Chip formation in machining hybrid components of SAE1020 and SAE5140

In the presented work hybrid components with two different steel alloys, SAE1020 and SAE5140, were investigated. The chemical composition of the materials used are shown in Table 1. The materials differ significantly in their carbon and chromium content.

At first the blanks were joined by a friction welding process. Friction welding (EN ISO 4063: process 42) is a welding process from the group of press-welding. In this process, two parts are moved relative to each other under pressure, with the components touching each other at the contact surfaces. The resulting friction causes the material to heat up and plasticise [4, 14]. For the friction welding process, shafts with a diameter of d = 40 mm, a length of l = 220 mm for SAE1020 and l = 150 mm for SAE5140, were used. The friction welding was carried out using a friction speed of n = 2000 min⁻¹, a friction pressure of p = 60 MPa and an infeed of s = 4 mm. The final bond was then generated with a compression of p = 150 MPa and a compression time of t = 6 s (Fig. 1b-1).

In the next step the friction weld bead was removed by longitudinal turning. The required contour for orthogonal cutting investigations was then produced by different milling processes. Subsequently, the hybrid components were heat-treated to achieve the required hardness. They were hardened at a temperature of 860 °C for 45 min, then quenched in a water bath and finally annealed at 150 °C for 60 min. In Fig. 1a the mechanical properties of the investigated materials after the heat treatment are summarized. The differences in hardness between the two materials are remarkable. SAE5140 with 612HV1 is almost twice as hard as SAE1020 with a hardness value of 258 HV1. The microstructure images in Fig. 1a show the differences between both materials. SAE1020 is characterized by a ferritic–pearlitic microstructure while SAE5140 consists of a homogeneous martensitic microstructure. This is the reason for the difference in hardness of both materials. To compensate the hardening distortions caused by heat treatment, the planed samples were ground plane–parallel in the final step (not depicted in Fig. 1).

Cutting edge rounded indexable inserts of type SNMA 150612 were used for the orthogonal cutting investigations. For a precise preparation of the cutting edge and for the production of different cutting edge roundings, the movement of the insert during brushing was realized by a robot. An industrial robot of type KUKA KR16 is used for this purpose (Fig. 2, right).

With a load capacity of 16 kg, six axes and a positioning accuracy of <  ± 0.05 mm, the robot has the necessary technical characteristics for precise cutting edge preparation. The indexable insert is mechanically clamped at the end of the robotic arm via a clamping device. In order to increase the accuracy, the positioning of the inserts is supported by additional measuring technology. An impact sound sensor is used to detect the contact of the indexable inserts with the brush. The applied tool is a brush containing SiC grains with a size of 240 µm. The bonding of the single fibres consists of a temperature-stable polymer. The setup for the preparation of the cutting edge is shown in Fig. 2 on the right. The process setting variables are brushing speed v_c, brushing time t_b, brushing depth a_z and brush setting angle φ_z. By varying these process parameters, two symmetrical cutting edge roundings with S_α = S_γ = 35 µm and S_α = S_γ = 75 µm were prepared. The cutting edge roundings were produced in sufficient numbers so that one cutting edge could be used for one test. Thus an influence of tool wear on the orthogonal cutting results could be excluded. After the cutting edge preparation the inserts were double-coated with Al₂O₃ and TiCN using a CVD process at Walter AG. Size and deviation of the actual geometry of the cutting edge roundings after coating in comparison to the target geometry are shown in Fig. 2 on the left. With a tolerance of ± 5 µm the produced geometries are within the intended range. In order to investigate the influence of different cutting edge roundings on chip formation mechanisms the coated SNMA 150612 WAK20 inserts were then used in the orthogonal cutting investigations.

The influence of cutting speed v_c, undeformed chip thickness h and cutting edge microgeometry S_α/S_γ = \({\overline{\text{S}}}\) on chip formation mechanisms in hybrid components is determined by means of microcinematography during orthogonal cutting. The orthogonal cutting process used here is a planing process (Fig. 3).

High-speed images of chip formation are taken during orthogonal cutting process. At the same time process forces and tool vibrations are measured simultaneously. The tool is rigidly clamped in a tool holder. The translatory movement of the hybrid samples is achieved by means of a jerk-decoupled linear direct drive moving the workpiece table. The workpiece table has a maximum stroke movement of s = 1.000 mm. The maximum cutting speed is v_c = 180 m/min with a force of F = 3.6 kN. A laser vibrometer from Polytec is used to record tool vibrations during the orthogonal cutting process. The system consists of a measuring head (type OFV-303) and a controller (OFV-3001). Axial vibrations of the tool during the orthogonal cutting process are recorded. The laser vibrometer uses the laser-Doppler-technique. This technique is characterized by the double shift of a coherent light beam that hits a vibrating surface and is reflected. The occurring interferences are digitized as path information by means of an A/D converter card of type BNC 2090 from National Instruments. The signals are processed, visualized and stored in the LabVIEW programming environment using software developed at the Institute of Production Engineering and Machine Tools. The sampling rate of the software used is 50 kHz. This allows even small vibrations to be reconstructed exactly. The vibrometer thus enables a non-contact measurement of the vibrations of an object based on laser interferometry. The workpiece is mechanically clamped on the table. This enables the absorption of process forces up to F_max = 10 kN. A sapphire glass plate is clamped between the sample and the lens of the high-speed camera to ensure a steady state of deformation. Here it is assumed that the simplification to a plane deformation state in the orthogonal section (two-dimensional case) is allowed due to the significantly larger cutting width b compared to the chip thickness h. In this case, an infinitely thin shear plane is assumed, in which the sliding of the material takes place in individual “segments” which have a homogeneous deformation state. The shear plane runs from the cutting edge to the surface of the workpiece in which the deformation takes place. At the same time, the sapphire glass plate prevents lateral material flow. To avoid material flow and contact of the tool with the sapphire glass, the corner radii of the tool insert are ground off. In addition, immersion oil is used to reduce friction between sapphire glass and tool. In the investigation on orthogonal cutting, the undeformed chip thickness h is varied in three steps, the cutting speed v_c in three steps and the cutting edge microgeometry \( {\overline{\text{S}}}\) in two steps. The workpiece length is L = 120 mm. The workpiece is chamfered at the front so that the cutting edge does not break when the tool enters the workpiece. The cutting width is set to b = 3 mm. The maximum workpiece height is H = 32 mm. The examined process parameters during the orthogonal cutting operation are summarized in Table 2.

In Fig. 4 the influence of undeformed chip thickness h on chip formation is presented. The chip formation depends on the material properties as well as on the undeformed chip thickness. In Fig. 4 the experimental microcinematographic and vibrometer results for SAE1020–SAE5140 hybrid components under variation of the undeformed chip thickness h from 0.05 to 0.15 mm are displayed.

The cutting speed v_c was held constant at 120 m/min. It can be seen that continuous chip formation in SAE1020 changes abruptly in segemented chip formation in SAE5140. The reason for this is the significantly higher hardness due to the martensitic microstructure of SAE5140 which leads to a segmented chip formation. When the material hits the cutting edge of the tool, it is compressed until, at a high degree of deformation, the fracture stress of the workpiece material is exceeded and the material tears at the cutting edge under the effect of normal stress. The following material causes the crack formation, until finally the surface is reached and the chip element is separated from the workpiece. According to the state of the art, the chip segmenting crack runs intercrystalline. It is therefore irregular, shape and size of the separated chip elements are random. During shear chip formation the chip thickness h′ varies over the chip. The chip thickness h′ is therefore given as an average value between maximum and minimum chip thickness h′. This must be taken into consideration for the evaluation. In SAE1020, on the other hand, the lower hardness and significantly lower thermal conduction coefficient leads to thermomechanical stress in the cutting zone and causes continuous chip formation. A change in chip formation from a continuous to a segmented chip, is also followed by a change in chip thickness h′ and shear angle ϕ. The shear angle increases during segmented chip formation while the chip thickness decreases. Consequently, the shear angle ϕ significantly influences chip formation. Large shear angles lead to segmented chip formation. If the undeformed chip thickness h is increased, the continous chip formation in SAE1020 is maintained, but the shear localization changes in SAE5140. A higher chip thickness therefore leads to a higher shear localization caused by an increased shear angle. While the chip compression λ_h = h′/h in SAE1020 remains almost constant as the chip thickness increases, the chip compression in SAE5140 decreases significantly. The chip compression represents the deformability of the material. As the chip compression λ_h decreases, the deformability of the material also decreases. Deformation reduction is accompanied by an increase in the shear angle, i.e. the cross-sectional area of the undeformed chip decreases. This also causes a decrease of the cutting force. In Fig. 4 (right) the results of the vibration tests are shown. As with chip formation, the vibration amplitude increases during segmented chip formation. The higher tool vibration in SAE5140 leads to higher roughness of the created surface and consequently to a reduction of surface quality. Furthermore, the higher vibration amplitude in SAE5140 leads to higher process force oscillations which directly influence the mechanical load of the tool insert used.

In Fig. 5 the results of chip formation mechanisms in dependence of the cutting speed are represented. Microkinematic images at two different cutting speeds v_c = 60 m/min and v_c = 180 m/min are shown. It is noticeable that with increasing cutting speed the shear angle increases. As the cutting speed increases, a local temperature increase occurs in front of the cutting edge. This leads to a softening of the material, which increases the formation ability of the material. However, the chip forms of SAE1020 (continous chips) and SAE5140 (segmented chips) remain unchanged. The influence of different cutting speeds (v_c = 60, 120, 180 m/min) and undeformed chip thicknesses (h = 0.05, 0.1, 0.15 mm) on tool vibrations is shown on the right in Fig. 5. The vibration amplitudes are always higher in SAE5140 than in SAE1020 due to segmented chip formation. The vibration amplitude decreases with increasing cutting speed. The reason for this is the softening of the material due to the higher process temperature. For this reason the yield point of the material decreases. A change in undeformed chip thickness leads to different effects depending on the material being machined. In SAE1020, an increase in undeformed chip thickness h leads to an increase in the vibration amplitude. The reason for this is the increasing mechanical stress with increasing undeformed chip thickness. In SAE5140, the vibration amplitude also increases with increasing undeformed chip thickness. However, an interaction with the cutting speed takes place here. As already mentioned, irregular chip formation could be the reason for this, which is why a clear correlation cannot be identified.

The influence of cutting speed v_c and undeformed chip thickness h using a cutting edge rounding of \( {\overline{\text{S}}}\) = 75 µm on the orthogonal cutting results is shown in Fig. 6. In analogy to a cutting edge rounding of 35 µm, the same chip formation mechanisms occur. Depending on the examined parameter range in SAE1020 continous chip formation and in SAE5140 segmented chip formation takes place. However, the chip compression λ_h in both material sections decreases with increasing undeformed chip thickness h. Consequently, the deformability decreases. The reduction in deformation is accompanied by an increase in shear angle, resulting in a decrease of the shear cross section. This also causes a decrease in the cutting force. Regarding the effect of cutting speed on chip formation, it can be seen that with a cutting edge rounding of 75 µm the chip thickness remains constant with increasing cutting speed. However, compared to cutting edge rounding with 35 µm, in which the shear angle increases in both material areas, it only increases in the area of SAE5140 with a cutting edge rounding of 75 µm. The reason for this is that with increasing cutting speed the process temperature rises and thus causes the material to soften. This softening effect is enhanced due to the additional temperature increase caused by higher frictional effects during machining of the harder material SAE5140. Consequently the significantly higher process temperature in the harder material area leads to a softening of the material and thus promotes material flow.

In Fig. 7 feed forces F_f for the examined cutting edge roundings \({\overline{\text{S}}}\) = 35 µm and \({\overline{\text{S}}}\) = 75 µm depending on the process parameters undeformed chip thickness h and cutting speed v_c are shown. Due to the higher material hardness in SAE5140 than in SAE1020 the feed force is always higher in SAE5140 and consequently leads to a higher tool displacement in this material range. The continuous chip formation in SAE1020 and the segmented chip formation in SAE5140 are recognizable in the force profiles. While the fluctuations in feed force during continuous chip formation are very low, the significantly higher oscillation of the feed forces during segmented chip formation is obvious. These oscillations are significantly higher at low cutting speeds. The reason for this is that at low cutting speeds the deformability of the material is lower and thus the resistance of the material to plastic deformation is higher. This effect is reduced with increasing cutting speed due to higher process temperatures.

In addition, a further dynamic cutting force component is generated as a result of friction between chip and rake face of the tool and between generated machined surface and tool flank face. However, this contribution is small, especially by using carbide tools at high cutting speeds. In the formation of segmented chips, the cutting force profiles thus represent a stochastic signal whose functional relationship cannot be specified analytically.

An interesting aspect is the force shift in the material transition area due to different material properties. By comparison of the influence of the undeformed chip thickness h on the measured feed forces in relation to the cutting edge rounding, different effects can be seen. When using a cutting edge rounding of \({\overline{\text{S}}}\) = 35 µm, the force shift in the transition area increases with increasing undeformed chip thickness. A possible reason for this could be the increase in tool deflection with increasing undeformed chip thickness. This is because the chip cross section increases with increasing undeformed chip thickness and consequently the specific cutting force increases according to Kienzle. In SAE5140, the tool deflection is higher due to the higher hardness of the material and leads to a greater force difference at a higher undeformed chip thickness. With a larger cutting edge rounding, the force step in the material transition zone remains almost the same. The process forces increase proportionally with the undeformed chip thickness. The force difference at an undeformed chip thickness of h = 0.15 mm is significantly lower compared to the smaller rounding. The tool displacement decreases with a larger cutting edge rounding in the transition zone of the hybrid component and therefore leads to a smaller force difference. This is a possible reason for the smaller force difference. It was also found that the chip compression decreases when using a larger cutting edge rounding, resulting in a decrease of the chip cross section and thus a reduction of the forces. This effect can also be observed under variation of the cutting speed. In addition, the different thermal conduction coefficients of the materials cause a different thermal conduction of the generated process temperature in the material. A larger cutting edge rounding leads to higher process temperatures due to the larger contact surface between tool and workpiece. The frictional power increases. Consequently, the heat in the shear cross section increases. This results in a reduction of the force difference in the material transition area. The effect of the heat input significantly increases with increasing cutting speed.

Finally, optical surface examinations of the planed hybrid samples were carried out to investigate the effect of the different chip formation mechanisms on the surface properties (Fig. 8).

Here too, the influence of different chip formation mechanisms on the resulting surface can be seen. For all samples, the surface roughness in SAE1020 is significantly lower than in SAE5140. The continuous chip formation results in a smoother surface with a lower roughness compared to the segmented chip formation. The vibrations that the tool experiences due to chip segmentation are also transmitted to the surface and explain the fluctuations in process forces which are depicted in Fig. 7.

In terms of surface quality, cutting speed has a significantly higher influence than undeformed chip thickness. The influence of undeformed chip thickness on the generated machined surface is shown as an example for a cutting edge rounding of \({\overline{\text{S}}}\) = 35 µm (Fig. 8). With increasing undeformed chip thickness, the surface roughness as well as the difference in the height profile ΔH in the material transition area increase. The reason for this is the decreasing deformability of the machined material with increasing undeformed chip thickness, which leads to a worsening of the new created surface. The influence of the cutting speed is evident (Fig. 8, right). As the cutting speed increases, the surface roughness in SAE1020 is significantly reduced from Rz = 4.19 to 0.63 µm, as the material flows better at higher temperatures, leaving a smooth surface. Due to the extreme variations in both chip segmentation and feed forces, the single influences on surface roughness of SAE5140 cannot be clearly explained. While an improvement of the surface roughness in SAE5140 can be seen with a cutting edge rounding of \({\overline{\text{S}}}\) = 35 µm, the use of a larger cutting edge roundingleads to an increase in surface roughness. However, similar to the process forces, the difference in the height profile in the material transition zone is significantly smaller when using a large cutting edge rounding.