Residual stresses in surface layer after dry and MQL turning of AISI 316L steel
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- Leppert, T. & Peng, R.L. Prod. Eng. Res. Devel. (2012) 6: 367. doi:10.1007/s11740-012-0389-3
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Residual stresses in the surface layer exert a significant impact on functional aspects of machined parts. Their type and value depend on the workpiece and tool material properties, cutting parameters and cooling and lubrication conditions in the tool-chip-machined surface interface. As the effects of material properties and cutting parameters have been widely studied, the influence of cooling and lubrication conditions, especially minimum quantity lubrication (MQL) on the surface layer residual stresses and the relationships between them have not been investigated. In this paper the effects of dry, MQL cutting and cutting with emulsion conditions together with cutting parameters on residual stresses after turning AISI 316L steel were investigated. X-ray diffraction method was used for measuring superficial residual stresses in the cutting (hoop) and feed (axial) directions. Tensile residual stresses were detected in both directions and the values in the cutting direction turned out to be higher than in the feed direction. The effects of cooling and lubrication conditions largely depend on the selected cutting parameters, whose influence is linked to the cutting zone cooling and lubrication mode. Elaborated regression functions allow calculation and optimization of residual stresses in turning AISI 316L steel, depending on cooling and lubrication conditions as well as cutting parameters.
KeywordsResidual stresses Surface layer MQL Dry turning
Residual stresses in the surface layer determine many application related characteristics of a machined part. Their type and value depend on the work piece material and tool properties, cutting parameters as well as on the cooling and lubrication conditions in the tool-chip-machined surface interface. Replacing the traditional method of cooling and lubrication of the cutting zone with emulsion with dry and MQL cutting significantly changes the prevailing cutting conditions in the chip and machined surface formation zone. An increase in the heat energy transferred into a machined component which affects the surface layer tensile residual stresses is particularly important. The residual stress in the surface layer originates from two sources: the interactions between factors characteristic of the employed technological process and the stress present in the part before the actual machining starts. The level of stresses in the surface layer results from complex phenomena of the following processes: mechanical—leading to inhomogeneous plastic deformation, thermal—causing thermal plastic flow, and physical—leading to phase transformations and specific volume variation. The experimental results indicate that residual stresses in the surface layer are mostly a consequence of complex interactions of the mechanical and thermal effects leading to inhomogeneous plastic deformation associated with the process of chip formation and an interaction between the tool and the freshly machined surface [1, 2, 3, 4, 5, 6, 7, 8].
The type of residual stresses and their level deeply depend on the mechanical properties of the machined material. With an increase of mechanical properties (material hardness, tensile strength, strain rate dependency, thermal conductivity, mechanics of plastification), the level of residual stresses also increases [6, 7, 9, 10]. High mechanical properties and severe work hardening of austenitic stainless steels during the chip formation process, combined with low thermal conductivity generate high cutting forces along with high localized interfacial temperatures and adhesion in the cutting zone which are the main reasons for high level tensile residual stresses both in the hoop and feed direction [5, 10, 11, 12, 13, 14].
It has been generally accepted that for a given material as well as cooling and lubrication method of the cutting zone the nature of the residual stress distribution in the surface layer depends on the cutting parameters [2, 7, 11]. With an increase of the cutting speed, the magnitude of all the stress components also increases [10, 11, 12, 15]. Experiments [11, 15] showed that higher cutting speeds in dry turning of austenitic stainless steels AISI 304 and 316L cause higher tensile superficial stresses in the hoop and axial direction. The great influence of the cutting speed on the thermal effects and its variation in making large differences in the residual stresses generated in the machined surface was confirmed by . However, in wet turning with an increase in the cutting speed the residual stress values can change from compressive to tensile because as the cutting speed increases machining process becomes more adiabatic . The influence of the feed rate on their type and value is not conclusive. An increase in tensile residual stresses alongside a higher feed rate has been showed in papers [3, 6, 12, 17]. An analysis of residual superficial stresses in the axial direction after dry turning has demonstrated that an increased feed rate leads to an increase in residual stresses and may also cause them to change from tensile to compressive . On the other hand, it has been found that in dry turning of AISI 316L steel the surface residual stresses barely vary as the feed rate increases , or its influence on surface residual stresses in turning stainless steel depends on other cutting parameter values, which can cause their values to increase or decrease and its effect should be analyzed in combination with all parameters and not just one . A large influence of depth of cut in turning stainless steel on the tensile residual stresses both in the primary and feed motion direction was demonstrated in [6, 13, 14, 15], however the research performed by  did not show any unique correlation between the depth of cut and residual stresses.
Apart from cutting parameters also the tool point geometry exerts a significant influence on residual stresses in the surface layer. An important element of its geometry, which determines the level of residual stresses, is the effective rake angle. An increase of this angle value from 0° to 5° in dry turning of AISI 316L steel caused a decrease in the tensile stresses both on the surface and inside the surface layer [15, 18]. On the other hand, an increase of the negative rake angle leads to greater compressive stresses, which are an effect of the mechanical stress factor . The experiments performed by  and  have shown that the surface residual stresses in the hoop and axial direction become higher as the tool edge radius rn or edge hone increase. The tool nose radius rε and the tool cutting edge angle κr have a similar influence on the residual stresses [6, 20]. The tool nose radius affects significantly impact on the surface residual stresses both in dry and traditional cooling with liquid . However, its influence in the initial phase of the cutting process becomes less pronounced as the tool wears out [22, 23].
The interactions of the above mentioned factors which are responsible for generating residual stresses largely depend on the cutting environment, including the cooling and lubrication mode of the cutting zone [10, 16, 21]. The advantageous effects of emulsion on the tribological phenomena in turning stainless steel and the improved lubrication in the tool—work piece interface was showed in . The use of coolant reduces friction and increases heat removal from the surface, resulting in the dominance of mechanical effects, which cause compressive stresses. According to , an application of cooling liquids in turning at low cutting speeds reduces the maximum of superficial residual stresses compared to dry cutting. At high cutting speeds, its influence on residual stresses is negligible. Eliminating emulsion leads to an increase in the values of tensile stresses, reduction of compressive stresses as well as a lower dispersion of values as compared to turning with emulsion [16, 21].
Although the problem of residual stresses in the surface and subsurface layer has been widely investigated, the research performed so far has showed that the effects of the cutting environment on residual stresses are still not fully recognized. Residual stresses in turning stainless steels were studied in a specific cooling and lubricating condition (mostly dry or wet) and for a limited range of cutting parameters. Investigations of the effects of MQL on residual stresses as well as comparative investigations of residual stresses generated in dry, MQL and wet turning conditions are still scarce. Therefore, considering the significance of residual stresses in machining, the paper presents a more comprehensive study, which examines the effects of MQL and dry cutting technique in relation to traditional cooling and lubrication by copious supply of emulsion. To allow evaluation and optimization of the effects of cutting conditions on hoop and axial components of residual stresses in turning AISI 316L steel, the correlations between residual stresses and cutting parameters for dry, MQL and with emulsion cutting conditions have been determined.
2 Experimental procedure
Material: AISI 316L (austenitic stainless steel), diameter: 60 mm, length of cut 15 mm
Composition: C < 0.03 %, Si ≤ 1.0 %, Mn < 2.0 %, P ≤ 0.045 %, S ≤ 0.015 %, N ≤ 0.1 %, Cr 16.5–18.5 %, Mo 2–2.5 %, Ni 10–14 %
Tool holder MSS 2525–12-EB (Mircona AB)
Carbide insert SNMG 120408-TF, grade IC 907, PVD coating composition TiAlN, (Iscar Ltd)
Rake angle γ0 = 5°, clearance angle α0 = 10°, cutting edge angle χr = 45°, cutting inclination angle λs = 0°, nose radius rε = 0.8 mm
Cutting speed: 82, 164, 255 m/min, feed rate: 0.08, 0.27, 0.47 mm/rev, depth of cut: 0.5, 1 mm
Cooling and lubrication conditions: D-dry, MQL-minimum lubrication, E-emulsion
The MQL technique was executed by a Minibooster II applicator (produced by Accu-Lube Manufacturing GmbH). The lubricant from the oil reservoir was transported by the pump at an air pressure of 0.6 MPa to the booster chamber where the aerosol was produced and transferred to the reservoir. From there, at a pressure of 0.35 MPa, it was supplied to the cutting edge through holes built inside the tool holder and two nozzles of 0.8 mm in diameter, targeting the rake face and principal and auxiliary flanks. As a lubrication medium an Accu-Lube LB 8000 biodegradable vegetable oil with kinematic viscosity of 37 mm2/s at 40 °C was used. The oil consumption rate was set at 50 ml/h. In wet turning, 6 % emulsion was prepared on the basis of emulsifying oil ARTEsol Super EP, supplied to the cutting zone with 4 l/min flow volume. The emulsifying oil comprised up to 35 % of mineral oil and additives increasing lubricity in the quantity of 15 % and was recommended for cutting operations of steel, cast iron and nonferrous metals.
d-sin2Ψ plot was generally less than 50 MPa. The error bars shown in the graphs present their values for specific turning conditions.
3 Results and discussion
The cooling and lubrication method had the highest influence on the hoop and axial stresses in machining with a low feed rate of 0.08 mm/rev and a cutting speed of 82 m/min. In such cutting conditions the highest values of hoop and axial residual stresses were observed in turning with emulsion (494 and 370 MPa) and the lowest in dry machining (327 and 108 MPa), while their values in cutting with MQL lay in between. The lower values in both directions after dry turning compared to other cooling and lubricating methods may have resulted from an increase of the cutting temperature in the chip formation zone. The temperature increase caused softening of the material in this area and eventually led to a decrease in the main cutting force and lower resultant stresses. The application of lubricating aerosol in MQL turning reduced the friction and temperature in the cutting zone which led to increased plastic deformation and higher residual stress values as compared to dry turning . Intensive flood cooling and easer penetration of emulsion in the tool- chip- machined part interface at low cutting speed significantly improved heat evacuation from the cutting zone and increased work hardening of the surface layer. A great tendency of AISI 316L steel to undergo plastic deformation, work hardening and formation of micro structural defects caused high tensile stresses even though the mechanical factor usually led to compressive residual stress after completing cutting process . The existing relations between the values of residual stresses and the cooling and lubrication technique pointed to the mechanical factor as the crucial reason for tensile stresses in machining with emulsion . With the increase of cutting speed to 164 m/min the influence of the applied cooling and lubrication methods on hoop stresses did not change. However, the increase of cutting speed to 255 m/min generated higher residual stresses both in the hoop and axial direction in dry turning (466 and 296 MPa) and MQL turning (508 and 320 MPa), than in turning with emulsion (407 and 212 MPa), indicating a change in the main mechanism of residual stress generation. An increase in the cutting speed from 82 to 255 m/min in dry turning increased the temperature and heat concentration in the cutting zone due to low heat conductivity of austenitic stainless steels. The absorbed heat remained longer in the surface layer and the increased influence of the thermal factor could have a decisive role in the generation of higher tensile stresses [11, 15]. The application of MQL and the lubricating action of oil aerosol did not change significantly the residual stress values as compared to dry cutting. Applying emulsion to the cutting zone in these conditions reduced the influence of heat in generating tensile residual stresses. These results pointed to a combined influence of the mechanical and thermal factors on constitution of residual stresses in the superficial surface layer after turning austenitic stainless steel, which remains consistent with the research by .
With an increase of the feed rate up to 0.27 mm/rev at the applied cutting speeds the influence of the cooling and lubrication conditions on residual stresses in both directions was reduced. The changes of stress values remained in the range of standard errors, which pointed out to a conclusion that the thermal and mechanical action remained in the relative balance.
A further increase in the feed up to 0.47 mm/rev did not differentiate significantly residual stresses between turning dry and with emulsion. However, lower values in both directions were observed in the MQL conditions, especially in low cutting speed (82 m/min). The low values of the hoop (430 MPa) and axial (411 MPa) stresses could be attributed to the penetration and lubrication action of the oil aerosol. With an increase of the cutting speed to 255 m/min, the effect of cooling and lubrication became less pronounced, slightly lower in MQL turning than in dry turning. Similarly to , the obtained results confirmed a considerable influence of the cutting speed and feed rate on the process of generating residual stresses.
The research has revealed that dry and MQL turning at a low feed rate, recommended for finish turning, and at a low and medium cutting speed (82, 164 m/min) facilitated reduction of residual stresses compared to emulsion turning. However, a greater reduction of residual stresses can be achieved in dry turning. In higher cutting speed (255 m/min) lower values of residual stresses were generated in turning with emulsion than in dry and MQL turning.
Considerable differences between both stress component values were observed in the whole range of the used cutting parameters. The cutting speed exerted the highest influence on the increase of the values of hoop and axial residual stresses within a speed range of 82–164 m/min, at a low feed rate, and along with an increase of the cutting speed to 255 m/min its impact was limited. Increasing the feed rate value to 0.27 and 0.47 mm/rev resulted in increased values of residual stresses but reduced the influence of the cutting speed on the hoop and axial stresses. The reason may have been related to a greater chip cross-section and greater heat evacuation from the cutting zone by the chip, which led to reduced impact of the thermal factor on the generating process of residual stresses. The results partly confirmed conclusions presented in papers [7, 10, 15].
Regression functions for hoop σhoop and axial σaxial residual stresses
Cooling and lubrication conditions
Regression functions σhoop, σaxial = f (vc,f,ap)
σhoop = 137.0921 + 2.3979*vc − 2.0373*vc*f − 0.5834*vc*ap + 1,276.998*f*ap − 0.003*vc2 − 1101.03*f2
σaxial = − 81.869 + 3.4281*vc − 2.8811*vc*f − 0.8969*vc*ap + 1,191.931*f*ap − 0.0047*vc2 − 252.857*f2
σhoop = 224.1831 + 0.1811*vc + 0.2319*vc*f − 0.4884*vc*ap + 1,168.171*f*ap + 0.0014* vc2 − 1,592.67*f2
σaxial = 224.1831 + 0.181101*vc + 0.2319*vc*f − 0.4884*vc*ap + 1,168.171*f*ap + 0.0014* vc2 − 1,592.67*f2
σhoop = 353.3383 + 2.1756*vc + 0.6793*vc*f − 0.2049*vc*ap + 365.2077*f*ap − 0.007* vc2 − 698.403*f2
σaxial = 276.6404 + 1.5182*vc − 0.0057*vc2 + 1.747*vc*f − 616.622*f2 − 0.51737*vc*ap + 420.3824*f*ap
The cutting zone cooling and lubrication conditions exert a substantial influence on the superficial residual stresses in the machined surface layer. The values of the hoop stress component are larger than those of the axial component and depend to a large extent, in addition to cooling and lubrication, on the applied cutting speed and feed rate. The level of residual stresses results from a joint action of the thermal and mechanical factors. It is recommended to perform further research into residual stresses appearing inside the surface layer, depending on the cooling and lubrication mode.
The influence of the cutting speed, feed rate and depth of cut on residual stresses depends on cooling and lubricating conditions in the cutting zone.
An increase of the cutting speed during dry turning causes an increase in hoop residual stresses when a low feed rate is used (0.08 mm/rev). When the feed rate increases (0.47 mm/rev), the influence of this parameter becomes less pronounced. An increase in the feed rate causes higher hoop and axial residual stresses.
In MQL turning, an increase in the cutting speed results in higher hoop and axial residual stresses. Increasing the feed rate does not considerably affect the hoop stress but causes higher axial residual stresses.
In turning with emulsion, the influence of the cutting speed and feed rate on hoop residual stresses did not present any conclusive trend. An increased cutting speed may cause higher or lower hoop and axial residual stresses, depending on the used feed rate. Regarding axial residual stresses, the increase of cutting speed allows reducing their value at low feed rate (0.08 mm/rev) but with an increase of feed rate the effect of cutting speed decreases but values of axial residual stresses raise.
In the applied conditions of cooling and lubrication an increase of the depth from 0.5 to 1 mm does not influence significantly hoop and axial residual stresses.
The research showed that at low feed rate (0.08 mm/rev), recommended for finish turning, and a low and medium cutting speed (82, 164 m/min) elimination of emulsion or application of MQL allows reduction of residual stresses. At these cutting speeds lower values of hoop stress than after turning with emulsion (494, 555 MPa) occurred after dry turning (327, 438 MPa) and with MQL (428, 497 MPa). Increase of cutting speed to 255 m/min at low feed rate generates higher residual stresses in dry and MQL turning than in turning with emulsion.
The results of the presented investigation and regression functions can make up guidelines for selection and optimization of cutting parameters taking into account the residual stresses produced in turning the AISI 316L steel as well as cooling and lubrication conditions.
In the properly selected cutting parameters, the values of the residual stresses after turning the AISI316L steel dry and with MQL can be lower or comparable to those in emulsion turning and elimination of cooling liquid is advisable.
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