E. Brinksmeier

Residual Stresses — Measurement and Causes in Machining Processes

Summary The functional behaviour of a machined component is substantially determined by the physical state of its surface including the residual stress distribution near the surface. The necessity to come to more exact methods of layout for machined components compels one to take “surface integrity” as defined by M. Field /1/ and herewith residual stresses into consideration. Residual stresses directly influence the deformation of workpieces, their static and dynamic strength, and their chemical and electrical properties. According to the relevance of residual stresses generated by machining, several laboratories affiliated with CIRP worked on a cooperative investigation of measurement techniques to define the state of the art. Measurements of residual stress distributions generated by some important machining processes have been made. This paper gives a report and tries to show how the possible sources for development of residual stresses are involved.

Capability Profile of Hard Cutting and Grinding Processes

This keynote paper aims at matching the supply of research results with the industrial demands in hard cutting and grinding. The capability profiles of the processes are characterised and several manufacturing solutions are discussed. The comparison of hard cutting and grinding operations is carried out with regard to certain evaluation criteria based on the functionality of the machined workpiece itself, discussed at different levels, and the process economical efficiency. The basis for a roadmap of future development of hard machining technology is provided, e. g. the main technological developments associated with multi-processing hard machining concepts are given detailed consideration.

Friction, Cooling and Lubrication in Grinding

Abstract It is generally considered that the heat produced during the grinding process is critical in terms of workpiece quality. Relatively high friction effects in abrasive machining cause heat generation which can lead to thermal damage in the surface layer of a machined part; cooling and lubrication therefore play a decisive role during grinding. The keynote paper shows the effect of coolant type, coolant composition and coolant supply on grinding processes and process results. Further investigations in the fields of fluid dynamic processes in supply nozzles and in the grinding zone are the key to optimization of cooling and lubrication during grinding, thus offering the chance to minimize the amount of coolant in circulation, leading to a reduction of adverse environmental effects and cost.

Drilling of Multi-Layer Composite Materials consisting of Carbon Fiber Reinforced Plastics (CFRP), Titanium and Aluminum Alloys

Abstract In this paper results are presented concerning the realization of economical drilling processes of multi-layer materials. Different carbide drill designs with improved geometries and coatings were investigated and compared by characterizing the cutting forces, tool wear, hole quality, and chip formation. Investigations have shown that dry machining of titanium workpiece layers leads to increased tool wear, chip formation problems, and surface damage in the aluminum and CFRP-layers. Consequently, the drilling experiments were carried out with minimum quantity lubrication (MQL) using different cutting fluids and supply strategies. The investigations were mainly focused on the development of the optimum drilling condition with respect to tool shape, tool material, and machining parameters. Another objective of the investigations was to analyze surface defects of the hole and the resulting diameter tolerances due to the high mechanical and thermal loads when machining titanium.

Utilization of Grinding Heat as a New Heat Treatment Process

Abstract In the past many scientists investigated the heat dissipation in grinding and the resulting influences on the surface integrity of the workpieces. Under abusive grinding conditions the formation of a heat-affected zone was observed which damaged the ground surface of hardened steels. The aim of this paper is to introduce a new surface heat treatment process, namely by making use of the heat flux generated in grinding. In this case the grinding conditions have to be optimized to induce martensitic phase transformations in the surface layers of annealed or tempered steels, such as it is achieved by other surface strengthening processes. The fundamentals of this new heat treatment method called grind-hardening are given in this paper.

Grind-Hardening: A Comprehensive View

Abstract The invention of advanced grinding processes enabling the surface hardening of steel parts was described for the first time in 1994 [1]. In such operations, named grind-hardening the dissipated heat in grinding is utilized to induce martensitic phase transformations in the surface layer of components. A grinding process then becomes a heat treatment operation like induction or flame hardening. The fundamentals of this new process, which had been developed up to first industrial applications, will be illustrated in this paper. Especially the impact of different grinding parameters on the structure and the achievable hardness penetration depth are discussed in detail.

Progress in Assessing Surface and Subsurface Integrity

Abstract Recent progress in both the development of characterization tools for the assessment of surface integrity, and the experimental examination of surface alteration, is reported. Emphasis has been placed on emerging characterization techniques. Examples of newly developed methods, with the potential for assessing surface integrity, are also included. Experimental results on the nature and extent of surface alteration have been presented for the processing of metals, ceramics and glasses, and single crystal materials. Particular attention has been given to characterizing surface and subsurface integrity at fine length scales.

Determination of the Mechanical and Thermal Influences on Machined Surfaces by Microhardness and Residual Stress Analysis

Summary There is a great demand for informations about the functional behaviour of technical surfaces dependent from machininq processes. Thermal, mechanical and chemical influences determine the relevant physical properties of the surface. Besides metallographical methods and microprobe analysis, microhardness and residual stress measurements are suitable tools to investigate the physical state of surface layers. It is reported about the fundamentals of these techniques, their experimental execution and the accuracy to be expected. The combined application offers the determination of causes for the generation of the surface state which are thermal or mechanical induced plastic deformation, structural transformation, or chemical reaction.

Ultra-Precision Diamond Cutting of Steel Molds

Excessive chemical tool wear occurs when steel alloys are machined with monocrystalline diamond tools. The basic idea of the presented research work is to avoid chemical reactions between the carbon of the diamond tool and the iron of the workpiece by establishing a chemical bond between the iron and another chemical element. Therefore a thermo-chemical process for modifying the chemical composition of the subsurface zone was developed. As a result the diamond tool wear was reduced by more than three orders of magnitude which by no other method has been achieved so far. The surface roughness obtained in diamond cutting of carbon steel was better than 10 nm Ra.

Aspects of cooling lubrication reduction in machining advanced materials

Abstract Coolant lubricants are recognized as undesirable factors in metal cutting, especially in machining advanced and difficult-to-cut materials. For both economic and ecological reasons, as well as because of increasing legislation, efforts are being made to reduce the use of coolants. On account of this, the introduction of dry machining and minimum quantity lubrication (MQL) techniques in machining processes is increasing. The results of the investigations carried out give an overview of the possibilities for influencing the machining of advanced materials, e.g. titanium alloys and extreme low-sulphur steels. The research topics focus on cutting tool performance and wear mechanisms at high cutting speeds while using different lubricants and cooling supply strategies. The investigations and verification experiments contribute to increasing process stability and tool life, improvement of machined surface finish and avoidance of tensile residual stresses.