WO2010009862A1 - Method for finishing a surface of a workpiece by means of forming a third body - Google Patents

Abstract

The invention relates to a method for finishing a surface of a workpiece by means of forming a third body utilizing a metal-cutting surface processing tool. The invention is characterized by the combination of the following process steps: providing a surface processing tool having a blade, the blade edge rounding rß of which approximately corresponds to a cutting depth h by which the blade penetrates into the tool during the surface processing, contacting the surface processing tool at the surface to be processed at a prescribed processing speed vc such that a) a plastic deformation occurs in the region of a deformation advancing zone advancing the surface processing tool at a shear stress that causes a flow stress to occur which can be associated with the surface region,  b) the surface end region to be finished is subjected to a strain rate increase to at least 10 s-1, followed by a strain rate decrease of the same magnitude as the strain rate increase, and c) the surface end region to be finished is subjected to a temperature increase of at least 1-103 Ks-1, followed by a cooling phase having a temperature change speed comparable to that of the heating phase.

Description

 Process for finishing a surface of a workpiece under

Training a third body

Technical area

The invention relates to a method for finishing a surface of a workpiece to form a third body using a cutting surface finishing tool.

State of the art

The power density in modern mechanical machines, such as internal combustion engines, is steadily increasing due to weight reduction and performance enhancement. For this reason, materials and functional surfaces subject to sliding or rolling friction are subject to greatly increased friction losses or friction energy densities, see M. Dienwiebel, M. Scherge, Nanotribology in automotive industry, Fundamentals of Friction and Wear, Eds. E. Gnecco, E. Meyer, Springer, Berlin Heidelberg, 2007, p. 549-560. In many mechanical applications, classical materials have reached the limit of their tribological load capacity.

To increase the robustness or resistance of tribologically relevant components, these were conditioned in the context of a so-called intake process of the entire mechanical system in which the respective component is integrated. This running process was used on large engines, eg commercial vehicles and Ship engines, represented by a specific intake program, which takes a few minutes to several hours, depending on the size of the engine, and is therefore time- and cost-intensive. With the help of extensive tribological and surface analytical methods, it was shown that under favorable shrinkage conditions, ie with suitable friction, a near-surface modified zone is formed which has a structure, chemical composition and mechanical properties, eg shear strength, compared to the microstructure of the base material is changed. This zone is known in the literature under the term "third body", see also Godet M., "The third body approach: an mechanical view of wear", Wear 1984; 100: 437, Godet M. "Third-bodies in tribology", Wear 1990; 136: 29 or Kragelski IV, Dobychin MM, "Basics of the calculation of friction and wear", Berlin: VEB-Verlag Technik, 1977. Also found in the literature Description of this zone terms such as "Mechanically mixed layer", Tribologically Transformed Structure (TTS) or in German-speaking countries partly also "Tribomutation".

From DE 10 2006 017 990 A1 u.a. a method for machining bores, in particular for machining cylinder surfaces of engine blocks in internal combustion engines forth, with which it should be possible to produce tribomutation layers or third body targeted in the final phase of machining a workpiece to the machined workpieces before the intended use especially To give advantageous tribological properties and thus to avoid the otherwise necessary inlet process. For this purpose, it is proposed to use a machining tool that is adaptable to a honing machine and has a working surface that is smooth and free of cutting means.

The documents DE 103 55 685 A1 and DE 10 2006 036 151 A1 describe methods for forming tribologically stressable surfaces on a workpiece, which provide for two processing steps. In a first Processing step, the workpiece surface is pre-processed, for example by means of honing, roughing or fine spindles. In a second, subsequent work step, the pre-machined workpiece surface is nitrided by laser to form a pocket structure.

Through own work it could be shown that the expression or the presence of the third body is directly correlated with small wear rates, in the range of about 1-50 nm / h, and low friction, see Scherge M, Shakhvorostov D, Pöhlmann K. Fundamental wear mechanism of metals. Wear 2003; 255: 395th

In many applications, however, the enema can not be systematically controlled but is subject to the conditions imposed by the end user on the component at the beginning of its use. As a result, the tribological system or the formation of the third body is subject to a stochastic fluctuation with respect to its microstructure, its thickness and chemical composition, which has an effect on the expected friction and wear behavior and makes its predictability even more difficult or impossible, or in the worst case Case, the life of the component is rapidly degraded.

Presentation of the invention

It is therefore the object of developing a method for finishing a surface of a workpiece to form a third body using a machining surface processing tool so that it is possible to produce a tribomutation on the surface of a workpiece under specification defining the tributation layer defining properties intended, ie it applies Specify procedural conditions concretely under which the tribomutation layer forms with certainty and this in a predetermined form. The solution of the problem underlying the invention is set forth in claim 1. The concept of the invention advantageously further features are the subject of the dependent claims and the description in particular with reference to said embodiments can be seen.

According to the invention, a method for finishing a surface of a workpiece to form a third body using a cutting surface treatment tool is characterized in that a machining surface finishing tool is used, with the local energy input during the removal of at least one finished surface end area is created, of which near-surface Tribomutationsschicht starts or adjacent to the near-surface Tribomutationsschicht. For this purpose, the surface treatment tool is provided with a cutting edge, whose cutting edge rounding r _ß largely corresponds to a depth of cut h, with which the cutting edge penetrates into the workpiece during the surface treatment. The cutting edge rounding r _β may differ materially from the depth of cut h in a range of 0.1 h <r _ß <4 h. The surface treatment tool interacts with the finished surface to be machined, with the specification of a machining speed v _c, such that a) the plastic surface is subjected to a plastic deformation in which it undergoes a shear stress, by which a yield stress that can be assigned to the surface region is achieved, b ) the final surface area to be completed is subjected to an expansion rate increase to at least 10 s ^-1 , followed by a strain rate decrease of the same order of magnitude as the strain rate increase, and c) the surface end area to be completed is subjected to a temperature increase of at least 1 -10 ³ Ks ^-1 followed by a cooling phase with a rate of temperature change comparable to that in the heating phase.

The term "cutting surface finishing tool" is to be understood as meaning all tools that have geometrically determined or indefinite Have cutting that interact with the workpiece surface in a material-removing interaction.

By combining experimental work with numerical simulations, it has been shown that the production of a nanocrystalline microstructure can be specifically adjusted in the process of shaping with the choice of the process parameters identified in solution. Indenter and microstructural analyzes were used to determine a solidification due to plastic deformation in the micrometer range or to find a tribologically favorable nanocrystalline structure near the surface. Thus, tribological conditioning can be implemented in the final step of manufacturing. In the following the experimental setup and the result are presented.

Brief description of the invention

The invention will now be described by way of example without limitation of the general inventive idea by means of embodiments with reference to the drawings. Show it:

Fig. 1 representation of the geometry and process parameters using the example of a

Zerspanprozesses, Fig. 2a diagram illustrating the load history of a near-surface material point after a one-time

Passage of the Zerspanprozesses, Fig. 2b Diagram showing the time course of the

Vergleichdehnrate, Fig. 2c diagram showing the time course of the

Effective strain,

FIG. 3 shows a diagram of metrologically determined coefficients of friction and FIG

Fig. 4 Diagrammatic metrological determined

Wear rates. Ways to carry out the invention, industrial usability

FIG. 1 schematically illustrates a machining process in which a chip is separated from the surface of a workpiece using a geometrically determined cutting edge. The figure here serves only to explain the principle and is therefore only to be regarded as a special case as will be explained below. Other processes in the field of manufacturing processes in which the principle can be applied are in particular the separation with geometrically-undetermined cutting edge such as grinding, but also in their characteristics similar processes such as extrusion, where by the targeted strong plastic Stress in the near-surface area can be achieved by correspondingly selected process parameters comparable combinations of load and state variables and their temporal and local courses.

The selectable parameters during the cutting process (see HK Tönshoff and B. Denkena: Spanen, Springer-Verlag, Berlin, 2004) include the cutting speed v, with which the cutting edge is moved over the workpiece surface to be machined, the cutting depth h, the chip and Open space angle y or a (stochastically distributed and thus not explicitly determined for geom. - undetermined cutting edges), selected cutting edge rounding r _ß (stochastically distributed for geom. - undetermined cutting edges) and the possible use of coolants and lubricants. The latter influences on the one hand the friction F _R applied during the process in the contact area between tool and workpiece, and on the other hand the chemical-physical configuration of the surface layer.

As soon as the cutting edge comes into contact with the surface of the workpiece to be machined at a predetermined machining speed and a predetermined infeed depth, the workpiece undergoes a plastic surface in a surface region leading the cutting edge, the area of the so-called primary shearing zone or the deformation leading zone transformation essentially by a shear stress. If the cutting edge has passed over a surface point on the newly forming surface of the workpiece, then there is essentially a strong, local elongation at this point. If the temporal change takes place between the plastic deformation and the final state with its characteristic elongation in a short temporal succession, then intrinsic material structure changes are initiated, which ultimately lead to the formation of third bodies.

During the machining process, a point that later comes close to the surface of the finished workpiece and thus potentially in the future direct contact area, goes through a complex load history. This solution can be characterized in more detail by the following processing states:

a) a very pronounced and early onset of shear stress until reaching the yield stress that precedes the cutting edge as a plastic shaft b) an extremely high strain rate increase in the material to> 10 s ^"1 and subsequent equally pronounced decay to ~ 0 s ^{" 1} and c) a strong local temperature increase ≥ 1 -10 ³ Ks -1 and subsequent pronounced cooling phase whose temporal temperature change is of comparable order of magnitude as the temperature rise of the warm-up phase.

The above processing conditions must be performed within a very short period of time, preferably within fractions of a millisecond, in the immediate temporal sequence at the location of the workpiece surface to be machined.

For the intended formation of a third body as a near-surface layer on a workpiece, it is therefore the process parameters with which the surface treatment is performed to coordinate exactly with each other and this in particular taking into account the material properties of each workpiece to be machined. In Figure 2a is an example of a characteristic Stress history of a near-surface point on a finished surface of a workpiece shown by means of thermo-mechanically coupled

FE simulation was won.

FIG. 2 a shows a diagram in which a plurality of function curves are shown in overlay. Here it is assumed that a workpiece - to which said near-surface point belongs - is driven from the left in one direction relatively and at a predetermined speed against a fixed-mounted cutting tool, which is equivalent to the process as shown in Figure 1. In this case, the fixed tool is moved relative to the workpiece on the surface, wherein the tool causes a material removal by means of machining on the workpiece surface. The constant values at x = 0 therefore correspond to the parameters in the material point to the beginning of the simulation. It can be seen from the temperature curve T that starting from room temperature, the workpiece surface undergoes a rapid increase in temperature, due to the leading edge of the cutting plastic deformation and the subsequent separation process by the cutting edge. In the case shown, surface temperatures of up to 400 ^° C. are locally achieved. After passing over the cutting edge over the surface point on the newly formed workpiece surface, the latter undergoes a rapid temperature drop, depending on the simulation parameter, at least in the example shown here, to half the maximum temperature and within fractions of a millisecond. If one starts from a machining speed with which the tool is guided over the workpiece surface of a few m / min, then the process section shown in the diagram represents a time interval of approximately 1 to 2 ten thousandths of a second.

If one considers the function of the von Mises stress σ _v , which is likewise shown in the diagram, the high stress value at the left edge of the diagram is due to the plastic deformation in the region in front of the cutting edge. After the separation process von-Mises tension decreases significantly. However, the stress induced by the cutting stress induced in the workpiece surface by machining and the comparative expansion rate causing it, which as a peak-shaped functional curve D is largely superimposed on the location of the temperature maximum, are to be emphasized.

FIG. 2 b shows this functional progression along a time axis representing the comparative elongation rate D, from which it can be read that the time duration for the process of local material removal at a machining point on the workpiece surface is only one or several ten thousandths of a second. This illustrates the high dynamics with which the workpiece surface is subjected to the processing history described and which is largely responsible for the solution-based formation of a near-surface layer of a third body.

It can be seen from the graph in Figure 2c that a comparative strain of greater than 1 occurs within the process time span described above, i. Workpiece areas undergo deformation within a few ten-thousandths of a second, which would be twice as long in the case of one-dimensional expansion. This also illustrates the extremely short-term effect on the surface material of the workpiece, which is associated with a significant material structure distortion and helps to generate a material structure of a third body.

Experimentally, the desired formation of a third body on a workpiece made of 90MnCrV8 could be demonstrated by treating the workpiece on its surface to be machined with a geometrically determined cutting edge in the following six process variants:

As a result of this complex and, in particular, highly dynamic loading history, the structure of the workpiece is converted from an ordinary microstructure into a nanocrystalline microstructure in a near-surface area, cf. Third body, which has an excellent effect on the service life of tribologically stressed components.

Experimental investigations on the 90MnCrV8 existing workpieces, which can be seen in the table above

Surface treatment parameters have been completed in the manner according to the solution surfaces, confirm the improvement of the mechanical wear properties. Thus, both the friction and the wear properties of the workpiece surfaces treated in the context of tests i to vi with the parameters specified in each case were measured. For this purpose, in each case a friction body was guided over the workpiece surface treated in accordance with the solution, and the coefficient of friction μ and the wear rate v _v [nm / h] were determined. In FIG. 3, the determined coefficients of friction μ for the individual workpieces treated in tests i to vi are shown in relation to the machining speeds vc with which the respective surface treatment tool has been guided over the workpiece surface to be treated. On the one hand, it turns out that the respectively determined friction value μ decreases the greater the machining speed vc is selected. On the other hand, it can be seen that the use of a cutting tool with a cutting edge rounding of 60 μm causes significantly lower coefficients of friction on the workpiece surface than the use of a cutting tool with one Cutting edge rounding of 10 μm. A comparable tendency is also reflected in view of the wear rate, which can be seen in the diagram in Figure 4. Also in this case, it can be said that the wear is lower, that is, the wear rate is lower when using a cutting tool with a cutting edge rounding of 60 μm (see cases ii, iv, vi). An increase in the processing speed vc also leads to improved, ie lower wear.

The experimentally obtained results support the current understanding of the formation of a third body influencing the wear of a workpiece in such a way that the choice of a

Surface treatment tool with a cutting edge rounding in the range of the selected depth of cut leads to significantly improved wear properties on the workpiece surface. Nevertheless, it has been shown that cutting edge roundings deviating from the size of the cutting depth also have a metrologically detectable influence on the wear behavior in an improving manner. Thus, it can be seen that a surface treatment tool with a cutting edge rounding, which is preferably in the range of depth of cut, but may deviate from the depth of cut by a factor of 10 downwards or by a factor of 4, leading to the formation of a third body on a workpiece surface, in particular taking into account the fact that the training behavior for a third body depends on the particular workpiece material. It is conceivable, therefore, quite possible that for certain workpiece materials or for certain preconditioned workpieces, the bpsw. have been subjected to a heat treatment, the choice of a cutting edge rounding, which deviates from the selected depth of cut h can lead to better wear properties than in the case of rp = h. This must be suitably adapted depending on the workpiece material.

The advantage of the solution according to the approach lies in the fact that the last step of a shaping processing and the tribological conditioning under choice of optimal process parameters of the forming process are combined to form a process step. This makes a significant cost and effort reduction possible.

Since the stress history is decisive for the expression of the third body, it is apparent that a setting of a tribologically optimal third body can be achieved by different but in their principle similar procedures. Since own investigations are dedicated specifically to the process of Mikrozerspanens (among other things to the finishing process) and the structure of the third body could be detected in the vicinity of the so-called minimum depth of cut, however, in addition to the separation with geometrically defined cutting edge also methods of separating geometrically undetermined cutting edge according to the solution and used analogously to the described method.

The new finishing process thus makes it possible either to completely omit an intake program or to significantly shorten this process, since the third body can already be formed during machining to the geometric final dimension. Other finishing steps such as polishing or surface layer technologies may also be omitted, since the method can achieve a very high dimensional accuracy and surface quality, right up to the direct end installation of movable components treated in their functional groups. The new method results in improved stability as well as a larger operational and application corridor at the beginning of the use phase of the mechanical system.

The method according to the invention can basically be applied to all tribologically loaded functional surfaces in mechanical systems, such as shaft journals, slide bearings, piston rings, cylinder liners or even ball bearings etc. LIST OF REFERENCE NUMBERS

T Temperature profile σ _v von Mises stress D Comparison strain rate

Claims

claims 1. A method for finishing a surface of a workpiece to form a third body using a cutting surface finishing tool, characterized by the combination of the following method steps: Providing a surface treatment tool with a cutting edge whose cutting edge rounding rp corresponds approximately to a depth of cut h, with which the Cutting edge penetrates into the workpiece during the surface treatment, contacting the surface processing tool with the surface to be machined under a machining speed v _c such that a) in the region of the surface leading tool leading deformation lead zone plastic deformation takes place with a shear stress through which a surface region assignable Yield stress is reached, b) the surface finish to be completed is subjected to a strain rate increase to at least 10 s ^-1 , followed by a strain rate decrease of the same order of magnitude as the strain rate increase, and c) the surface end area to be completed, a temperature increase of at least 1-10 ³ Ks ^"1 followed by a cooling phase with a temperature change rate comparable to that in the heating phase. 2. The method according to claim 1, characterized in that a tool for cutting with geometrically defined cutting edge is used as a surface processing tool. 3. The method according to claim 1, characterized in that a cutting or cutting tool with geometrically indefinite cutting edge is used as a surface processing tool. 4. The method according to claim 2 or 3, characterized in that the surface processing tool penetrates with a penetration depth of 1 micron to 500 microns in the workpiece. 5. The method according to any one of claims 1 to 4, characterized in that the cutting edge rounding a radius between 5 microns and 500 microns is selected. 6. The method according to any one of claims 1 to 5, characterized in that during the surface treatment between the workpiece and the surface processing tool, a coolant and / or lubricant is used. 7. The method according to any one of claims 1 to 6, characterized in that workpieces are used, represent the finished functional surfaces tribologically resilient functional surfaces. 8. The method of claims 1 to 7, characterized in that the processing speed v _c from a speed range between 1 and 1000 m / min, preferably 1 and 150 m / min is selected. 9. The method according to any one of claims 1 to 8, characterized in that for the cutting edge rounding rp is: 0.1 h <r _ß ≤ 4h