Figure 2 shows the SEM images of the GA Zn-22wt%Al powders, which contained spherical particles of various sizes below 40 μm. The microstructure of the powder used to manufacture miniature helical gears exhibited a fine two-phase structure. The light and dark phases are Zn and Al, respectively [9][11].

Figure 2: 
SEM images of Zn-22wt%Al powders

The compaction equipment and the appearance of the powder preform are shown in Figure 3. The alloy powders were compacted into a conical shape (D4.9×h14 mm) under a compressive force of 30 kN; the relative density of the compacted specimens was reported to be approximately 0.85 in the previous study [11].

Figure 3: 
Powder compaction equipment and compacted preform

Subsequently, the specimens were consolidated using the three processes shown in Figure 4. First, the compacted powders were homogenized at 375 °C for 22 h, solution heat treated at 340 °C for 90 min, quenched in liquid nitrogen, and stabilized at 250 °C for 30 min (Figure 4(a)). Second, the specimens were sintered at 350 °C for 2 h and then cooled in a furnace (Figure 4(b)). Third, the specimens were heated and stabilized at 250 °C for 15 min and then quenched in water (Figure 4(c)) to improve the formability of mechanically alloyed (MA) Zn-22wt%Al powder [12]. The microstructures of the consolidated specimens were investigated using SEM.

Figure 4: 
Powder consolidation processes

Extrusion experiments with consolidated powder preforms were performed to evaluate the effects of different microstructures on the extrusion behavior and mechanical properties of helical gears in a hot-powder extrusion process.

Figure 5 shows the configuration of the helical gear die and extrusion equipment used in this study. The specifications of the helical gears were a pitch diameter of 3.727 mm, 12 number of teeth, module of 0.3, reduction ratio of 50%, and helix angle of 15°. AISI H13 was used for the extrusion die.

Figure 5: 
Extrusion equipment and helical gear die

The temperature and strain rate in hot extrusion are the most important parameters that influence the deformation behavior of the powders. Extrusion temperature of 250 °C and punch speed of 0.1 mm/min under an average strain rate of 2.36×10^-3s^-1 were hot working conditions for superplastic deformation behavior [13].

Figure 6 shows the symbols used to calculate the average strain rate. In forward extrusion, the strain rate is defined by Equation (1).

Figure 6: 
Symbol used in calculation of strain rate

The average strain rate inside the extrusion die was obtained by averaging the volume in Equation (1). The volume of the deformation region between the die inlet and outlet is equal to Equation (2).

The above average strain rate is given by Equation (3):

where v₀ denotes the extrusion velocity, α denotes the half-die angle, and R denotes the extrusion ratio. r₀ and r_f are the radii of the powder preform and the helical gear, respectively.

The microstructure of powder preforms is a significant parameter that influences the forming behaviors [8][9]. To determine the optimal microstructures of the Zn-22wt%Al powder billet, it was extruded from various microstructures obtained via three different consolidation processes. During powder extrusion, a graphite lubricant (Klüber Press DC5-01KR) was applied to the extrusion tool, and a green compact was used to prevent adhesion. Extrusion experiments were carried out at MTS (100 kN), and the billet temperature (250 °C) in the container was directly measured using a contact-type T-type thermocouple. The general conditions of powder extrusion are listed in Table 1.

Figure 7 shows the microstructures of the powder billets consolidated by various processes. The SEM micrographs of the consolidated billets revealed three different microstructures. As shown in Figure 7(a), the solution heat treatment result in a very fine equiaxed grain structure with a Vickers hardness of 67.1 (Hv. 100 gf). The microstructure of the sintered billets was a typical lamellar eutectoid structure with a 63.3 Hv. The thicknesses of the individual layers were 1-2 μm, as shown in Figure 7(b). A mixed structure, with lamellar and equiaxed grain structures, was obtained via dwelling and quenching processes. The final consolidation process provided internal porosity. In all SEM micrographs, the bright phase was Zn and the dark phase was Al [9].

Figure 7: 
Microstructures of the consolidated billet

Figure 8 shows the surfaces of the helical gears with different consolidated billets (fine equiaxed grain, lamellar, and mixed structures). The surfaces of the gear teeth were fine, and no surface defects were observed during the extrusion process.

Figure 8: 
Surface of the extruded helical gear

The microstructure of Zn-22wt%Al alloys has a considerable influence on the SPF. The three basic requirements for SPF are (i) a temperature higher than approximately one-half of the melting temperature, T_m, (ii) a strain rate less than 10^-2 s^-1, and (iii) a fine and equiaxed grain size (<10 μm), which did not undergo significant grain growth during deformation. The conditions for the extrusion experiments fundamentally satisfy requirements (a) and (b).

Figure 9 shows the typical microstructures of the extruded Zn-22wt%Al specimens from different consolidated billets. The microstructure was finer, but substantially unchanged from that of the initially consolidated billets. A fine equiaxed grain structure was maintained during the extrusion process, and no microcracks were observed in the cross-sections of the extruded helical gears. These results might be attributed to the grain boundary sliding and diffusion creep mechanism, which play an important role in the fine-grained Zn-22wt%Al eutectoid alloy during the forming process under compression stress states [8]. The microstructure changed from a mixed to a fine equiaxed grain structure when the extrusion experiments were dwelled and quenched. However, there were some microcracks and pores in the case of the dwelled and quenched billets.

Figure 9: 
Microstructures of the helical gears extruded using different consolidated billets

Figure 10 shows the extrusion load-punch stroke curves of the Zn-22wt%Al alloy prepared using different consolidation processes. The powder preforms were extruded into elongated gear shapes after reaching their maximum extrusion load. The extrusion load of the lamellar Zn-22wt%Al alloy was dramatically higher than that of the fine equiaxed-grained Zn-22wt%Al alloy under the same extrusion conditions. A greater extrusion load (> 1.2 kN) was required for specimens with lamellar structures resulting from sintering, and a drop in the extrusion load was observed after a certain punch stroke. Consequently, a higher deformation energy is required to fill the fine teeth of the helical gear during extrusion compared with forming using the fine equiaxed-grained Zn-22wt%Al alloy preform [11]. The maximum extrusion load with the fine equiaxed grain structure resulting from solution heat treatment was only 0.69 kN, and the extrusion load curve was quite flat throughout the whole extrusion process. This illustrates that a fine equiaxed microstructure of Zn-22wt%Al alloys leads to a forming load lower than 60%, which is a typical superplastic flow.

Figure 10: 
Extrusion loads of the helical gears extruded using different consolidated billets

The Vickers hardness (Hv. 100gf) of the deformed helical gears is shown in Figure 11. The hardness was measured at three different points on the cross-section of the helical gears. The distribution of hardness was uniform over all measured points for individual cases. Despite the lowest extrusion load, the fine-grained billet obtained by solution heat treatment provided a greater hardness, about 83 Hv, after extrusion. However, owing to the residual porosity, cracks, and relatively large grain size in the mixed structure, a lower hardness (approximately 68 Hv) resulted in the extruded gear.

Figure 11: 
Vickers hardness of the helical gears extruded using different consolidated billets