WO2018193242A1

WO2018193242A1 - Method of applying dry sliding friction on a metallic material

Info

Publication number: WO2018193242A1
Application number: PCT/GB2018/051009
Authority: WO
Inventors: Andreas Chrysanthou; Anatolii BABUTSKYI
Original assignee: University Of Hertfordshire Higher Education Corporation
Priority date: 2017-04-18
Filing date: 2018-04-17
Publication date: 2018-10-25
Also published as: GB2561564A; GB2561564B; GB201706079D0

Abstract

The invention relates to a method for the reduction of tensile stresses which arise in components made from metallic materials during (or after) processing, and/or the creation of compressive stresses in components made from such metallic materials through creation, generation or introduction of micro-plastic deformations under the surface of a metallic material. The invention particularly relates to a method where the creation or introduction of micro-plastic deformations under the surface of a metallic material is by the application of friction to the surface of the metallic material, wherein the application of friction is more particularly dry sliding friction.

Description

METHOD OF APPLYING DRY SLIDING FRICTION ON A METALLIC MATERIAL FIELD OF THE INVENTION

BACKGROUND OF THE INVENTION

Tensile stresses arise in components made from metallic materials during metalworking processes such as stretching and bending due to the application of exterior loads. These stresses are also known as common mechanical stresses. In addition sheet metal forming processes such as deep drawing, stretching, bending include operations that apply exterior loads which in turn causes the increase of tensile mechanical stresses in the metallic material above the yield point of the metallic material. As a result the plastic deformation of the metallic material occurs and forming takes place. Preliminary heating of the components can potentially reduce the value of the exterior loads needed for such manufacturing.

In addition when a metallic material is bent, after being released from the forces which caused the metallic material to bend, the metallic material will typically try to return to its original shape, this process is called springback. Springback in a metallic sheet material which is formed for example by deep-drawn or stretch-drawn, can have an impact on the dimensional accuracy of the finished component. In order to reduce the impact of the springback, metallic materials need to be over bent in order to compensate for the expected springback. However, if metallic materials are over bent significantly to overcome any expected springback, instead the integrity of the metallic material can be compromised. In some cases after the original cause of the stresses has been removed residual stresses remain in components. Typically residual stresses arise due to the development of deformation gradients caused by localized yielding of the material, volumetric changes during solidification or solid state transformations and from differences in the coefficient of thermal expansion in components manufactured from different materials. Examples of processes during which residual stresses are formed include cold drawing, rolling, welding, and coating.

Residual stresses can cause distortion or warping of the structures during manufacturing or service. Tensile residual stresses can reduce fatigue life, initiate cracks and increase speed of their propagation, increase general and pitting corrosion, cause stress corrosion cracking.

The value of tensile residual stresses and springback can be reduced using thermal and mechanical (or non-thermal) methods. Usually the methods involve processing the component to be stress relieved as a whole.

For example, the most common method to reduce such residual stresses is stress relief baking (annealing) and the problem with this method is that a part with residual stresses needs to be placed into a furnace. This can be problematic with large components, or where the treatment needs to be carried out whilst the component is in situ, such as in the case of a repair.

Creation of residual compressive stresses in the surface of a component is beneficial. Such residual compressive stresses increase fatigue strength, prevent crack initiation, reduce general and pitting corrosion as well as stress corrosion cracking of the surface.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of reducing tensile stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(b) applying a force to the metallic material to put the metallic material under tension; (c) applying dry sliding friction to the surface of the metallic material wherein the dry sliding friction is applied by placing a working tool in contact with the surface of the metallic material and moving the working tool with respect to the surface of the metallic material wherein the pressure applied by the working tool to the surface of the metallic material is between 0.5 MPa and 0.5σγ wherein σγ is the yield limit of the metallic material.

The method works by creating micro-plastic deformations under the surface of a metallic material using dry sliding friction, which in turn results in the reduction of tensile stresses in a metallic material. The tensile stresses reduced may be active tensile stresses, i.e. the material is actually under tension when the method is applied or they may be residual tensile stresses, i.e. they were created in the material through a force being applied in the past, which includes metal working operations, forming operations, joining operations and heating/cooling operations such as seen in welding.

The dry sliding friction causes wear on the surface of the metallic material which subsequently creates the micro-plastic deformations under the surface of the metallic material which subsequently reduces the tensile stresses in the metallic material.

Preferably the wear on the surface of the metallic material is the order of tens of micrometres.

Preferably the method reduces residual tensile stresses that arise during processing of the metallic material.

Preferably the method results in a reduction of the load required in the processing of the material. Preferably the processing includes metalworking operations.

Preferably the force applied to the metallic material creates tensile stresses in the metallic material. Preferably the thickness of the metallic material is about 0.5mm to about 3mm or less than about 3mm.

Preferably the force is continuously applied whilst the dry sliding friction is applied.

Preferably the tensile stresses are active tensile stresses. Alternatively the force is removed before the dry sliding friction is applied. Alternatively the tensile stresses are residual tensile stresses. Alternatively the tensile stresses are welding stresses.

Preferably the dry sliding friction is applied to the surface of the metallic material that has been put under tension.

Preferably the force creates a deformation in the metallic material.

Preferably the deformation in the metallic material comprises a deformation formed from a forming operation, preferably the forming operation includes but is not limited to bending, punching, extrusion, stretching, drawing, riveting, clinching, and welding.

Preferably the dry sliding friction is applied to the surface of the metallic material in the area of the deformation.

Preferably spring back of the metallic material is also reduced.

Preferably stress corrosion cracking of the metallic material is also reduced. Preferably welding stress of the metallic material is also reduced.

According to a second aspect of the present invention there is provided a method of creating compressive stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material; and (b) applying a force to the metallic material to put the metallic material under tension;

(c) continuing to apply the force to the metallic material whilst at the same time applying dry sliding friction to the surface of the metallic material; (d) removing the force after the application of the dry sliding friction is complete

wherein the dry sliding friction is applied by placing a working tool in contact with the surface of the metallic material and moving the working tool with respect to the surface of the metallic material wherein the pressure applied by the working tool to the surface of the metallic material is between 0.5 MPa and 0.5σγ wherein σγ is the yield limit of the metallic material.

Preferably the thickness of the metallic material is about 0.5mm to about 20mm or greater than about 0.5mm.

According to a third aspect of the present invention there is provided a method of reducing spring back in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(b) applying a force to the metallic material to put the metallic material under tension;

(c) continuing to apply the force to the metallic material whilst at the same time applying dry sliding friction to the surface of the metallic material;

(d) removing the force after the application of the dry sliding friction is complete

It is of note that the forming of sheet metal using any method at room temperature can result in springback. Preferably the thickness of the metallic material is about 0.5mm to about 3mm or less than 3mm.

Preferably the force creates a deformation in the metallic material.

According to a fourth aspect of the present invention there is provided a method of reducing stress corrosion cracking in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(c) applying dry sliding friction to the surface of the metallic material wherein the dry sliding friction is applied by placing a working tool in contact with the surface of the metallic material and moving the working tool with respect to the surface of the metallic material wherein the pressure applied by the working tool to the surface of the metallic material is between 0.5 MPa and 0.5σγ wherein σγ is the yield limit of the metallic material. Preferably the thickness of the metallic material is about 0.5mm to about 5mm or less than 5mm

Preferably the dry sliding friction is applied to the surface of the metallic material in the area where the material has been put under tension. Preferably the force creates a deformation in the metallic material.

Preferably the dry sliding friction is applied to the surface of the metallic material in the area of the deformation. Preferably the force is continuously applied whilst the dry sliding friction is applied.

Alternatively the force is removed before the dry sliding friction is applied.

According to a fifth aspect of the present invention there is provided a method of reducing tensile stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(b) heating the metallic material;

(c) cooling the metallic material;

(d) applying dry sliding friction to the surface of the metallic material wherein the dry sliding friction is applied by placing a working tool in contact with the surface of the metallic material and moving the working tool with respect to the surface of the metallic material wherein the pressure applied by the working tool to the surface of the metallic material is between 0.5 MPa and 0.5σγ wherein σγ is the yield limit of the metallic material.

Preferably the thickness of the metallic material is about 0.5mm to about 3mm or less than about 3mm.

Preferably the tensile stresses are residual tensile stresses.

Preferably residual tensile stresses are created in the metallic material through the heating and cooling thereof. Preferably the dry sliding friction is applied to the surface of the metallic material that has been heated and cooled.

Preferably the heating and cooling comprises welding of the metallic material to create a weld.

Preferably the dry sliding friction is applied to the surface of the metallic material in the area of the weld. In accordance with each of the first, second, third, fourth and fifth aspects of the present invention:

Preferably the working tool is slid backwards and forwards over the surface of the metallic material.

Preferably the working tool is slid side to side over the surface of the metallic material.

Preferably the working tool is rotated about a central axis over a portion of the surface of the metallic material.

Preferably the working tool is rotated about a central axis over a plurality of portions of the surface of the metallic material.

Preferably the working tool is rotated about a central axis over a portion of the surface of the metallic material at the same time as the working tool is slid over the surface of the metallic material.

Preferably the working tool is harder than the metallic material. Preferably the working tool is at least 50% harder than the metallic material.

Preferably the working tool comprises a magnet. In one alternative the magnet is a permanent magnet, in another alternative the magnet is an electromagnet. This is advantageous as a constant force is applied by the working tool to the surface of the metallic material, and the working tool simply needs to be moved about the surface of the metallic material. In the case of a permanent magnet the magnet may be an NdFeB magnet.

Preferably the working tool is slid over the surface of the metallic material with a velocity of sliding between 1 mm/s and 10OOmm/s, more preferably 200mm/s.

Preferably the working tool is rotated with a velocity of between Orpm and l OOOrpm, more preferably 100rpm. Preferably there are a plurality of working tools spaced along the surface of metallic material. Preferably the plurality of working tools are identical. Preferably the plurality of working tools are operated simultaneously.

Preferably the method reduces welding stresses.

Preferably the method reduces the load required to form a metallic material.

Preferably the method reduces the mismatch between coatings applied to the surface of the metallic material and the underlying material.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

Figure 1 illustrates a graph showing the dependence of F(t);

Figure 2a illustrates a front view of dry slide friction processing of pre-tensioned sample; Figure 2b illustrates a side view of dry slide friction processing of pre-tensioned sample; Figure 3 illustrates a graph for stress reduction and temperature vs. applied pressure under dry sliding friction processing of steel strip;

Figure 4a illustrates a front view of dry slide friction processing of pre-tensioned sample; Figure 4b illustrates a side view of dry slide friction processing of pre-tensioned sample; Figure 5a illustrates the method of bending samples;

Figure 5b illustrates the method of bending samples and dry slide friction processing; Figure 6 illustrates loading curves for samples bent with and without dry slide processing; Figure 7a illustrates an appearance of a sample after bending without dry slide processing;

Figure 7b illustrates an appearance of a sample after bending with dry slide processing; Figure 8 illustrates the method of bending samples;

Figure 9 illustrates the clamping positions used for the bent samples;

Figure 10a illustrates a method of applying dry sliding friction by sliding perpendicularly to the main tensile residual stresses in the sample;

Figure 10b illustrates a method of applying dry sliding friction by sliding along the main tensile residual stresses in the sample;

Figure 1 1 illustrates the appearance of samples after they have been undamped and allowed to springback after processing according to the methods of Figures 10a and 10b;

Figure 12a illustrates a front view of slide friction processing of a tubular sample;

Figure 12b illustrates a side view of slide friction processing of a tubular sample;

Figure 13a illustrates the condition of a sample which has not been processed after 24 days exposure in sodium nitrite solution;

Figure 13b illustrates the condition of a sample which has been processed after 24 days exposure in sodium nitrite solution;

Figure 14a illustrates a front view of a clamped welded sample;

Figure 14b illustrates a side view of a clamped welded sample;

Figure 15 illustrates a front view of a welded sample released from the clamps which has not been processed;

Figure 16a illustrates a front view of slide friction processing of a welded sample;

Figure 16b illustrates a side view of slide friction processing of a welded sample;

Figure 17 illustrates the method used to measure the deflection of the welded sample; Figure 18 illustrates the creation of compressive residual stresses in the metallic material after dry friction processing;

Figure 19 illustrates the surface of the sample after processing; and

Figure 20 illustrates a magnified surface of the sample after processing. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It was observed for the first time that the application of dry sliding friction can dramatically extend the relaxation of tensile stresses in metallic materials. These observations were confirmed following tests using sheet steels and aluminium alloys as well as 70/30 brass tubes. The mechanism of this phenomenon is due to the generation of micro-plastic deformation (shear) under the surface of the metal as a result of friction. If the treated portion is thin (thickness 0.5mm - 1 mm) and already under tensile stresses, micro-plastic deformation develops through the whole thickness of material when friction is applied, which in turn leads to a decrease in the tensile stresses through the whole thickness of the material. If the treated portion is thicker (thicker than 1 mm), the process will be the same, but the micro-plastic deformations will only form close to the surface of the material and the remainder of the material will still try to restore itself to its original shape, this results in the formation of a compressed layer under the surface of the material. The method of the present invention does have some similarities to the method of slide burnishing, however, there is a significant difference between the two. Slide burnishing relies on the application of a stress to a material that exceeds the yield stress of the material which leads to deformation of the material. In the present invention however, the stress that is applied to the material is below the yield stress of the material, and instead relies on the build-up of a frictional force which in turn results in the observed micro-plastic deformation. In the present invention dry sliding friction is used in contrast to slide burnishing which typically uses lubrication or diamond like coatings with a low friction co-efficient.

In addition tests were conducted where the temperature as a result of friction was measured; the temperature increase varied depending on the speed at which the frictional force was applied. The temperature increase (from 21 °C) at low speed was as low as 1 0 °C and as high as 63 °C at higher speed values. In order to compare mild steel, aluminium alloys and 70/30 brass were heat-treated in the furnace at temperatures up to 84 °C for 2hrs and no changes to the tensile residual stress were observed. Clearly the effect is not due to frictional heat, but is the result of dry friction itself.

Generally the method comprises:

(a) providing a metallic material;

(b) applying a force to the metallic material to create a tensile stress in the material or using a metallic component where residual tensile stresses are present after processing ; and

(c) applying dry sliding friction to the surface of the metallic material wherein the dry sliding friction causes micro-plastic deformation under the surface of the metallic material to which the dry sliding friction has been applied which in turn decreases acting (or residual) tensile stresses in the metallic material. The following conditions are preferred to both obtain the effect of the relaxation of tensile stresses and at the same time to prevent any damage of metallic material forming the processed component:

1 . Dry friction. Addition any lubricants reduces the effect.

2. Surface of the tool. The surface of the tool used for processing (i.e. the surface that contacts processed component) can be any shape (flat, cylindrical, spherical, etc.), but it must be smooth enough to prevent the substantial wear of processed component.

3. The normal force (the net force of the compressing tool and processed component together). Typically, the greater the force the greater the relaxation, however, there is a force upper limit. The stresses, σ_ρ, that develop in the processed component as the surface of the tool and the component being processed come in contact with each other under the imposed normal force, have to be below the yield limit, σγ, of the material of the processed component such that, σ_ρ ≤ σ_Υ to avoid substantial wear of processed component. In addition the normal force has to deliver normal pressure on the surface at least 0.5 MPa.

4. Direction of processing. Typically, greater results are obtained when the tool is moved along the direction of the highest main tensile stresses, however, movement of the tool in any other direction will also lead to the expected results. 5. Speed of sliding (speed of relative motion of tool and processed component). Under processing the tool can be slid transversely to the component being processed and can in addition or in the alternative be rotated. Therefore, the speed of sliding

is considered as a combination of feed velocity

(speed of the tool traversing relatively the processed component) and the speed rotation V_m (in case of rotational tool). In general, the greater the e greater the levels of relaxation observed, however, there is an upper limit of th

The needs to be less that the speed which would cause an increase of local temperature of processed area T_pr above (i) temperature T₀ under which local thermal stresses overcome yield limit σ_Υ of material of the component (T_a = σγ/(α ), where a and E are respectively a coefficient of thermal expansion and Young modulus of material of the component), or (ii) temperature T₀₂ under which surface oxidation takes place, or (iii) temperature 7^" _transf under which structural transformations in the material of the component take place. Therefore 7pr < min ( T_a, T₀₂, 7^" _transf). Typically, under the processing, T_a has a minimal value among three of them. 6. Duration of processing. Duration of processing is defined by the feed velocity V\ and geometry of the component being processed. All the tensioned surface areas of the component being processed must be traversed during the processing at least 1 time, more preferably at least 10 times, even more preferably at least 100 times. 1. REDUCTION OF TENSILE STRESSES

1.1 Tensile stress reduction in tensioned strip using dry sliding friction processing

A 0.85 mm thickness sheet of low carbon steel was used for manufacturing samples with 10 mm width and 250 mm length. The samples were mounted on a 10 kN Hounsfield machine, wherein the distance between jaws was 200 mm.

Figure 1 illustrates the dependence of F(t) during the test which demonstrates in particular the natural stress relaxation 10, the friction processing 14 and the stress recovery 12.

Since the tensile machine used was mechanical and the load in the machine was applied by screws, once the machine was stopped the distance between the jaws was kept constant. This meant that after reaching the required load and stopping the motor of the machine, any further F(t) behavior was due to the reduction or relaxation of stresses in the sample.

During the test a sample was loaded initially up to a force level of to obtain a stress level in the sample of about 180 MPa (which is lower than the yield limit) and then the machine was stopped. Some minor natural relaxation of stresses and their stabilization took place during 3 min period after initial loading, and as a result the sample was loaded finally by force of F₂, however, the most significant drop of the load from F₂ took place due to applied friction processing.

The application of dry sliding friction to a sample is illustrated in Figures 2a and 2b. Two hardened steel rods (working tools) 16 with a diameter of 10 mm and flat ends were pressed coaxially to the surface 22 of the sample 18 with a normal force P ranging from 15 N to 60 N. The normal force was created using springs 20.

The pressure p applied to the sample surface due to the force Pwas calculated as:

p

P = ^>

m

(2) where P- is a force applied to the sample surfaces, r- is a radius of the rods.

During the friction processing both rods were moved along the sample simultaneously in vertical direction up and down the sample between the jaws of the machine with velocity of sliding v = 200 mm/s for 1 min.

During the friction processing the sample became heated as a result of the application of the friction, and after the friction processing had finished the dependence F(t) indicated that there was a small recovery of the load due to shrinkage of the sample resulting from the sample cooling-down. The temperature of the samples was measured immediately after the processing using multimeter TEN A 72-7745 with K-type thermocouple.

For each sample the same time exposure of 5 min for recovery of stresses and their final stabilization was used. The level of stress reduction P was calculated as:

A F

R = 100 %

(1 )

Stress reduction data is presented in Table 1 below, and illustrated in Figure 3, where it can be seen that after processing the stress reduction R reached about 50%. Table 1

Data for stress reduction and heating of samples during the processing

Applied pressure Increase of temperature

Samples Stress reduction R, % p, MPa AT, °C

1 0.19 was not measured 1 5.6

2 0.19 2.6 14.9

3 0.38 was not measured 30.4

4 0.38 3.1 30.0

5 0.57 6.6 42.1

6 0.57 7.0 41 .0

7 0.76 9.7 52.1

8 0.76 1 1 .0 50.0

1 .2 Tensile stress reduction in tensioned strip using dry sliding friction processing

As with example 1 .1 above a 0.85 mm thickness sheet of low carbon steel was used for manufacturing a sample with 10 mm width and 250 mm length. The samples were mounted on a 10 kN Hounsfield machine, wherein the distance between jaws was 200 mm.

The testing procedure used was similar to that used in example 1 .1 , except that the application of the dry sliding friction processing, illustrated in Figures 4a and 4b, used a working tool 1 16 formed from a hardened steel cylinder with a channel and a flat butt which was pressed to the sample 1 18 which was in turn supported by support 124 to avoid bending of the sample. The tool 1 1 6 was rotated at a speed ω during the friction processing. The processing of the sample during test was localized within two contact patches 1 26. Again each of the samples were loaded initially up to a force level of Fi to obtain a stress level in the sample of around 180 MPa. In the tests the pressure p applied to each of the samples surface contact patches due to normal force P ranged from 1 MPa to 1 .5 MPa, the rotation speed ranged from 25 rpm to 200 rpm and the duration of processing ranged from 1 min to 3 min.

During these tests the stress reduction R which was registered (in the same way as in the example 1 .1 ) was within a range from 2% to 15%. The stress reduction R is lower than in example 1 .1 because of the localized area of processing in example 1 b of only the contact patches of the samples, compared with the full length and both sides of samples in example 1 .1 . The maximum temperature registered during the tests was about 50°C. It was found that the level of stress reduction increased significantly when the rotation speed and duration of processing were increased. The pressure p applied to the sample surface within the range used did not substantially impact the level of stress reduction. Furthermore, the addition of lubricant (oil) onto the contact patches during the friction processing decreased the level of stress reduction two fold. 1.3 Using dry sliding friction to reduce tensile stresses in a metallic material during forming operations

Tensile stresses arise in parts and structural elements during different forming metalworking processes due to application of exterior loads. These stresses are also known as common mechanical stresses. All sheet metal forming processes like bending, flanging, stretching and other include operations with application of exterior loads which cause increase of tensile mechanical stresses in the material of parts above the yield point. As a result plastic deformation of material occurs and forming takes place.

Application of dry sliding friction upon areas with tensile stresses during forming processes reduces these stresses.

As a result reduction of the applied force (and corresponding tensile stresses) takes place during forming processes. This means that the energy needed for forming processes will be reduced. Therefore efficiency of forming processes will be increased. Also springback can be eliminated.

Examples explaining this aspect are presented in Figures 5 to 1 1 .

During tests presented in Figures 5 to 6 an example of reduction of tensile stresses during forming processes is presented. A 1 mm thickness sheet of mild steel was used for manufacturing of samples with 65 mm width and 200 mm length. Each sample was bent using a 25 kN Tinius-Olsen 25ST machine according to the scheme presented in Figures 5a and 5b. In Figures 5a and 5b the sheet material 518 is provided, fixed support 530 is provided, and moving supports 532 are provided which apply a bending force equal 2F to the sheet material resulting in a bent sheet material.

The samples 518 were bent under 125 mm distance between moving supports, supports tip radius R = 5mm. Bending was stopped under 30 mm displacement and samples were finally unloaded.

Dry sliding friction was applied to the tensioned (bent) area of the sample, which is the area which contains tensile stresses. The application of dry sliding friction to the sample is presented schematically in Figure 5b wherein a cylindrical (20mm diameter) working tool 516 with a flat butt was pressed to a sample 518 with normal force P = 130 N, under processing the tool was slid traversely relative to the processed sample. Sliding was applied perpendicularly 522 to the main tensile stresses in the sample (illustrated in Figure 5b). The velocity of sliding ranged between v = 80 and 100 mm/s, and the total friction processing time per sample was 5 min. The tensile stresses in the contact patch were calculated using formulas for Hertz contact stresses as set out in Shigley's Mechanical Engineering Design, Richard Budynas, Keith Nisbett, (McGraw-Hill Series in Mechanical Engineering) 10th Edition, and maximum Von Mises stress in the sample was about 85 MPa (which is lower than the yield limit of the material of sample).

Figure 6 illustrates loading curves for samples bent without and with dry slide processing according to this example. One can see substantial (more than double) reduction of the bending force (and corresponding tensile stresses) during bending with application of dry friction processing. This means that the energy needed for bending of the sample was reduced accordingly (shaded area 604 on Figure 6 corresponds to the energy advantage obtained using the processing). So, efficiency of the bending operation was increased. The appearance of unloaded samples without and with processing are illustrated in Figure 7a and 7b accordingly. The sample without any processing shows a slightly higher springback and a larger bending radius. During tests presented in Figures 8 to 1 1 , an example of reduction of springback during forming processes is presented.

A 0.4 mm thickness sheet of aluminium alloy 2014T4 was used for manufacturing of samples with 35 mm width and 250 mm length. Each sample was bent using a 10 kN Hounsfield machine, according to the scheme presented in Figure 8. In Figure 8 a V-die 30 is provided, sheet material 31 is provided, a punch 32 presses the sheet material into the die opening which applies a bending force F to the sheet material resulting in a bent sheet material.

The samples 218 were bent using V-die 230 and punch 240 with a 90° opening angle, punch tip radius R = 5mm and bending force F = 5 kN (see Figure 9). The samples 218 were fixed on the punch 240 under application of a bending force using two clampings 244. Due to such fixing certain tensile residual stresses were introduced into the bent area.

Dry sliding friction was applied to the tensioned (bent) area which is the area which contains tensile residual stresses. The application of dry sliding friction to the samples is presented schematically in Figures 10a and 10b wherein a cylindrical (20mm diameter) working tool 216 with a flat butt was pressed to a sample 218 with normal force P = 130 N, under processing the tool was slid traversing relatively the processed sample. Two variants of sliding were applied: sliding perpendicularly 246a to the main tensile residual stresses in the samples (illustrated in Figure 10a), and sliding along 246b the main tensile residual stresses in the samples (illustrated in Figure 10b). The velocity of sliding ranged between v = 80 and 100 mm/s, and the total friction processing time per sample was 60s. The tensile stresses in the contact patch were calculated using formulae for Hertz contact stresses as set out in Shigley's Mechanical Engineering Design, Richard Budynas, Keith Nisbett, (McGraw-Hill Series in Mechanical Engineering) 10th Edition, and maximum Von Mises stress in the sample was about 85 MPa (which is lower than the yield limit of the material of sample).

A view of undamped samples without and after processing are illustrated in Figure 1 1 . Sample 48 without any processing shows maximal springback with an opening angle of 125°. Sample 50 processed in accordance with Figure 10a shows the opening angle of 99° and the sample 52 processed in accordance with Figures 10b shows minimal opening angle of 94°. (Note: full removal of the springback means opening angle of 90°).

1.4 Using dry sliding friction to reduce residual stresses in a metallic material

In some cases after manufacturing operations, tensile residual stresses remain in parts and structural elements. Usually tensile residual stresses arise due to the development of deformation gradients in various sections of parts and structural elements caused by localized yielding of the material, from differences in the coefficient of thermal expansion in parts manufactured from different materials and other reasons. Examples of technological processes during which tensile residual stresses are formed are cold drawing, welding, coating and other processes.

Application of dry sliding friction upon areas with tensile residual stresses reduces these stresses. Reduction of tensile residual stresses is useful for increase of fatigue life, reduce crack initiation and decrease speed of its propagation, reduce general and pitting corrosion, prevent stress corrosion cracking.

Examples explaining this aspect are presented in Figures 12 to 17. During tests presented in Figures 12 to 13, an example of reduction of tensile residual stresses that existed in cold drawn 70/30 brass tube is presented.

Hard cold drawn brass tube with 19 mm outer diameter and 1 .27 mm wall thickness was used for manufacturing of samples. Due to fabrication process (cold drawing) the tube is characterized by high level of circumferential tensile residual stresses. Samples with 30 mm length were cut from the tube and used for further tests as unprocessed ones.

Dry sliding friction was applied to a portion of the tube according to Figures 12a and 12b, tube 318 was rotated with speed ω = 600 rpm between two working tools 316 with flat butts pressed to the tube by force P = 260 N. Additionally, the both working tools 316 were slid back and forth with linear speed v = 10 mm/min along the tube 318 within end area / = 30 mm. Overall processing of a sample took place 2.5 min. Stresses in the contact patch during the processing can be calculated using formulas for Hertz contact stresses (Shigley's Mechanical Engineering Design, Richard Budynas, Keith Nisbett, (McGraw-Hill Series in Mechanical Engineering)) and maximum Von Mises stress in the sample was about 146 MPa (which is lower than the yield limit of the material of sample). The maximum temperature achieved during the processing was not higher than 85 °C. After processing the treated portion of the tube was cut off and used as sample for further testing (residual stress determination and corrosion tests) as processed ones.

The level of the residual stresses can be estimated using Hatfield and Thirkell formula ATSM E1928 - 99 Standard Practice for Estimating the Approximate Residual Circumferential Stress in Straight Thin-walled Tubing. Residual stresses are readily calculated from the change of the outside diameter of a sample that occurs upon splitting its length.

For stress corrosion tests the samples were immersed individually in containers with 100 ml of solution of sodium nitrite in deionized water and further exposed at room temperature.

Using above mentioned formula maximal tensile residual stress for samples without any processing was determined to be 300 MPa. After use of dry sliding friction processing maximal tensile residual stress for the samples was determined to be 145 MPa. Results of stress corrosion tests of the samples without and with processing after their exposure in sodium nitrite solution within 24 days are illustrated in Figures 13a and 13b, wherein Figure 13a illustrates a sample which has not been processed using the method of the invention, and Figure 13b illustrates a sample which has been processed using the method of the invention.

During tests presented in Figures 14 to 17 an example of reduction of tensile residual stresses in AA5083 plate existed after its welding is presented.

A plate 458 with 300 mm x 300 mm x 3 mm dimensions was made by butt welding two 150 mm x 300 mm x 3 mm rectangular blanks of AA5083. During welding the blanks were clamped as illustrated in Figures 14a and 14b, wherein the blanks 418 are clamped with solid clampings 444 to rigid base 454 and welded 456 along the joint between the two blanks 418. After welding and cooling down the plate 458 was released and some deflection w was seen as illustrated in Figure 15. The cause of the deflection is due to residual tensile stresses which are formed during welding. Dry sliding friction was applied to plate 458 in the clamped condition as illustrated in Figures 16a and 16b wherein a hard steel roller 416 with 25 mm diameter was slid (without rotation) back and forth with linear speed v = 200 mm/min along front surface of the weld 456 with normal force P = 55 N. The same procedure was repeated for the back surface of the weld 456. Processing of plate 458 took place for 10 min for each surface. The stresses in the contact patch during the processing were calculated using formulas for Hertz contact stresses (Shigley's Mechanical Engineering Design, Richard Budynas, Keith Nisbett, (McGraw-Hill Series in Mechanical Engineering)) and maximum Von Mises stress in the sample was about 50 MPa (which is lower than the yield limit of the material of the plate).

Deflection w of the plate before and after the friction processing was measured accordingly to a scheme illustrated in Figure 17 wherein the plate 458 is placed on a block 460 which itself is placed on a base 462. For each condition (before and after the friction processing) measurements of R values were calculated at three points A, B and C and measurements of lvalues were calculated at three points D, E and F.

Using the above technique, averaged deflections w_ave for plate 458 before and after the friction processing were calculated based on formula:

₌ (^hQA + KB + ^hoc ) - (^HD + _E + h_F )

ave ^

The average deflection for plate 258 before the friction processing was w_ave_before = 5.7 mm. The average deflection for the plate 258 after the friction processing was w_ave__an_er = - 0.7 mm. It can therefore be concluded that the processing resulted in a decrease of post-welding deflection (the plate was nearly flat after the processing).

Therefore, if we clamp the plate as illustrated in Figures 14a and 14b, much higher levels of tensile welding residual stresses will be obtained for plate 258 before the friction processing and substantially lower levels of tensile welding residual stresses for the plate 258 after the friction processing. Taking into account the linear ratio between displacements in the elastic body and the corresponding elastic stresses it can concluded qualitatively that tensile welding residual stresses were substantially decreased after use of dry sliding friction processing. 2. CREATION OF COMPRESSIVE RESIDUAL STRESSES IN A METALLIC MATERIAL

Creation of compressive residual stresses in the surface of parts and structural element is beneficial. They increase fatigue strength, prevent crack initiation, reduce general and pitting corrosion as well as stress corrosion cracking of the surface, etc.

Application of dry sliding friction under some conditions can create compressive residual stresses localized in processed area.

This technique preferably includes initial preliminary mechanical loading of metallic component to obtain some level of tensile stresses or shear stresses in material, next dry friction processing and final full removal of the initial mechanical load in order to observe more substantive results. Preferably the stresses created in the material are 0.5σ_Υ or less than σ_Υ (σ_Υ is yield limit of material); more preferably the stresses are equal 0.9σ_Υ. Then dry friction processing is applied and finally full removal of the initial mechanical load should be done.

Wherein the dry sliding friction causes wear and subsequent generation of micro-plastic deformation under the surface of the metallic material to which the dry sliding friction has been applied to a part or structural element loaded which in turn decreases the tensile stresses (or shear stresses) in the metallic material. After unloading the part or the structural element will contain compressive residual stresses in the processed areas.

To understand the technique see Figure 18 where the stress-strain behaviour of the material is presented. Suppose zero initial conditions {σ = 0, ε = 0). During preliminary mechanical loading the stresses in the material will amount to σ = σ (loading from 0 to A). Next dry friction processing is applied and stresses will drop to σ= σ₂ (unloading from A to B). After final full removal of the initial mechanical loading (unloading from B to C) the compressive residual stresses σ= ¾ will be created in the processed areas. This technique can be applied to thin walled components to obtain local compressive residual stresses through the wall thickness or thick wall parts and structure element to obtain surface compressive residual stresses in processed areas and in layers under the processed areas.

3. CONCLUSION

In each of the examples above significant reductions of tensile stress were obtained as a result of the application of dry sliding friction processing. The dry sliding friction causes micro-plastic deformation under the surface of the metallic material to which the dry sliding friction has been applied which in turn decreases acting (or residual) tensile stresses in the metallic material.

The physical mechanism of such reduction lies in micro-plastic deformations which take place under application of dry friction processing and wear. These deformations start from the treated surface and penetrate deeply into the material. For example, as can be seen from Figure 19, which illustrates the area indicated "A" in Figure 4a. In this Figure one can see Luders bands 602 which penetrate the whole thickness of the sample. Usually the Luders bands accompany plastic deformation of low carbon steels. It can be concluded that micro-plastic deformation initiated on the surface by the dry friction processing has spread through the whole thickness of the sample.

The same behaviour can be observed in the polished butt surface of brass sample after the processing. For example, as can be seen from Figure 20, which illustrates the area indicated "A" in Figure 9a there are signs of plastic deformation 600 which have spread through the thickness of the preliminary polished butt surface of the sample wall.

The processing can be done using simultaneously several identical working tools spread along the surface of metallic material. To simplify application of the force during the processing of thin-wall structure elements the working tool comprises a permanent magnet. This is advantageous as a constant force is applied by the working tool to the surface of the metallic material, and the working tool simply needs to be slid transversely relative to the processed surface. The strongest available permanent magnets are used (e.g. NdFeB magnet). In the case of processing of non-magnetic materials, backing with a magnetic material is necessary with a magnet (working tool) applying dry friction in the opposite site.

Claims

1 . A method of reducing tensile stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(c) applying dry sliding friction to the surface of the metallic material wherein the dry sliding friction is applied by placing a working tool in contact with the surface of the metallic material and moving the working tool with respect to the surface of the metallic material wherein the pressure applied by the working tool to the surface of the metallic material is between 0.5 MPa and 0.5σγ wherein σγ is the yield limit of the metallic material.

2. A method as claimed in Claim 1 wherein the thickness of the metallic material is about 0.5mm to about 3mm or less than about 3mm.

3. A method as claimed in any preceding claim wherein the force is continuously applied whilst the dry sliding friction is applied.

4. A method as claimed in any preceding claim wherein the tensile stresses are active tensile stresses.

5. A method as claimed in any preceding claim wherein the force is removed before the dry sliding friction is applied.

6. A method as claimed in any preceding claim wherein the tensile stresses are residual tensile stresses.

7. A method as claimed in any preceding claim where the dry sliding friction is applied to the surface of the metallic material that has been put under tension.

8. A method as claimed in any preceding claim wherein the force creates a deformation in the metallic material.

9. A method as claimed in Claim 8 wherein the deformation in the metallic material comprises a deformation formed from a forming operation, preferably the forming operation includes but is not limited to bending, punching, extrusion, stretching, drawing, riveting, clinching, and welding.

10. A method as claimed in Claim 8 or Claim 9 where the dry sliding friction is applied to the surface of the metallic material in the area of the deformation.

1 1 . A method as claimed in any of claims 1 to 10 wherein spring back of the metallic material is also reduced.

12. A method as claimed in any of claims 1 to 1 1 wherein stress corrosion cracking of the metallic material is also reduced.

13. A method of creating compressive stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material; and

14. A method as claimed in Claim 13 wherein the thickness of the metallic material is about 0.5mm to about 20mm or greater than about 0.5mm.

15. A method of reducing spring back in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

16. A method as claimed in Claim 15 wherein the thickness of the metallic material is about 0.5mm to about 3mm or less than 3mm.

17. A method as claimed in Claim 15 or Claim 16 where the dry sliding friction is applied to the surface of the metallic material that has been put under tension.

18. A method as claimed in any of claims 15 to 17 wherein the force creates a deformation in the metallic material.

19. A method as claimed in Claim 18 wherein the deformation in the metallic material comprises a deformation formed from a forming operation, preferably the forming operation includes but is not limited to bending, punching, extrusion, stretching, drawing, riveting, clinching, and welding.

20. A method as claimed in Claim 18 or Claim 19 where the dry sliding friction is applied to the surface of the metallic material in the area of the deformation.

21 . A method of reducing stress corrosion cracking in a metallic material using dry sliding friction wherein the method comprises: (a) providing a metallic material;

22. A method as claimed in Claim 21 wherein the thickness of the metallic material is about 0.5mm to about 5mm or less than 5mm

23. A method as claimed in Claim 21 or Claim 22 where the dry sliding friction is applied to the surface of the metallic material in the area where the material has been put under tension.

24. A method as claimed in any of claims 21 to 23 wherein the force creates a deformation in the metallic material.

25. A method as claimed in Claim 24 wherein the deformation in the metallic material comprises a deformation formed from a forming operation, preferably the forming operation includes but is not limited to bending, punching, extrusion, stretching, drawing, riveting, clinching, and welding.

26. A method as claimed in Claim 24 or Claim 25 where the dry sliding friction is applied to the surface of the metallic material in the area of the deformation.

27. A method as claimed in any of claims 21 to 26 wherein the force is continuously applied whilst the dry sliding friction is applied.

28. A method as claimed in any of claims 21 to 26 wherein the force is removed before the dry sliding friction is applied.

29. A method of reducing tensile stresses in a metallic material using dry sliding friction wherein the method comprises:

(a) providing a metallic material;

(b) heating the metallic material;

(c) cooling the metallic material;

30. A method as claimed in Claim 29 wherein the thickness of the metallic material is about 0.5mm to about 3mm or less than about 3mm.

31 . A method as claimed in Claim 29 or Claim 30 wherein the tensile stresses are residual tensile stresses.

32. A method as claimed in Claim 31 wherein the residual tensile stresses are created in the metallic material through the heating and cooling thereof.

33. A method as claimed in any of claims 29 to 32 wherein the dry sliding friction is applied to the surface of the metallic material that has been heated and cooled.

34. A method as claimed in any of claims 29 to 33 wherein the heating and cooling comprises welding of the metallic material to create a weld.

35. A method as claimed in Claim 34 wherein the dry sliding friction is applied to the surface of the metallic material in the area of the weld.

36. A method as claimed in any preceding claim wherein the working tool backwards and forwards over the surface of the metallic material.

37. A method as claimed in any preceding claim wherein the working tool is slid side to side over the surface of the metallic material.

38. A method as claimed in any preceding claim wherein the working tool is rotated about a central axis over a portion of the surface of the metallic material.

39. A method as claimed in any preceding claim wherein the working tool is rotated about a central axis over a plurality of portions of the surface of the metallic material.

40. A method as claimed in any preceding claim wherein working tool is rotated about a central axis over a portion of the surface of the metallic material at the same time as the working tool is slid over the surface of the metallic material.

41 . A method as claimed in any preceding claim wherein the working tool is harder than the metallic material.

42. A method as claimed in any preceding claim wherein the working tool comprises a magnet.

43. A method as claimed in any preceding claim wherein the working tool is slid over the surface of the metallic material with a velocity of sliding between I mm/s and 1000mm/s, more preferably 200mm/s.

44. A method as claimed in any preceding claim wherein the working tool is rotated with a velocity of between Orpm and 10OOrpm, more preferably 10Orpm.

45. A method as claimed in any preceding claim wherein there are a plurality of working tools spaced along the surface of metallic material.