Dieter Lung

Influence of a High-Pressure Lubricoolant Supply on Thermo-Mechanical Tool Load and Tool Wear Behaviour in the Turning of Aerospace Materials

In the field of machining difficult-to-cut materials like titanium or nickel-based alloys, the use of a high-pressure lubricoolant supply may result in a significant increase of productivity and process stability. Due to enhanced cooling and lubrication of the cutting zone and thus reduced thermal tool load, tool wear can be decreased which allows higher applicable cutting speeds. Furthermore, the process stability can be increased as a result of effective chip breaking and chip evacuation. The present paper investigates the effect of high-pressure lubricoolant jets, directed into the tool—chip interface, in a longitudinal turning process with cemented carbide tools. For this type of lubricoolant supply the reduction of the contact length between tool and chip is an important detail. In turning of Inconel 718 and Ti6Al4V, the cutting tool temperature, tool wear, and resulting chip forms as well as the ratio of cutting forces and tool—chip contact area were analysed as a function of the lubricoolant supply pressure and flowrate (up to 300 bar, 55 l/min). To study the effect of the high-pressure lubricoolant supply, reference tests were carried out using conventional flood cooling. The results suggest that the tool temperature can be decreased by almost 25 per cent by the use of a high-pressure lubricoolant supply for both machined workpiece materials. However, due to the different tool wear mechanisms of the presented materials and the change in the specific load on the cutting edge during machining, the resulting tool wear was influenced differently. In the best case, tool wear could be reduced in an order of magnitude of 50 per cent, while in other cases tool wear even increased significantly due to the use of a high-pressure lubricoolant supply.

A new experimental methodology to analyse the friction behaviour at the tool-chip interface in metal cutting

This paper investigates a new test to analyse the friction behaviour of the tool-chip interface under conditions that usually appear in metal cutting. The developed test is basically an orthogonal cutting process, that was modified to a high speed forming and friction process by using an extreme negative rake angle and a very high feed. The negative rake angle suppresses chip formation and results in plastic metal flow on the tool rake face. Through the modified kinematics and in combination with a feed velocity that is five to ten times higher than in conventional metal cutting, the shear and normal stresses are only acting in a simple inclined plane, allowing to calculate the mean friction coefficient analytically. In addition, the test setup allows to obtain the coefficient of friction for different temperatures, forces and sliding velocities. Experiments showed, that the coefficient of friction is strongly dependent on the sliding velocity for the example workpiece/tool material combination of C45E+N (AISI 1045) and uncoated cemented carbide.

Finite-element-analysis of the relationship between chip geometry and stress triaxiality distribution in the chip breakage location of metal cutting operations

Abstract Chip breakage is a major machinability criterion of metal cutting operations. Favorable broken chips enable their efficient removal and prevent mechanical damages to the machined surface. Ductile failure on the chip free surface initiates chip breakage. The ductility of most materials depends on the stress triaxiality. Its relationship to the manufacturing parameters has to be understood in order to develop predictive methodologies of tool/process design. The problem can be approached by assessing the relationship between triaxiality and chip geometry, which is an integral representation of all tool/process parameters and material properties. This work presents a novel Finite-Element (FE) modeling approach of the relationship between the 3D chip geometry and the distribution of the stress triaxiality in the chip breakage location. For the derivation of the proposed approach it is shown that the stress state in the chip breakage location mainly develops during the chip bending phase. The approach enables for the first time to study the separate impacts of chip helical radius, helical angle, helical pitch and the shape of the deformed chip cross section on the triaxiality distribution. A sensitivity analysis of all input parameters is conducted and it is shown that all parameters but the helical pitch have characteristic impacts on the triaxiality distribution. The computational effort of the proposed modeling approach is significantly lower of than of available FE-models of metal cutting processes. The validation includes longitudinal turning experiments on steel AISI 1045 and 3D FE-process simulation modeling.

Broaching of Inconel 718 with cemented carbide

Broaching is the standard process for machining complex-shaped slots in turbine discs. More flexible processes such as milling, wire EDM machining and water-jet cutting are under investigation and show promising results. In order to further use existing resources and process knowledge, the broaching process has to be improved towards higher material removal rates. Taking into account that the state-of-the-art broaching process is working with high-speed-steel tools, the higher thermal resistant cemented carbide cutting materials offer the potential to significantly increase cutting speeds, which lead to increased process productivity. The following article presents a broad study on broaching with cemented carbide tools. Different cutting edge geometries are discussed on the basis of process forces, chip formation and tool wear mechanisms. Furthermore, a detailed comparison to the standard process is drawn.

Material-related aspects of the machinability of Austempered Ductile Iron

The exceptional properties of Austempered Ductile Iron (ADI) are widely known and can be ascribed to its austenitic-ferritic microstructure. Strain hardening of this material is exceptional and an advantage for many applications where high wear resistance is required, as well as the extraordinary combination of ductility and tensile strength. One reason which restricts the introduction of this material in practical applications is its poor machinability. This paper describes the material-sided influences on machinability, especially on the acting wear mechanism. The heat treatment factor austempering time, ADI grade and the alloying elements nickel and molybdenum are varied and investigated in external longitudinal turning operations.

Sustainabilty in Manufacturing – Energy Consumption of Cutting Processes

Rising prices on the international commodity markets as well as the increasing demand on environmental friendly products are generating new needs on product development and process planning. This includes also the development of an energetic product-life-cycle, which does not only comprise the use of a product, but also considers the production and refinement of raw materials at its beginning and possible recycling or end of life scenarios. Therefore within this paper an approach is presented for the evaluation of energy consumption for manufacturing. The results are discussed and evaluated for drilling operations. Final conclusions how cutting processes can be optimised are drawn at the end.

From Orthogonal Cutting Experiments towards Easy-to-Implement and Accurate Flow Stress Data

Despite recent advances in numerical modeling of machining processes showing a significant potential for shortening product and process design activities, the broader application of the Finite Element Method (FEM)–based modeling approaches in the manufacturing industry is still limited by the expensive and time-consuming techniques involved in obtaining accurate material flow stress data. This study proposes a new efficient approach consisting of a combined numerical-empirical methodology for inversely identifying the thermomechanical material behavior of AISI 316L stainless steel from machining experiments. In order to establish the work materials flow stress properties under the extreme conditions of machining, the Johnson–Cook (JC) constitutive equation is chosen to consider the impacts of strain, strain-rate, and temperature. Due to the unstable material behavior of AISI 316L stainless steel under certain thermomechanical conditions in the primary shear zone, the calibration of a damage model is integrated into the process for evaluating the flow stress data. The methodology approximates the material constants of the JC relationship by systematically comparing experimentally measured machining results with those predicted equivalents of a 2D implicit FEM process model. The methodology is experimentally verified on AISI 316L stainless steel with respect to cutting forces, chip geometries and temperatures in the tool-chip interface.

High Performance Cutting of Aerospace Materials

Titanium and nickel-based alloys belong to the group of difficult-to-cut materials. The machining of these high-temperature alloys is characterized by low productivity and low process stability as a result of their physical and mechanical properties. Major problems during the machining of these materials are low applicable cutting speeds due to excessive tool wear, long machining times, and thus high manufacturing costs, as well as the formation of ribbon and snarled chips. Under these conditions automation of the production process is limited. This paper deals with strategies to improve machinability of titanium and nickel-based alloys. Using the example of the nickel-based alloy Inconel 718 high performance cutting with advanced cutting materials, such as PCBN and cutting ceramics, is presented. Afterwards the influence of different cooling strategies, like high-pressure lubricoolant supply and cryogenic cooling, during machining of TiAl6V4 is shown.

High performance cutting of aircraft and turbine components

Titanium and nickel-based alloys belong to the group of difficult-to-cut materials. The machining of these high-temperature alloys is characterized by low productivity and low process stability as a result of their physical and mechanical properties. Major problems during the machining of these materials are low applicable cutting speeds due to excessive tool wear, long machining times, and thus high manufacturing costs, as well as the formation of ribbon and snarled chips. Under these conditions automation of the production process is limited. This paper deals with strategies to improve machinability of titanium and nickel-based alloys. Using the example of the nickel-based alloy Inconel 718 high performance cutting with advanced cutting materials, such as PCBN and cutting ceramics, is presented. Afterwards the influence of different cooling strategies, like high-pressure lubricoolant supply and cryogenic cooling, during machining of TiAl6V4 is shown.

3D FEM Model for the Prediction of Chip Breakage

The aim of the presented work was to define a criterion for the prediction of chip breakage in turning C45E+N (AISI 1045). The chip formation, the chip flow and the expansion of the chip due to collision with the periphery were modelled three-dimensionally using the Finite Element Method (FEM). The mechanical loads in the chip breakage zone were determined by comparing the modelled chip with high speed filming records of the real chip breakage cycle. Based on the calculated loads in the chip breakage zone a novel damage criterion based on an approach of Johnson and Cook was developed. This criterion enables the FEM-model to simulate chip breakage three-dimensionally for different tool geometries and varying cutting parameters. The enhanced FE model correlated well with high speed filming records of the chip flow and breakage as well as with the empirical determined cutting forces and chip temperatures.