Cutting Force in Milling of Additive Manufacturing AISI 420 Stainless Steel

Abstract. In manufacturing, hybrid systems of metal additive manufacturing and cutting in the same platform have been attractive in terms of low volume production of customized parts, complex shape, and fine surface finish. Milling is conducted to finish rough surface fabricated in additive process. The fundamental machinability of the additive workpiece should be studied because the material properties are different from metals produced in the conventional process. The paper discusses the cutting forces in milling of AISI 420 stainless steel fabricated in additive process. The cutting tests were conducted to measure the cutting forces and the chip morphologies for tool geometries. The cutting forces were also analyzed in an energy-based force model. In the analysis model, three-dimensional chip flow is interpreted as a piling up of orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities, where the cutting model is made by the orthogonal cutting data acquired in cutting tests. The chip flow direction is determined to minimize the cutting energy. The cutting forces, then, were predicted in the determined chip flow model. The cutting force model was validated in comparison of simulated forces with the actual ones.


Intr Introduction oduction
Martensitic stainless steel has been widely used for applications including injection molds and dies, dental and surgical instruments due to the high strength with the good corrosion resistance. This grade of steel is suitable to hardening after heat treatments of quenching and tempering. After hardening, the material properties include high strength, toughness, and corrosion resistance [1]. In terms of machining, however, Stainless steels are one of the difficult-cut metals due to the high strength, the high rate of work hardening, and the low thermal conductivity [2]. Tamimi and Hossainy measured the cutting force, specific cutting energy, shear angle, friction coefficient on the rake face, the shear stress on the shear plane, the shear strain in turning of AISI 420 of 260 HV hardness at various cutting speeds, feed rates, and the rake angles. The cutting force decreases in low cutting speed due to build-up-edge [3]. The cutting force and the power consumption in turning of AISI 420 stainless steel were predicted with finite element method [2]. The result shows the effect of cutting speed on power consumption is less than the cutting depth effect due to small cutting forces at high cutting speeds. Tsai et al. applied ultrasonic vibration to milling of STAVAX (modified AISI 420) stainless steel. The surface finish was improved with an end mill at a large helix angle in an optimum amplitude of the ultrasonic vibration [4].
As an effective manufacturing of functional parts with complex geometries and low volumes, the additive process has recently been applied to high value materials such as stainless steels, maraging steel, cobalt chromium alloys, titanium alloys, and Inconel in several industries [5] [6]. Several manners have been developed, as selective laser melting (SLM), selective laser sintering (SLS), electron beam melting (EBM), laser engineered net shaping (LENS), and direct metal deposition (DMD) [6][7][8] [9]. For instance, an optimal design of a cooling channels system in an injection mold has a possibility to achieve high production rates with high product qualities [10]. Zhao et al. fabricated an injection mold insert of AISI 420 with complex cooling channels with SLM, and the hardness increased at a high laser power for phase composition [11]. Krakhmalev, et.al investigated in situ microstructural evolution in AISI 420 stainless steel during SLM. The hardnesses in upper and inner regions were measured 750 HV and 500-550, respectively [5]. Regarding surface integrity of the metallic additive manufacturing (AM) products, additive/subtractive hybrid manufacturing was taken to control surface roughness and accuracy [12]. The subtractive process in the hybrid AM is economical in terms of the removal material volume and the tool wear [13]. Montevecchi et al. compared the cutting forces in milling of AISI H13 steels processed in LENS and wire-arc additive manufacturing (WAAM). The tangential and the radial cutting coefficients on WAAM material becomes higher and lower than in the LENS one, respectively [14]. Heigel et al. [12] discussed residual stress of AISI 17-4 stainless steel fabricated into cylindrical geometry in a hybrid AM. The process promotes compressive stresses on the inside surface and tensile stresses on the outside one of the cylinder. After AM process, the compressive stress, in turn, appears on the outside by finishing with an endmill.
Allegri et al. [15] reported hardness on the machined surface increases with the feed per tooth in micro milling of medical CoCrMo alloys made by SLM. The. The cutting process of Ti-5553 fabricated in SLM was compared with the original Ti-5553 [16].
The paper investigates machinability of AISI 420 stainless fabricated by SLM steel in milling process. The cutting force and chip morphology of workpiece fabricated in SLM are compared to wrought workpiece in the cutting tests. An analytical model, then, is applied to discuss the difference in the cutting models.   Table 1. The chip morphologies were observed with an optical microscope (KEYENCE VHX6000). In order to investigate the effect of tool geometry on the machinability, straight and helical square end mills shown in Table 2 were employed for the cutting tests.   The large helix angle should be taken to reduce tool failure induced by the tool vibration. It is notes that the cutting forces of SLM workpiece in Fig. 4(a) and (c) were similar to those of wrought workpiece in Fig. 4(b) and (d), even though the SLM workpiece is much harder than wrought workpiece. Figure 5 compares the average cutting force at the edge engagement of both workpieces as: (1) Little difference appears in the cutting force of SLM workpiece compared to the wrought workpiece in the same cutting conditions.
(2) The cutting force becomes small at a high cutting speed when comparing the results of Condition 3 with Condition 4.  where t1, tc, and α are the uncut chip thickness, the chip thickness, and the radial rake angle, respectively. larger than that of wrought workpiece. Since the cutting force of SLM workpiece is almost equal to those of wrought workpiece, the shear angle and the shear stress on the shear plane is expected to be large. Although the shear stress on the shear plane slightly increases with uncut chip thickness. the average in milling of SLM workpiece in Fig. 9(a) is estimated as 1447 MPa, which is 1.86 times larger than 778 MPa of the wrought workpiece in Fig. 9(b). The friction angle goes up slightly as the cutting thickness decreases due to the relatively increase of edge roundness effect.

Conclusion Conclusion
Machinability of AISI 420 martensitic stainless steel fabricated SLM has been discussed in the cutting tests and the simulation. The force model based on the minimum cutting energy was applied to the simulation of milling.
The cutting force of SLM workpiece is nearly equal to that of wrought workpiece. The chip morphologies, however, are different between both workpieces. The large shear angle is expected in milling of SLM workpiece in comparison to the wrought workpiece. In the simulation, the shear angle of the SLM is larger than that of the wrought workpieces based on the orthogonal cutting data. The shear stress on the shear plane is estimated as 1447 MPa, which is 1.86 times larger than that of wrought workpiece. Although the shear stress on the shear plane is large in the SLM workpiece in comparison to the wrought workpiece, the result shows the cutting force does not become large significantly due to a large shear angle.