Research on the Characteristics of Deformation and Axial Force Changes During Drilling of Thin-Walled AF1410 High-Strength Steel

Abstract

Axial force and deformation during drilling significantly impact the hole quality of thin-walled high-strength steel components. This study analyzed the drilling process of thin-walled AF1410 steel, focusing on axial force, deformation, drill cap formation, and hole exit edge characteristics. The effects of cutting speed (12.6–37.7 m/min) and feed rate (0.01–0.1 mm/r) were also examined. Initially, the steel plate undergoes elastic, outward bulging deformation. Axial force, driven by elastic resistance, rises from 114.9 N to 322.1 N as feed rate increases from 0.025 mm/r to 0.1 mm/r, with minimal influence from cutting speed. As drilling progresses, axial force increases slowly. Near the hole exit, plastic deformation occurs beneath the drill bit, causing material to yield and form a drill cap. This results in a sharp rise in axial force, with maximum values increasing from 314.2 N to 525.3 N at higher cutting speeds and from 314.2 N to 840.1 N at higher feed rates. The formation characteristics of the drill cap directly affect hole edge defects, with larger thickness and width leading to more pronounced burrs.

1. Introduction

TAF1410 high-strength steel is an iron-based, high-alloyed, low-carbon martensitic steel, which is a typical secondary hardening ultra-high-strength steel. It has outstanding advantages such as high yield strength and tensile strength, high hardness, good plasticity (section shrinkage 68%, elongation 15%), high fracture toughness (154 MPa·m^1/2), good corrosion resistance, etc., and it has a wide range of applications in the fields of weaponry, aeronautics, ships, automobiles, and special equipment, etc. [1,2,3,4]. However, due to its high strength and low thermal conductivity, it is a typically difficult-to-machine material due to the existence of a large cutting force, high cutting temperature, serious tool wear, low machining efficiency, and other problems during the cutting process. In recent years, the AF1410 high-strength steel thin-walled structure has been more widely used in various fields, but due to the special mechanical and mechanical properties of AF1410 high-strength steel, the difficult-to-machine characteristics and thin-walled structure of the cutting and processing of deformation make its thin-walled structure of high-efficiency precision machining more difficult. The cutting force and deformation characteristics of the cutting process are an important basis for further research on cutting mechanism, deformation control, and machining quality, which is of great research significance.

In order to meet the application requirements of efficient and precise machining of AF1410 high-strength steel, scholars at home and abroad have carried out a lot of research on the cutting and machining of AF1410 high-strength steel in recent years. Jin Xu et al. established a prediction model for the surface topography of AF1410 high-strength steel based on the dynamic characteristics of the milling system, which revealed the formation mechanism of the milling surface topography from the geometrical and physical perspectives [1], and carried out a multi-objective optimization study of the milling parameters for the milling of AF1410 steel based on the Non-dominated Sorting Genetic Algorithm II (NSGA-II) [5]; Ge Song and others studied the cutting force, tool life, wear mechanism, and the effect of milling parameters on residual stresses in milling AF1410 high-strength steel with carbide tools [2,3]. A research team from Shenyang Ligong University studied the changing law of tool wear, milling temperature, milling force, machined surface quality, etc., during high-speed and high-efficiency milling of AF1410 steel [6]. Zhiyuan Sun [7,8] proposed the application of shallow-cutting-fast-feed isometric layer-descending cutting to achieve high-precision and high-efficiency machining of AF1410 before hardening and strengthening as well as high-precision drilling of AF1410 after hardening and strengthening of steel parts by chip-breaking drilling and cyclic drilling, with regard to the characteristics of the drilling of AF1410 steel parts in the two material states of pre-quenching and post-quenching. Yahong Li [9] used an orthogonal experimental method to study the tool materials, cutting parameters, and ultra-deep structure large shaft machining method adapted to AF1410 steel machining.

In addition to AF1410 high-strength steel, many scholars have carried out numerous studies on the cutting and machining mechanisms of other high-strength steel, etc. Lukáš Pelikán [10] investigated the cutting conditions suitable for dry drilling of high-strength steel S960QL based on the stability of the process and the quality of the part by using an experimental method. Hui Zhang [11] investigated the influence of the law of drilling parameters on cutting temperature, torque, and axial drilling force when drilling 300 M high-strength steel by predictive modeling and experimental validation. A research team from the Beijing Institute of Technology conducted a study on the influence of the law of machining surface integrity on the flexural fatigue life of 34CrNiMo6 high-strength steel and the hard turning machinability of 45CrNiMoVA hardened ultra-high-strength steel [12,13]. Comprehensive research literature has shown that the machining of high-strength steels is mainly carried out on thick-walled parts.

In thin-walled part cutting, a lot of research has been carried out mainly on milling of aluminum alloys [14,15], titanium alloys [16,17], high-temperature alloys [18], and metal matrix particle-reinforced composites [19], and in thin-walled part drilling, the research team of Dalian University of Technology has carried out a lot of fruitful studies on fiber-reinforced composites [20,21,22]. Vipan kumar [23] investigated the effect of drilling parameters such as feed rate, cutting speed and drill diameter on the drilling quality of glass fiber reinforced plastic composites. Michela Dalle Mura [24] carried out experimental studies on pre-cooling of carbon fiber reinforced plastic plates prior to drilling and obtained better drilling results. Some scholars have also studied the drilling process and drilling mechanism for thin-walled parts of aluminum alloys [25], titanium alloys [26], and metal matrix particle-reinforced composites [27]. However, the SiCp/Al composite material is relatively brittle, and the edges of drilled holes are prone to chipping defects. Titanium alloy materials have high specific strength, low thermal conductivity, and work hardening properties, but their strength is much lower than that of AF1410 high-strength steel. Therefore, the variation characteristics and effects of axial forces during drilling of SiCp/Al composite materials and titanium alloys are different from those of AF1410 high-strength steel. However, there are very few research papers currently available on the drilling of thin-walled, high-strength steel components.

High-strength steel possesses unique mechanical properties such as high mechanical strength and toughness, and its thin-walled structures are increasingly widely applied. Drilling is an important machining process for most of the thin-walled parts of high-strength steel, and the axial force and deformation during the drilling process have an important influence on the drilling accuracy and edge quality. In this paper, the axial force and deformation characteristics of the drilling process of AF1410 thin-walled parts of high-strength steel, the formation mechanism of the drilling cap, edge crimping defects, and the influence of drilling speed and feed rate on it are investigated through experiments. It provides a foundation for the research on the drilling mechanism and deformation control of thin-walled high-strength steel components and also offers theoretical support for the development of precision machining processes for thin-walled high-strength steel components.

2. Experimental Conditions and Experimental Program

The experimental system is shown in Figure 1. The machine tool is a VMC850E vertical machining center manufactured by Shenyang Machine Tool Co. In order to facilitate the measurement of drilling temperature, a vertical-horizontal converter head is installed on the spindle of the vertical machining center to transform vertical machining into horizontal machining. The drilling force measurement system consists of a Kistler 9257B plate force gauge, a Kistler 5070A charge amplifier, a Kistler 5697A data collector, and a computer. The temperature field on the underside of the workpiece during drilling was observed and measured by a Fotric 388 thermal imaging camera. This paper employs the contact thermometry method, measuring the emissivity of the workpiece surface to be 0.30. The drill was a SANDVIK monolithic cemented carbide TiAlN-coated drill with a diameter of Φ8 mm, a tip angle of 140°, and a tip height of 1.4 mm. The experimental workpiece was a thin-walled plate of AF1410 high-strength steel, and its physical and mechanical properties are shown in

Table 1. Material physical properties of AF1410 steel [6].

Physical Property Index	Numerical Value
Density ρ (kg/m3)	7860
Modulus of elasticity (GPa)	203
Yield strength σ0.2 (MPa)	1580
Thermal conductivity (W/mK)	27.8
Specific heat capacity (J·Kg−1K−1)	490
Poisson’s ratio μ	0.29
Tensile strength σ (MPa)	1700
Melting temperature (K)	1623~1695

. To facilitate temperature measurement, we employed dry cutting. Although this method differs from the wet cutting commonly used in industrial environments where coolant is applied, the coolant in wet cutting often fails to reach the cutting zone effectively. Therefore, the cutting temperatures obtained through dry cutting still offer significant reference value for actual machining processes.

On the basis of the research on the characteristics of axial force change and the mechanism of workpiece deformation and drilling cap formation during the drilling process of a thin-walled plate of AF1410 high-strength steel, a single-factor experimental method was used to study the effects of cutting speed and feed on axial force, deformation, drilling cap formation, and hole edge defects, respectively. Since the drilling temperature has an important influence on the formation of the drill cap and the change in axial force, the experiment was carried out under dry cutting conditions, and a thermal imaging camera was applied to analyze the influence of cutting speed and feed on the temperature field of the workpiece bottom surface when the drill bit was close to drilling out. The center of the drill hole is the geometric center of the workpiece, and the constraint method is a 2-edge (left and right to right) constraint, and the experimental scheme is shown in

Table 2. Experimental plan.

Overhanging Dimensions of Workpiece Length × Width × Thickness (mm)	Feedrate (mm/r)	Cutting Peed(m/min)/ Spindle Speed (r/min)
70 × 70 × 1.5	0.025	12.6/(500)
		18.8/(750)
		25.1/(1000)
		31.4/(1250)
		37.7/(1500)
70 × 70 × 1.5	0.01	18.8/(750)
	0.025
	0.05
	0.075
	0.1

.

3. Experimental Results and Analysis

3.1. Characteristics of Axial Force Changes and Workpiece Deformation Mechanism During Drilling of Thin-Walled Plates of AF1410 High-Tensile Steel

When the thickness of the workpiece is greater than the height of the drill tip, the drilling process can be divided into several characteristic stages as shown in Figure 2 if the deformation of the thin-walled plate during the drilling process is not considered. In the figure, A is the top edge of the drill just touching the upper surface of the workpiece, B is the intersection point of the main and secondary cutting edges of the drill drilling to the upper surface of the workpiece, C is the top edge of the drill drilling to the bottom surface of the workpiece, D is the intersection point of the main and secondary cutting edges of the drill drilling to the bottom surface of the workpiece, and E is the point at which the drill drills out the bottom surface of the workpiece completely, and the axial force is zero. If the friction between the drill bit and the hole wall is not taken into account, the axial force is theoretically 0 when the drill bit is in the position of the D point.

If the deformation of the workpiece in the drilling process is not considered, the axial traveling of the drill from state A to state B is the height of the drill tip; the axial traveling from state B to state C is the difference between the thickness of the workpiece and the height of the drill tip; the axial traveling from state C to state D is the height of the drill tip. The theoretical drilling time for each characteristic stage of the drilling process is calculated as follows:

$$\Delta t_i={\displaystyle \frac{60h_i}{f\cdot n}},$$

(1)

where h_i is the theoretical stroke of the drill at a certain stage, in mm; f is the feed, in mm/r; n is the spindle speed, in r/min; and Δt_i is the theoretical drilling time, in s.

Drilling experiments were carried out on the center of a thin-walled plate of AF1410 high-tensile steel under the cutting speed v = 25.1 m/min (rotational speed n = 1000 r/min), feed f = 0.025 mm/r, workpiece overhang size (length × width × thickness) of 70 mm × 70 mm × 1.5 mm, and dry cutting conditions, and the variation in axial force with time is shown in Figure 3, and the points labeled in the figures. Points A, B, C, D, and E correspond to the theoretical drilling characteristic points shown in Figure 2.

As can be seen from Figure 3, from the beginning of drilling to the axial force decreasing to 0 (Figure 3, A–E), the actual drilling time lagged about 0.63 s compared with the theoretical drilling time without considering the deformation of the thin-walled plate (Figure 2, A–D), which indicates that the thin-walled plate had undergone overall convex deformation under the action of axial force during the drilling process, so that the time from the contact of the drill bit with the workpiece’s upper surface to the intersection of the drill bit main and vice cutting edges drilling to the bottom of the workpiece was increased.

From the curve of axial force with drilling time in Figure 3, the change in axial force during the drilling process can be divided into four stages in general: In the first stage (corresponding to the straight line segment between point A and point P in the figure), the axial force rises in a straight line with a large slope, and the axial force is mainly caused by elastic deformation resistance of the workpiece, which lasts significantly longer than the difference between actual and theoretical drilling time (0.63 s). The axial force is mainly caused by the elastic deformation resistance of the workpiece, and the duration (1.19 s) is significantly larger than the difference between the actual drilling and theoretical drilling time (0.63 s). In the second stage, the axial force shows a curved increase at the initial stage and basically increases linearly at the later stage; when the elastic resistance of the workpiece increases to a certain value in the first stage, the drill starts to drill into the workpiece, and at the initial stage, the main cutting edge has just cut into the workpiece, and the axial force is mainly caused by the extrusion of the top cutting edge; at the later stage, with the drill drilling, the role of the top cutting edge stabilizes, and the length of the main cutting edge involved in the cutting increases gradually, and the axial force increases approximately and linearly at a relatively small slope. In the second stage, the increase in axial force after drilling is smaller than the elastic resistance generated in the first stage, and it can be assumed that this stage is mainly dominated by the drilling of the drill bit, and at the same time, due to the production of a certain amount of elastic recovery of the thin-walled plate, the actual drilling thickness is greater than the feed amount, which is one of the reasons why the difference between the actual drilling and the theoretical drilling time is less than the duration of the overall convex deformation at the early stage of the drilling process. In the third stage (between points M–Q in the figure), the axial force increases rapidly with a large slope to the maximum value of the whole drilling process. In the fourth stage, the axial force decreases rapidly after reaching the maximum value. The variation characteristics of the axial force during the drilling of thin-walled AF1410 high-strength steel plates are completely different from those of the brittle SiCp/Al composite material, which experiences a sudden drop in axial force when the drill bit is about to exit the bottom surface of the workpiece [27].

Theoretically, if the deformation of the workpiece is not taken into account, the maximum value of axial force should occur between the characteristic points B–C (Figure 2), i.e., the main cutting edge enters into the workpiece completely, and the top edge is not yet drilled out of the position; however, from the axial force waveform in Figure 3, the maximum value of the axial force is obviously lagging behind that at the points B However, from the axial force waveform in Figure 3, the maximum value of axial force is obviously lagging behind the point B–C, and at the theoretical characteristic points B and C, the axial force is in the state of steady increase, which indicates that the top edge of the drill bit is not drilled to the bottom surface of the workpiece, which is mainly caused by the fact that under the action of the axial force, the overall cambering deformation of the workpiece lags behind the relative position of the drill bit and the workpiece.

In addition, AF1410 high-strength steel is a high-strength plastic material with poor thermal conductivity. Under the action of axial force and drilling temperature, not only does the overall convex deformation occur during the drilling process, but also a large local plastic deformation occurs when the drill tip is about to drill out of the bottom surface of the workpiece, resulting in the formation of a complete conical cap whose inner surface coincides with the rotating cone surface of the drill tip as shown in Figure 4. Due to the high strength of AF1410 high-strength steel, the drill caps and edge profiles formed during thin-walled drilling are significantly different from the exit profiles of titanium alloy drilling reported in Reference [26]. This also results in distinct characteristics of the drilling axial force compared to titanium alloy drilling.

Based on the analysis of the characteristics of axial force and deformation changes during the drilling process of high-strength steel thin-wall panels, the schematic diagram of workpiece deformation during the drilling process is shown in Figure 5.

In Figure 5, a is the top edge of the drill bit just contacting the upper surface of the workpiece; a–b is the stage of elastic deformation of the lower convexity of the workpiece; at this stage, although the top edge of the drill bit exerts a large extrusion friction on the workpiece, it is not drilled into the workpiece, and the main body manifests itself as the lower convexity elastic deformation of the workpiece, and the axial force grows linearly, and the elastic deformation and the elastic resistance of the workpiece in the position of b reach the maximum; b–c is the stage of the drill bit drilling into the workpiece, and at this stage, the lower convex elastic deformation of the workpiece is basically in a stable state, under the support of elastic resistance, the drill bit gradually drills into the workpiece, and the axial force increases with the increase in the drilling length of the main cutting edge; d is the stage of local deformation under the tip of the drill bit, and the center of the drill bit cracks after the top edge of the tip is pushed out from the bottom of the workpiece; e is the stage of the local plastic shaped of the workpiece under the main cutting edge of the drill bit, which gradually forms the drilling cap, and the axial force first starts to increase rapidly and reaches its maximum. The axial force at this stage first starts to increase rapidly, reaches the maximum value, and then decreases rapidly. f is the intersection of the main and secondary cutting edges of the drill bit drilling to the bottom surface of the workpiece, and the drill cap is separated from the bottom surface of the workpiece by fracture, and the axial force decreases to zero.

From Figure 4, the morphology of the drill cap can be seen. AF1410 high-strength steel thin-walled plate drilling formed by the inner surface of the drill cap and the drill tip rotary taper coincides with the formation of the drill cap process, mainly manifested as the tip of the drill below the workpiece to be processed layer of local plastic deformation. The formation of the cap first occurs in the top edge of the layer to be machined below the top edge, in the top edge drilling to the position near the bottom surface of the workpiece, in the axial force and drilling temperature under the action of the top edge of the layer to be machined under the action of the local plastic convex deformation; at the same time, the top edge of the top edge and the main cutting edge of the ring edge machining layer adjacent to the role of the part of the material in the tip of the taper of the action of the material under the stress to the material bending yield strength, the occurrence of the outward turning of the plastic deformation. With the feed of the drill bit, the material of the machining layer near the bottom surface of the workpiece in contact with the main cutting edge of the annular region continuously yields and turns out, forming the drill cap. In the process of forming the drill cap, the top edge of the drill bit is mainly used for the rotary extrusion of the layer material to be processed underneath, and the main cutting edge and the main back face are mainly used for cutting and rotary pressure; for the part farther away from the bottom surface, the thickness is larger, and the rigidity can still support the cutting force to produce a certain degree of cutting, and for the part close to the bottom surface, the thickness is small, and the rigidity is not enough to support the cutting when the rotary pressure is mainly used for extruding. At the beginning of the formation of the drill cap, the axial force increases rapidly with a large growth slope; with the continued feeding of the drill, the thickness of the workpiece machining layer corresponding to the bottom of the main cutting edge of the drill and the remaining annular width decrease, the stiffness decreases, and the axial force reaches its maximum value when its stiffness is not enough to support the cutting action of the main cutting edge and the overall yield occurs; the maximum thickness of the workpiece machining layer corresponding to the bottom of the main cutting edge at this point in time is known as the critical thickness. The maximum thickness of the workpiece layer below the main cutting edge at this time is called the critical thickness. Subsequently, as the drill continues to feed, the axial force decreases rapidly and is conical to expand outward under the action of the taper surface of the drill tip. When the intersection of the main and secondary cutting edges of the drill is actually drilled to the bottom surface of the thin-walled plate, the edge of the taper surface is fractured, the drilling cap and the substrate are separated, the deformed workpiece is rebounded, and the axial force rapidly decreases to 0. Yield outward flopping of the edge of the borehole wall under the tip of the drill speeds up the elastic recovery of the thin-walled plate, and this also makes the actual drilling lag time smaller than the overall convex deformation duration at the beginning of drilling.

3.2. Influence of Cutting Speed (Spindle Speed) on Axial Force and Deformation in Drilling

Under the conditions of feed f = 0.025 mm/r and dry cutting, the cutting speed v = 12.6~37.7 m/min (spindle speed n = 500~1500 r/min) was varied to study the effect of cutting speed on the axial force of drilling. The waveforms of axial force with time at different cutting speeds (rpm) are shown in Figure 6.

As can be seen from Figure 6, the axial force of drilling thin-walled plates of AF1410 high-strength steel at different drilling speeds generally exhibits the same change characteristics as those described before. Among them, in the range of higher cutting speed v = 25.1–37.7 m/min (n = 1000–1500 r/min), the axial force waveform graphs have higher similarity, and there is an obvious rapid increase in the process of drill cap formation in the late stage of drilling; while in the range of lower cutting speed v = 12.6–18.8 m/min (n = 500–750 r/min), the axial force in the increase in axial force in the process of drilling cap formation is more gentle, and the lower the speed, the more gentle.

From the time of generation of the maximum axial force, at the cutting speed v = 25.1–37.7 m/min (n = 1000–1500 r/min), the generation of the maximum axial force is closer to the intersection point of the main and secondary cutting edges where the bottom surface of the workpiece is to be drilled out; and at the low cutting speed range of v = 12.6–18.8 m/min (n = 500–750 r/min), the generation of the maximum axial force slightly deviates from the theoretical position of the top edge of the drill bit where the bottom surface of the workpiece is to be drilled out. In the low cutting speed range of v = 126–18 m/min (n = 500–750 r/min), the maximum axial force is slightly deviated from the theoretical position of the top edge of the drill bit to drill out the bottom surface of the workpiece, and considering the effect of convex deformation of the workpiece at the initial stage of drilling, the maximum axial force occurs at the position of the top edge of the drill bit when the drill bit is actually about to be drilled out, which indicates that the maximum axial force occurs due to different reasons at different cutting speeds.

This change in axial force with cutting speed is characterized by the temperature of the bottom of the workpiece corresponding to the drill tip during the formation of the drill cap. Figure 7 shows the morphology of the drill cap formed at different cutting speeds and the temperature field at the time of maximum axial force.

From the color of the drill cap and the temperature field of the bottom surface of the workpiece, it can be seen that the temperature of the bottom surface of the drill cap increases greatly with the increase in the cutting speed. Due to the increase in temperature in the process of cap formation at higher cutting speed, the yield strength of AF1410 high-strength steel decreases with the increase in temperature [28], the bottom material of the workpiece corresponding to the drill tip is more prone to thermoplastic deformation, and the cap formation is dominated by the rotational extrusion of the drill tip on the bottom material of the workpiece, so that the thickness and width of the machined layer of the workpiece corresponding to the main cutting edge below the main cutting edge under the process of cap formation increase, and the increase in the axial force is mainly caused by the rotational extrusion of the drill tip on the bottom material of the workpiece, and the axial force increases in the intersection point of the main and secondary cutting edges close to the bottom surface of the workpiece. The increase in axial force is mainly caused by the rotational extrusion of the drill tip on the workpiece material at its bottom, and the axial force reaches the maximum when the intersection point of the main and vice cutting edges is close to the bottom surface of the workpiece, and the workpiece machining layer corresponding to the workpiece below the main cutting edge undergoes the overall outward plastic deformation, and then it decreases rapidly with the plastic yielding. When the cutting speed is low, during the formation of the drill cap, the temperature of the material on the bottom surface of the workpiece corresponding to the drill tip is low, and the hardness and stiffness are large, so that the bearing capacity of the axial drilling force during the formation of the drill cap is large, and the thickness and width of the plastic deformation of the whole outward turning of the main and vice cutting edges are small when the main cutting edge is about to be drilled out of the bottom surface of the workpiece, at this time, the axial force is also small, and the largest axial force occurs in the early stage of the formation of the drill cap, and the top cutting edge of the drill bit is about to be drilled out of the bottom surface of the workpiece. The maximum axial force occurs at the early stage of cap formation, when the top edge of the drill bit is about to drill out the bottom surface of the workpiece. From the photo of the morphology of the drill cap, when the cutting speed is lower, the rupture opening in the center of the drill cap is larger, and with the increase in cutting speed, the drilling temperature rises, the material softens, the plasticity increases, the cracks at the top of the drill cap become smaller, and the drill cap tends to be complete.

Different cutting speeds under the formation of the characteristics of the drill cap directly affect the formation of edge defects in the workpiece; if the cutting speed is higher, the main cutting edge below the corresponding workpiece processing layer occurs as a whole when the thickness of the plastic deformation of the flap is greater; the edge of the plastic deformation caused by the crimping and other defects is greater. When the cutting speed is lower, the edge of the main cutting edge by the drill cutting edge cutting is completed, and the edge is more flat, as shown in Figure 8.

The effect of cutting speed on the maximum axial force during drilling and the maximum value of axial force at the initial stage of linear variation in axial force during cutting is shown in Figure 9.

As can be seen from Figure 9, the maximum values of the axial force at the stage of linear change at the early stage of drilling do not differ much, and the cutting speed has less influence on it. The previous analysis shows that, due to the low stiffness of the thin-walled plate of AF1410 high-strength steel, at the initial stage of the drill contacting the workpiece, the workpiece undergoes elastic convex deformation under the action of the axial force of the top edge of the drill, and the drill does not cut into the workpiece obviously, and the axial force is mainly caused by the elastic resistance of the workpiece, and the maximum axial force at this stage depends on the support force required for the drill to cut into the workpiece, and the drill only starts to cut into the workpiece when the elastic resistance is sufficient to support the drill to cut into it. When the elastic resistance force is enough to support the drill bit to drill into the workpiece, the drill bit starts to cut into the workpiece. The results in Figure 9 show that the cutting speed has little effect on the support condition of the drill bit to drill into the workpiece. The previous analysis shows that the maximum axial force occurs when the cutting speed v = 25.1~37.7 m/min (n = 1000~1500 r/min) is about to be drilled out of the bottom surface of the workpiece at the intersection point of the main and vice cutting edges, which is caused by the plastic flanging deformation that occurs underneath the cutting edge, and the thickness and width of the flanging deformation occurring underneath the drill bit is not affected by the increase of the cutting speed from 25.1 m/min to 31.4 m/min due to the increase in temperature of the material underneath the drill tip. When the cutting speed increases from 25.1 m/min to 31.4 m/min due to the increase in temperature of the material below the drill tip, the thickness and width of the flanging deformation increase, and the maximum axial force increases, but with the further increase in cutting speed to 37.7 m/min, due to the further increase in temperature, the material softens, and the maximum axial force decreases to a certain extent. Considering Figure 6 and Figure 8, if one wants to achieve better hole exit edge quality with smaller edge defect dimensions, the cutting speed should be less than 25.1 m/min.

The previous analysis shows that at the early stage of drilling, due to the low stiffness of the workpiece, the elastic convex deformation occurs under the pressure of the top edge, the drill bit does not drill into the workpiece, the axial force grows linearly, and the moving distance of the drill bit at this stage is approximated to the amount of convex deformation of the center of the workpiece. If the time of linear growth of axial force in the early stage of drilling is △t, then the convex deformation Δ (mm) is as follows:

$$\delta ={\displaystyle \frac{△t\times n\times f}{60}}$$

(2)

In Equation (2), n is the spindle speed (r/min) and f is the feed (mm/r).

The effect of cutting speed on the amount of convex deformation of the workpiece at the initial stage of drilling is thus calculated as shown in Figure 10.

As seen in Figure 10, the cutting speed has little effect on the elastic convex deformation of the workpiece before the drill bit drills into the thin-walled plate at the initial stage of drilling. Since the elastic convex deformation of the workpiece before drilling in directly affects the size and shape accuracy of the hole after the workpiece elasticity is recovered, the results in Figure 10 also show that the cutting speed has little effect on the size and shape accuracy of the hole due to elastic deformation of the drilling process of the thin-walled plate of high-strength steel.

3.3. Influence of Feed on Axial Drilling Force

Under the conditions of a workpiece overhang size of 70 mm × 70 mm, a thickness of 1.5 mm, 2-side constraint (left and right opposite sides fixed), a cutting speed of v = 18.8 m/min (spindle speed n = 750 r/min), a feed of f = 0.01–0.1 mm/r, and dry cutting conditions, the variation in axial force with time under different feeds is shown in Figure 11.

For the same reason as mentioned above, the axial force at the beginning of drilling increases linearly under different feeds; after the drill bit is drilled into the workpiece, the axial force increases with the increase in the cutting length of the main cutting edge of the drill bit, and the slope of its increase is smaller than that at the beginning of the drilling; however, the position point of the maximum axial force and the characteristics of the axial force waveforms are different under different feeds.

The trend of the axial force waveforms is basically the same in the case of f = 0.01 mm/r and f ≥ 0.05 mm/r, and the latter part of the axial force waveform in the case of f = 0.01 mm/r is characterized by more obvious plastic deformation of the drill cap.

When f = 0.01 mm/r, the feed amount is too small; due to the influence of cutting edge radius, in the process of drilling, the extrusion friction effect is larger, the temperature of the bottom surface below the drill tip is higher (Figure 12), and the contact area between the drill tip and the workpiece is more prone to plastic deformation, and in the intersection point of the main and secondary cutting edges of the drill bit when it is about to be drilled out of the bottom surface of the workpiece, the plastic outward flap occurs so that the axial force increases rapidly, and the plastic deformation resistance decreases rapidly after the maximal value is reached. When f = 0.025 mm/r, as mentioned in Section 3.2, the process of drilling cap formation is dominated by cutting action, and the contact area between the main cutting edge and the workpiece machining surface undergoes plastic deformation in the small thickness to form the drilling cap. f = 0.05 mm/r, the axial force in the process of drilling cap formation shows a small increase, and no significant sudden change in axial force occurs due to the larger plastic deformation. There is no significant change in axial force due to large outward plastic deformation.

With the further increase in feed, after f ≥ 0.075 mm/r, the slope of the increase in the axial force in the second stage and the increase in the axial force in the third stage converge. With the increase in the feed volume, the drilling speed of the drill bit is fast, and the axial force increase rate after the main cutting edge is drilled in becomes larger, at the same time, from the temperature field of the bottom surface of the workpiece at the time of the maximum drilling force (Figure 12), the maximum temperature of the bottom surface temperature field increases correspondingly with the increase in the feed volume, and the degree of thermal softening of the workpiece material in the process of drilling is increased, and the axial force of plastic deformation of the drilling cap camber decreases, so that the axial force increase in the second stage and the axial force increase in the third stage are consistent. The slope of the increase in the axial force in the second stage and the third stage is close to the same.

Figure 12 shows the morphology of the drilling cap and the temperature field when the axial force is maximized at different feeds f.

From the temperature field of the bottom surface of the drill cap formation process, it can be seen that the temperature is higher after the feed amount f = 0.01 mm/r and f ≥ 0.05 mm/r. Due to the higher temperature in the cutting zone during the formation process of the drill cap, the material at the bottom surface of the workpiece corresponding to the drill tip undergoes localized softening, and the formation of the drill cap is dominated by the rotational extrusion of the drill tip on the material of the workpiece at the bottom surface of the drill tip.

In Figure 12, when the feed rate f = 0.025 mm/r, during the process of top hat formation, the temperature of the bottom material of the workpiece corresponding to the drill tip is low, and the hardness and strength are high, so in the early stage of top hat formation, it is mainly the cutting of the main cutting edge of the drill tip, and therefore the change in axial force is relatively smooth. When the top edge drills through the top of the cap, the axial force decreases. It can also be seen from the morphology of the drill cap that the top crack of the drill cap is larger when the feed f = 0.025 mm/r.

The characteristics of drilling cap formation under different feeds also directly affect the formation of workpiece edge defects. At f ≥ 0.05 mm/r, the axial force and temperature in the process of drilling cap formation are higher, and the overall increase with the increase in feed, the critical thickness and width of the workpiece machining layer corresponding to the overall outward plastic deformation below the main cutting edge increase accordingly, and the edge of the edge by the plastic deformation caused by the crimping and other defects is larger. At f = 0.01 mm/r, the temperature of the bottom surface below the drilling tip is higher, and the exit of the hole also has obvious crimping. As shown in Figure 13, the edge of the hole is relatively flat, 0.01 mm/r, the temperature of the bottom surface below the drill tip is higher, and the exit of the hole has an obvious curling edge. At f = 0.025 mm/r, the edge of the hole is more flat, as shown in Figure 13.

The effect of feed on the maximum axial force during drilling and the maximum value of axial force in the linear change stage at the beginning of cutting is shown in Figure 14.

From Figure 14, it can be seen that the axial force at the stage of overall convex deformation of the thin-walled plate at the early stage of cutting increases slowly with the increase in feed f. This indicates that the feed has a certain influence on the conditions of the drill bit drilling into the thin-walled plate at the early stage of drilling, and the increase in axial force at a smaller feed is smaller, and the support force required by the drill bit for drilling into the workpiece is smaller, so the drill bit can start drilling into the workpiece after the thin-walled plate produces a smaller convex deformation; with the increase in feed, the support force required for drilling into the workpiece increases, and the drill can only drill into the workpiece after the thin-walled plate produces a larger convex deformation. From Figure 13, the maximum axial force changes to see the overall maximum axial force with the increase in feed f and increase; the reason is the same as the aforementioned. At f = 0.01 mm/r, due to the feed being too small, the extrusion of the drilling edge of the role of the large, the maximum axial force is instead larger. Considering Figure 11 and Figure 13, if one wants to achieve better hole exit edge quality with smaller edge defect dimensions, the feed rate should be less than 0.05 mm/r.

Calculated according to Equation (2), the variation in convex deformation produced by the thin-walled plate before drilling into the drill bit with the feed amount is shown in Figure 15.

As can be seen from Figure 15, the amount of convex deformation of the thin-walled plate before drilling into the drill increases with the increase in feed; the axial force is small when the feed is small, and the support force required for the drill to drill into the workpiece is small, so the drill starts to drill into the workpiece after the thin-walled plate produces a small convex deformation; with the increase in feed, the support force required for the drill to drill into the drill increases, and the thin-walled plate can only be drilled into the workpiece after the drill produces a large convex deformation.

4. Conclusions

(1) In the early stage of drilling a thin-walled plate of AF1410 high-strength steel, the thin-walled plate produces overall convex elastic deformation; the drill bit does not drill into the workpiece at this stage; the axial force is mainly caused by the elastic deformation resistance of the workpiece, which grows linearly; at this stage, the maximum axial force and the amount of convex deformation increase with the increase in feed, but the drilling speed does not have much effect on it.

(2) When the overall convex elastic resistance of the thin-walled plate at the early stage of drilling increases to withstand the axial force of drilling, the drill starts to drill into the workpiece, the increase in axial force at the early stage of drilling is dominated by the extrusion of the top edge, and with the drilling, the role of the top edge is stabilized, the length of the main cutting edge involved in the cutting is gradually increased, and the axial force is increased by a small slope in an approximate linear manner.

(3) In the top edge drilling to the position near the lower surface of the workpiece, under the action of axial force, the top edge of the material to be processed under the local plastic convex deformation of the layer, with the feed of the drill, in the tip of the rotary pressure plus extrusion, near the bottom surface of the workpiece and the main cutting edge in contact with the annular area of the processed layer of the material, yielded to turn over continuously; the formation of the inner surface of the rotary surface of the tip and the tip coincided with the cap of the drilling. The critical thickness of yielding and flipping increases with the increase in cutting temperature.

(4) The characteristics of the formation of the drill cap under different cutting speeds directly affect the formation of edge defects of the workpiece; the higher the cutting speed or the larger the feed, the larger the edge of the plastic deformation caused by the crimping and other defects is, while the lower the cutting speed or the smaller the feed, the flatter the edge of the hole; however, the feed is too small due to the extrusion caused by the cutting edge radius of the bottom surface under the tip of the drill at higher temperatures, and there is a clear hole outlet. The hole exit also has obvious curled edges.

(5) The drilling temperature generally increases with the increase in drilling speed and feed rate, but at the feed rate f = 0.01 mm/r, the drilling temperature is higher due to the small feed rate and the influence of the cutting edge radius, which results in a larger extrusion friction in the drilling process of the drill bit.

(6) When the drilling temperature is higher and the critical yield flange thickness is larger, the drilling axial force increases sharply.

(7) When the cutting speed is less than 25.1 m/min and the feed rate is less than 0.05 mm/r, the size of the hole edge defects can be reduced, and the quality of the hole exit edge can be improved.