**5. Metal forming of FGSS**

*Engineering Steels and High Entropy-Alloys*

**4. Laser machining of FGSS**

burr size.

ignorable [17].

by 30 μm.

relatively shallow microgrooves.

In summary, mechanical machinability of FGSS is much improved by finegrained microstructure; smooth cutting edges, reduction of in situ vibrations and

Laser machining grew up as one of useful means to produce miniature parts. This laser processing has two different aspects; one is thermal process like welding, and the other is mechanical process. When producing accurate parts for medical or precision parts, thermal effects on the works surface became a main driving force in the CO2-laser machining. When the laser wave length became shorter or when using the pico- or femtosecond laser, the thermal effects are ignored because materials are removed before transferring the temperature [16]. Siegelet et al. investigated the picosecond pulse ablation behavior and industrial relevance in 2009. He noted that the accurate surface and structures were introduced and the thermal effect was

Since the picosecond laser reduces the thermal influence on cutting, the grain-size effect on the laser machining changes by its mechanical process. Using FGSS is one of the methods to apply to the enhanced parts of medical, because it has higher tensile strength and hardness without changing any chemical composition. Komatsu used the picosecond laser to investigate the grain-size effect on the laser machining of AISI 304 with the thickness of 1.5 mm [18]. **Figure 6(A)–(C)** shows the cross-sectional images after laser drilling AISI304 sheet with different grain sizes by 30 times. The laser machining system (PANASONIC AP-3220) was employed; the laser output was 500 mW, the wavelength, 1053 nm, the pulse duration, 15–25 ps, and the repetitive frequency, 5 kHz. The spot size was constant

There are significant differences of processing depth in different grain size. When processing smaller grains such as FGSS (1 or 2 μm), the depth of groove was from 0.1 to 0.12 mm. On the other hand, there are from 20 to 40% deeper groove in large grain's material. After 30 times laser processing, the material nearly cut. **Figure 7** describes the relationship between the one-shot machined depth and the grain size. Deeper groove is formed by machining the large grain AISI304, while shallower groove becomes unstable in case of FGSS. This might come not only from the hardness of material but also from the absorptivity, which related to surface roughness and structure [18]. The smaller grain size is preferable to micro deep laser cutting. In summary, laser machinability of FGSS is characterized by precise cutting of

*SEM cross-sectional image after laser processing machining the AISI304 with different grain size (d) by 30 times in the same condition. (A) Left: D = 1.1 μm, (B) center: D = 2.0 μm, and (C) right: D = 9.2 μm.*

**8**

**Figure 6.**

#### **5.1 Deformation and transformation in micropunching of FGSS304**

As well known, size effects were observed during metal forming [19–21]. The size effect also occurs in punching process. In this chapter, introducing the difference in deformation and transformation of austenitic stainless steel AISI304 when micropunching with grain size changing. AISI304 stainless steel causes strain-induced transformation during plasticity process [22]. The strain-induced transformation of AISI304 austenitic stainless steel was classified as non-diffusive shear transformation, and transformation occurs in grain-size units [23]. Therefore, shear deformation in micropunching became complicate.

#### **5.2 The stability of sheared surface length when changing a grain size**

**Figure 8** shows four IPFs of AISI304 stainless steels with different grain sizes, which were analyzed by EBSD [24]. Grain size (Gs) 7.5 μm was standard grain size of AISI304. The grain size from Gs 3.0 to Gs 1.0 μm were specially minimized AISI304 that were not changed chemical composition. The characteristics of materials were shown in **Table 3** [24]. From this table, as grain size decreasing, 0.2 % proof stress and tensile strength increasing and elongation became decreasing. The Hall-Petch relationship works effectively in these grain-size conditions.

**Figure 9** shows the constitution of sheared surface and cross-sectional SEM images of punched microhole [24]. The punching condition was following; material thickness 100 μm, punch diameter 80 μm, die diameter 87 μm, clearance between punch and die 3.5 μm, punching speed 4.2 mm/s. The sheared surfaces were consisted by shear droop, burnished surface, and fractured surface, respectively. The burrs were not shown in this figure. **Figure 10** shows the ratio of sheared surface that calculated from five samples [24]. From this figure, as grain size decreasing, the shear droop ratio became small and fractured surface ratio became large. However,

#### **Figure 8.**

*EBSD inverse pole figure of four grain size austenitic stainless steel AISI304 including FGSS304.*


#### **Table 3.**

*Mechanical characteristics of four grain size austenitic stainless steel AISI304 including FGSS304.*

#### **Figure 9.**

*Constitution of sheared surface and SEM images of cross section of micropunched hole. (A) Constitution of sheared surface, (B) Grain size 1.0μm, (C) Grain size 1.5μm, (D) Grain size 3.0μm, (E) Grain size 7.5μm.*

from **Figure 11**, the standard deviation of burnished surface ratio took a different trend [24]. The grain size 3.0 μm took a smallest standard deviation. It was found that the stability of the sheared surface length changing with grain size.
