presented model, the results are presented to gain insight on the influence of starvation on the friction for the textured ring–liner system.     

Lubricant Effects in the Transition from Boundary to Microelastohydrodynamic Lubrication

Lubricant effects in the friction transition from boundary to microelastohydrodynamic lubrication were investigated by using a ballon-flat tribotester at sliding speeds from 0.02 to 0.88 mm/sec. Three lubricants—cyclophosphazine (X-IP), poly-alpha-olefin (PAO) and Z-DOL—were used, in this investigation. When X-IP was used at room temperature, a drop in friction coefficient from 0.22 to 0.12 at sliding speeds ≥0.10 mm/sec. (an unusually low speed) was observed, accompanied by a rise in the contact electrical resistance across the ball-fiat interface. The friction drop did not occur at temperatures ≥100°C. The friction transition was achieved at lower speeds when sliding perpendicular to the surface roughness texture. No transition occurred when PAO and Z-DOL were substituted as the lubricant. The latter him lubricants were working in the boundary lubrication regime as indicated by the contact resistance measurement.     

润滑油粘度对缸套/活塞环摩擦学性能的影响

在内燃机实际运行中,润滑油的粘度直接影响到润滑油膜的状态,因而活塞环在缸套中不同位置时的摩擦、润滑状态各不相同。文中以缸套活塞环为研究对象,建立了润滑计算模型,并运用该模型对缸内压力、温度、油膜厚度和摩擦系数进行了分析。结果表明,润滑油膜厚度和摩擦系数随转速改变而发生变化,而剪切稀化导致润滑油粘度减小是引起该变化的主要原因。最后,通过对计算结果的分析,提出了适用于缸套活塞环的润滑油粘度指标。