Advanced Materials Research Vol. 769

Abstract: Titanium alloys offer outstanding properties with regard to its strength to density ratio and a good corrosive resistance in air atmospheres. Substantial advancements could be made by using titanium alloys, in particular for applications in the aerospace industry and medical engineering. However, no product innovation is possible without an appropriate machining technology. For example, low thermal conductivity and hot hardness lead to limitations regarding the applicable machining parameters, particularly for continuous cutting operations. Turning of high performance materials sets high demands on machine tools and especially on the used cutting tools. For conventional continuous cutting of titanium alloys the tool life time and therefore the tool life volume is limited due to the thermal mechanical behaviour. Depending on the chemical and structural composition of the alloy, conventional cutting operations can rarely be regarded as an economic solution. The Abrasive Waterjet Turning process (AWJT) represents a promising alternative manufacturing method to produce rotation-symmetrically or helical parts made of difficult to machine materials. The AWJT process combines the kinematics of conventional turning methods with process-specific advantages of the abrasive waterjet machining. The main advantages are the high variety of machinable materials, the long life time T of the focus nozzles of at least 300 minutes and its independence of the material to be processed. Furthermore, material-inhomogeneity or the initial geometrical contour of the workpiece cannot result in tool failures. An interaction of workpiece and tool known from conventional cutting processes cannot occur. An investigation on hyper eutectic aluminium alloys has shown that AWJT is an economic manufacturing process regarding the resulted material removal rates Q_w and tool life volumes. The resulting roughnesses and roundnesses are comparable to a rough turning operation. In addition, AWJT results in a lower hardness penetration depth t_w in comparison to conventional turning. Machining of titanium alloys with cylindrical and external turning operations as well as grooving is the next step in the experimental investigation of the machinability of difficult to machine materials with AWJT. Therefore, the objective of the presented work is to provide a model for predicting the material removal rate, the cylindrical roundness and the surface roughness of waterjet turning of the titanium alloy Ti6Al4V. In a screening experiment the significant setting parameters were identified and an adequate range of parameter settings for the response surface study was determined. The tested parameters were the feed rate v_f, the abrasive flow rate m and particle size d_p, the depth of cut d_c and the rotational speed n of the workpiece. It is shown that in relation to the material removal rate Q_w linear main effects as well as interaction effects are significant. The developed second-order-regression-model includes these linear main and interaction effects and the quadratic effects of the relevant setting parameters. Furthermore, the achieved material removal rates, tool life volumes, cylindrical roundness and surface quality are used as target values. Additionally the changes like plastic deformations and grain damages in the rim zone were compared to conventional machined parts. Relating to the material removal rate Q_w, up to 2.5 cm³/min could be achieved for AWJT at a maximum height of profile Rz below 100 microns. Furthermore, the investigation resulted in a maximum tool life volume of 750 cm³ at a given nozzle life time. The results show that AWJT can be used as an economic alternative manufacturing process for rough turning of titanium alloys.

Abstract: In this paper, a new method for the preparation of cutting edges via grinding is presented. This method enables the manufacturing of the tool macro and micro geometry in one setup without reclamping, allowing improved flexibility, repeatability and accuracy at reduced processing times. This new method is path controlled using a special elastic bond for the grinding wheels. By using elastic bond, a rounded cutting edge instead of undesired chamfers can be achieved, as the bond nestles around the cutting edge and elastically deforms. The elastic bond is specified by the grain concentration and its basic hardness. Besides the specifications of the bond, the process kinematics highly influences the properties of the cutting edge. The kinematics is a combination of the tool path (machining strategy) and the grinding wheel geometry. The presented experiments include the examination of three different kinematics using three different grinding wheel geometries, FEPA 1A1, 1V1 and 4A2. For each kinematics, three different grain concentrations and three degrees of basic bond hardness were tested, resulting in a complete amount of 27 parameter combinations. The outer diameter cutting edges of cemented carbide milling tools (end mills) were prepared in a 5-axis tool grinding machine. The shape and quality of the achieved cutting edge rounding was qualitatively evaluated by means of scanning electron microscopy (SEM).

Abstract: The machining at elevated workpiece temperatures has many technological advantages and was intensively addressed in research within the last 20 years. Hot machining is foremost applied for difficult to cut materials, such as stainless steels. The key reason of the advantages of hot machining of steels is a reduction of the shear strength and a prevention of phase transformation in austenitic steels. The main disadvantage of hot machining remains the requirement for heating of the workpieces in connection with large energy consumption. An optional method to avoid this disadvantage is to machine the workpieces directly after the heat treatment at elevated temperatures. This method is in focus of hot machining research at the IWT Foundation Institute of Materials Science in Bremen within the cluster project Ecoforge. Within the current work the results of turning of the precipitation hardening ferrite-pearlite (AFP-) steel in two heat treated conditions with tungsten carbide inserts are presented. The cutting forces, tool wear and the chip formation are analysed due to turning of steel 38MnVS6 in the soft condition (ferrite-pearlite) and in the heat treated condition (bainite). The results show the potential of utilisation of the heat energy from the hot forming integrated heat treatment for hot turning of forge steels with bainite structure. The experimental results pave the way to shorten the process chain for the investigated materials within the cluster project Ecoforge and reveal correlations of machining at different workpiece temperatures.

Abstract: With the help of an explicit finite element analysis, cutting of cold-rolled dual-phase steels for various tool clearances was studied. In the first part of the study, the influence of the element size in the shear zone of the sheet on the predicted cut edge geometry and punch force was assessed and the optimal simulation parameters were identified. In the second part, the fracture description was put into focus of the investigation. It is shown that the used mathematical description of the equivalent plastic strain at fracture as a function of the stress triaxiality does not yield accurate results for FEA-based prediction of the cut edge geometry. A need in a more accurate fracture characterisation and, possibly, a more advanced fracture description of dual-phase sheet steels and the directions of the future research are identified.

Abstract: Present work analyzes the influence of process and modified geometry parameters of an orthogonal final machining process (finishing) on the nanocrystalline surface layers generation by quantitative microstructural analysis. Thereby, AISI 4140 (German Steel 42CrMo4) in a state quenched and tempered at 450°C is used as workpiece material. Metallic materials used in technical applications are polycrystalline in nature and are composed of a large number of grains which are separated by grain boundaries. The grain size has a strong influence on the mechanical material properties. Metallic parts with a severe nanocrystalline grain refinement in the near-surface area show many beneficial properties. Such surface layers considerably influence the friction and wear characteristics of the workpiece in a subsequent usage as design elements working under tribological loads due to their extreme superplastic properties. The tribologically induced surface layers formation already starts during the manufacturing of the components, by leading to a change of workpiece material near the surface. Particularly when the depth of cut h becomes of the same order as the cutting edge radius r_ß, the ploughing process becomes increasingly important and strongly influences the chip formation process. The plastic zone depth within the surface layer is especially influenced by the design of the microgeometry of the cutting tools and increases almost linearly with the ratio of cutting edge radius r_ß to depth of cut h. The plastic zone is hereby approximately of the same order of magnitude as the cutting edge radius r_ß. Parameters that are studied and taken into account in the manufacturing process are cutting edge radius r_ß, depth of cut h and cutting velocity v_c. Variations of cutting depth h are performed in a range of 30 to 100 µm and variations of cutting edge radius r_ß are executed in a range of 30 to 150 µm. The microgeometries of the tools are preconditioned by abrasive grinding with a drag finishing machine and observed by a confocal light microscope. A cutting velocity v_c of 25 and 150 m/min is applied. The evaluation of the manufacturing process is carried out by detailed analyses of the microstructural conditions in the surface layer after processing using a Focused Ion Beam system. These material characterizations provide information about the surface engineering concerning the microstructural changes in the workpiece surface layer due to machining. Hereby, the grain size analysis is investigated by a line method based on the characterization of portions of several test-lines positioned across the two dimensional Focused Ion Beam images.

Abstract: In this paper the technological basics of machining the nickel-based superalloy Inconel 718 with a self-propelled rotary tool and an actively driven rotary tool are presented. The advantages compared to conventional machining with fixed round indexable inserts are discussed. In a series of experiments the influence of the tool rotation and modification of the process parameters to the chip formation process and characterization of the surface area of the workpiece are investigated. By using a multitasking lathe with a milling spindle further experiments are conducted. Thereby the influence of the direction of the tool rotation, the relative speed between tool and workpiece and the variation of the penetration angles are analysed.

Abstract: The contact pressure is one of the most important parameter in the industrial polishing process of ceramic tiles. The contact pressure is asically a function of the elasticity moduli of both tile and abrasive tool, the applied load, and also the curvature of the abrasive tool. Due to the wear, this curvature decreases during the polishing process, causing an increase in the contact pressure. The purpose of this work is to research the influence of contact pressure on the evolution of gloss and roughness of the polished ceramic tiles and to improve the quality of generated surface. The variation of curvature was replaced with the direct increment of three different normal forces onto the abrasive tool. It is known from literature that for fine abrasive grits higher tool loads increase gloss gain and decrease roughness. However, there are not many works that research the whole sequence of abrasives for different loads and compare the quality of the final surface. Polishing tests on a laboratory scale CNC-Tribometer have been used to study the industrial polishing process for unglazed porcelain ceramic tiles. Tests were carried out for three different tool loads with a sequence of progressively smaller silicon carbide abrasive particles embedded in a magnesia cement matrix. Tile surface quality was evaluated by roughness and optical gloss. The removed work piece material and the used abrasive were measured with a coordinate measuring machine. The distribution of gloss and roughness of the tile was measured before and during the experiments until a saturation of gloss and roughness for each grit number was achieved, respectively. The topography of the tile was measured before and after the polishing process with particularly grit number. The used abrasives show a general trend of increasing gloss and decreasing roughness during the process. The coarse abrasives caused the major effect on surface roughness and almost no effect on gloss. In opposite finer abrasives caused the major gloss enhancement and almost no effect on surface roughness. The results show the evolution of roughness and gloss for each load as a function of abrasive grit number and polishing time, as well as the material removal rate for each grit number and load.

Abstract: In the present study the vibration drilling process of light weight materials and compound stacks has been investigated. Fiber-metal compound materials provide excellent mechanical properties which make them a major choice in lightweight applications. Especially in aircraft industry the use of multi-layer materials is significantly increased during the last few years. To join parts of dissimilar materials usually rivets or bolts are applied as fasteners. Therefore it is necessary to machine boreholes with partially very high quality requirements. Because of the different material properties the machining process of serial stacks imposes high demands to the cutting tools and requires certain process strategies. Previous investigations revealed that the bore surface can be damaged during the extraction of the hot and sharp metallic chips. Besides the risk of thermal damage the main issue lies in an erosive expansion of the borehole diameter due to the reaming of metallic chips at the borehole surface. The chip extraction can be significantly improved by low frequency assisted vibration drilling. In that case the axial tool movement is superimposed by a sinusoidal oscillation (in this case 1.5 per revolution) which is provided by the tool holder. Under certain cutting conditions this leads to a controlled chip breakage. Compared to conventional drilling the process parameters, cutting speed v_c and feed f are supplemented with the amplitude A of the oscillation and the frequency f which represents the amount of vibrations per revolution of the tool. This causes radical changes to the kinematics of the process and therefore of the cutting conditions and chip formation. For a better understanding of the process a kinematic model for a two-flute cutter was developed which allows calculating the undeformed chip shape in dependency of the four cutting parameters v_c, f_z, A and f. The model also helps to predict whether a discontinuous cut will be achieved or not. To characterize the process and chip shape the following parameters are optionally calculated within the model: maximum chip thickness, chip radian, effective feed, feed speed at the moment of tool entrance and exit (for one chip). Experimental drilling trials in Al2024 T351 were used to evaluate the calculated parameters. The chip thickness and radian as well as the cutting time show a very good correlation to the calculations. It is interesting that the measured cutting forces are much lower compared to the theoretical values according to the Kienzle cutting force equation. Additionally it was found that the measured cutting force is strongly decreasing with an increasing cutting speed. Infrared images of the drilling process in Ti6Al4V were used to analyze the temperature close to the cutting zone and to observe the chip evacuation during the process. It was found that the cutting temperature is up to 50% lower when using vibration drilling. Furthermore it was shown that this effect is strongly dependent on the chip extraction. It is important that the chips do not stack in the drilling flute during the process. A chip breakage is facilitated by a decreasing ratio between feed and amplitude. At the same time an increasing material removal rate degrades the chip extraction even if the chips are separated. Besides the advantages of vibration drilling a major issue was found to be chipping at the cutting edges or even tool breakage. This could be avoided by a reduction of the oscillation amplitude and /or feed. Under consideration of these correlations the productivity of the drilling process and the bore hole quality in CFRP/Ti6Al4V-stacks could be significantly increased. The investigations have shown that vibration assisted drilling represents a huge opportunity, especially in the field of drilling composite materials. However further investigations are necessary to better understand this very complex process.

Abstract: Knowledge of temperature fields and heat flow evolving during metal cutting processes is of significant importance for ensuring and predicting the product`s quality. Furthermore, this knowledge enables an improved usage of resources, such as machine tools and tool deployment. The strength of the heat sources as a result of the process and the distribution of the temperature in the material directly influence the tool wear mechanisms, wear rate, thermo-elastic deflection of the tool centre point and the amount of heat flowing into the newly generated work piece surface. Especially the latter effect is of crucial importance when it comes to safety critical components as they are employed in aero-engines. In aviation industry, the surface integrity is used as a complex quality measure summarising several aspects at the machined surface and sub-surface out of which many issues are predominantly thermal issues (e.g. temperature driven hardening of the work piece material, re-cast and white etching layers as well as residual stress profiles).

Abstract: In aircrafts, hydraulic systems control moveable parts. For example parts like the front strut or the landing flaps. These parts are usually made from aluminium or titanium. Due to an increasing number of functions these valves show an increasing number of cross holes. The production process causes burrs at the intersection of the holes. Until now these burrs cant be removed reliably by an automated process. Remaining burrs can influence dimensional tolerances and reduce the efficiency and technical lifetime of the component. In some applications cross holes are used for the lubricant and coolant supply. In this case burrs can lead to blockades of critical passages or cause turbulences in the fluid. This can lead to leakage or bursting of the valve. Hence an uncontrolled removal of the burr during operation must be avoided. The consequence of these basic conditions is a time consuming manual deburring process. An automated deburring process of cross holes with industrial robots is usually performed with flexible abrasive brushes. Alternatively processes like AFM (Abrasive Flow Machining), ECM (Electro Chemical Machining) or TEM (Thermal Energy Machining) are used. Those processes are very efficient but require specialized equipment and cleaning processes for the used chemicals and the remaining abrasive paste. So they are not suitable for the deburring of safety related parts. This paper presents an experimental based approach for the robot based deburring of cross holes using industrial robots. For the deburring of cross holes several special tools are available. This article gives a short overview over the specific advantages and disadvantages of these tools. As the investigations revealed the best results can be achieved using the so called Orbitool developed by JWDone. The Orbitool is a tungsten carbide cutter developed for the deburring of cross holes. A better control of the required dimension at the intersection compared to brushes and other deburring methods is possible. Furthermore the tool can be used on machine tools and industrial robots and is flexible to a huge variety of bore diameters. The tool mainly consists of a ball shaped carbide milling cutter with a protective disk which is made of polished steel and a shaft of tool steel. To remove the burr the tool is moved along the bore axis into the smallest of the intersecting holes until the tip of the tool is close to the intersection. Then the tool is moved in radial direction to the bore surface until the tool axis corresponds to the interpolation diameter. This causes a deflection of the tool. In this situation only the protective disk is in contact with the bore surface. While the tool rotates it is moved towards the intersection in a helical motion. When the tool tip has reached the intersection the cutting edges get in contact with intersection and the deburring process begins. After the tool has passed the whole intersection it stops its rotation and is moved to the bore hole centre and then moved out of the workpiece. This paper deals with the optimization of the deburring process. The result mainly depends on the parameters movement speed of the robot, slope of the helical movement and rotational speed of the tool. The experiments are planned using DOE (Design Of Experiment) methods. Initial values for the optimization of the movement speed were determined by grid encoder measurements. Robotic specific parameters like the number of interpolating points and the influence of the path smoothing caused by the controller were also investigated. For the analysis of the burr and the secondary burr an optical 3-D measurement system is used. The results show that with the presented approach the burrs can be reliably removed. Before the deburring process the average burr height is about 60 μm and can be reduced so that there is no secondary burr visible. The result is a chamfer between 150 μm and 85 μm that depends on the process parameters. It can be demonstrated that a chamfer that is smaller than 100 μm leads to a secondary burr. Anyway the cycle time can be reduced from about 3 minutes for manual deburring to 30 seconds using an industrial robot. Additional wear analysis show that about 200 bore holes can safely be deburred.