Novel process combined extrusion and severe plastic deformation for plate component with rib-web structure of magnesium alloys

Plates with rib-web structure of magnesium alloys are hardly to be fabricated through the conventional method. In this work, the variable circular channel angular extrusion (VCCAE) combined extrusion and severe plastic deformation is proposed, in which billet undergoes severe deformation at several variable circular angular channel. The FE model for VCCAE of scaling plate with ribs is established, and the experimental setup is designed. The feasibility of the novel process and the accuracy of the FE model are validated by simulations and experiments. Stress state and the effective strain distribution in the longitudinal section and cross section are analyzed. The results show that stress and strain distribution in the longitudinal section is extremely inhomogeneous and obvious stress concentration at the shear plane is founded. The microstructure in the angular region is more uniform and finer, which validate that the severe shear deformation induced by the angular channel in the VCCAE process could promote the deformation uniformity and grain refinement. The results of numerical simulation and experiments demonstrate that the VCCAE process proposed in this paper is feasible and it has great potential to manufacture high-performance plate with ribs.


Introduction
Lightweight structures with lightweight materials were widely used in automobile and aerospace industries [1]. Magnesium alloys have received widespread attention in the aerospace field due to their advantages of low density, high specific strength, high specific stiffness, and good damping performance [2][3][4]. As a typical load-bearing component, the standards for the comprehensive performance of the plate part with rib-web structure of magnesium alloys are gradually elevated, and the traditional forming methods for this kind of part include welding, forging, casting, and machining. However, machining has poor performance in work efficiency and material utilization and often weakens the comprehensive mechanical properties because the metal streamlines was cut off [5][6][7]. And for welding, the comprehensive mechanical properties of welding joints are generally worse than the base material [8][9][10][11]. In our previous works, we proposed the multi-stage variable speed (MSV) isothermal forging process to form the large size rib-web component [12], and the results showed that the maximum load is 55.8% lower and the formed component has an average grain size of 9 μm, ultimate tensile strength of 342 MPa, and yield strength of 268 MPa, respectively. Although the simulation and experiments were carried out to present the advantages of reducing forming load, grain refinement and enhance the mechanical properties, forming efficiency is really far from high efficiency.
In addition, as a kind of local incremental forming technology, spinning has great advantages of high dimensional accuracy and low forming force in manufacturing rotational parts through accumulating multiple small strains [13][14][15][16]. Unfortunately, spinning prefers to cause the fracture and underfilling of ribs, so it is hard to manufacture high-performance parts with ribs [17,18]. Isothermal forging and extrusion are also the important way to form thin-walled component with ribs [19]. The axial closed extrusion (ACE) process was proposed to manufacture highperformance ring parts with mesh-like ribs, and the feasibility of ACE process in fabricating this kind of parts was validated through discussing the distribution of the stress state, metal flow behaviors, temperature, and effective strain [20]. An innovative radial envelope forming method was proposed to manufacture thin-walled cylindrical rings with inner web ribs (TCRIWR), and the motivation and principles were clarified that envelop movement and geometry relationship between the constraining roll, feeding roll, and TCRIWR [21]. The simulation and experiments were executed to verify the feasibility of the proposal method. The design method of the novel process, determination method, and modification method of interfering are also proposed to ensure the novel process working. The above processes have made many innovative contributions to the forming of thin walled component with ribs and provide an important reference for our works, but they have limited improvement in the comprehensive properties. Thus, for this typical plate with ribs, the traditional processing methods could no longer meet the higher standards.
Due to the hexagonal close-packed (HCP) structure of magnesium alloys, only the basal slip system can be activated, which is restrict the wider application of Mg alloys [22,23]. In the last decades, many improvement methods have been tried to enhance the comprehensive mechanical properties of materials, and the severe plastic deformation (SPD) technology is considered to be the most effective method to obtain high-performance materials with ultrafine grains (UFG). Several SPD technologies are widely used, for instance, equal channel angular pressing (ECAP) [24], friction stir processing (FSP) [25], accumulative rolling bonding (ARB) [26], high pressure torsion (HPT) [27,28], repetitive upsetting and extrusion (RUE), [29] and multi-directional forging (MDF) [30,31]. The radial forging actually is a multidirectional and simultaneous forging process in which four dies perform pressing operations through high-frequency radial movements to accumulating small strains for severe plastic deformation and is analogous to accumulative rolling bonding (ARB) and repetitive upsetting and extrusion (RUE) [32][33][34]. Differential velocity sideways extrusion (DVSE) was proposed to directly form billets into curved profiles/sections by extrusion alone without springback and cross-sectional distortion [35]. Interestingly, the further investigation found that the zone of shear could be generated along the profile edges [36]. Thus, the effective strains and strain tensor components on the longitudinal section were analyzed. The results showed that the shear intensive zones are different from that in ECAP, while shear strains are very small, and normal strains are dominant in welding line although the DVSE could approximately regarded as two reverse N-ECAP processes [37]. Furthermore, the DVSE process can also lead to a remarkable grain refinement, and the grains in the strain intensive zones are fine and equiaxed [38]. In addition, since ECAP could satisfy many expectations in homogeneous deformation, grain refinement, and industrial demands [39], several modified ECAP configurations have been proposed [40][41][42]. A dual equal channel lateral extrusion called T-ECAP process in which the two outgoing channel perpendicular to the ingoing channel like T-shape configuration was proposed, and the shear strain could obtained through upper-bound analysis [43]. Almost all ECAP and its modified process are benefit to grain refinement and mechanical properties [44][45][46]. However, most of the SPD methods mentioned above are generally used for the preparation of materials and rarely used for forming specific parts. Thus, in order to make use of the technical advantages of the SPD methods in refining the structure and improving the comprehensive mechanical properties on the manufactured parts, some traditional plastic forming processes are referenced in new combined process. KOBO process which combined the rotating extrusion and conventional extrusion was developed [47][48][49], and the mechanical properties and microstructure of AZ31 magnesium alloy during the new process at room temperature was investigated. The results indicated that the KOBO process can significantly improve the mechanical properties and refine the grains. The rotating backward extrusion (RBE) process which combined rotation and the conventional backward extrusion (CBE) was developed [50][51][52], and the metal flow behavior and strain distribution was investigated by finite element simulation. The results showed that effective strain distribution and deformation uniformity could be improved because of the additional shear deformation.
From the above, this paper will focus on the combination of conventional plastic deformation process and SPD method to fabricate the high-performance component. Therefore, for the plate parts with rib-web structure, we developed the circular variable channel angular extrusion process which combined conventional extrusion and ECAP process. The complex cavity of integrated die was designed to implement the combination. The deformation mechanism was revealed, and the feasibility of the developed process was validated by analyzing the metal flow behavior and forming force. In addition, the microstructure and stress state analysis show that the severe shear deformation could promote the deformation uniformity and grain refinement. The results demonstrate that the developed process is meaningful for providing a new feasible method for the manufacturing of large rib-web.

VCCAE process for typical component
A scheme of VCCAE process is described in Fig. 1, and the integrated die is composed of an upper die, a bottom die, a heating setup, and a punch. The circular block tightly holds down the integrated die so as to firmly fix the circular-shaped die. The VCCAE process can be briefly depicted as follows: In the initial stage, a proper single punch intrudes along its axial direction in a vertical channel of the circular cross section and forces the sample to extrude from the right angular region shown as region I into the horizontal channel. In the second stage, as the punch moves for more distance, the sample is squeezed into the second angular region as shown in region II from the horizontal channel. Subsequently, the material continues to flow into the next angular region shown as region III and eventually fills the entire channel area. In the final stage, under the continuous action of the extruding force, the material begins to complete the filling of the rib structure, as shown in area IV.
There are three sub-processes of in VCCAE process, i.e., regions I, II, and III, as shown in Fig. 2a. Considering the characteristic of the die, the deformation mechanism and metal flow behavior in region I are actually similar to T-ECAP process [53], which is considered two back to back typical channel angular extrusion processes as shown in Fig. 2b like ECAP process [54]. Therefore, it is effective to discuss the one-half of the region I. And regions II and III are the typical channel angular extrusion process as shown in Fig. 2c. In other words, the entire process of the VCCAE process is the accumulation of multiple channel angular extrusion processes. Thus, the microstructure evolution of the material in each region affected by VCCAE process show a similar trend as by ECAE. However, the differences in dimension and shape of two processes are supposed to affect the accumulation of strain and further cause some discrepancies in microstructure evolution.
The deformation mechanism in VCCAE process is a simple shear deformation as like ECAP. There is an important feature of metal flow at the channels angular forced by the shear stress, which has great influence on the microstructural evolution. After each channel angular extrusion, the main influence factor of the equivalent effect variable is the inner and outer angles between the die channels [55]. For ECAP process, in order to obtain a larger strain, multiple passes of extrusion are often performed, and the accumulated strain becomes larger as the extrusion passed increases.  Severe plastic deformation techniques such as equal channel angular extrusion, as a deformation method that can effectively refine grains and improve the comprehensive mechanical properties of materials, have obvious advantages in the preparation of bulk ultra-fine-grained metal materials. As the development of ECAP technology, VCCAE process inherits the same shear deformation mode and has the potential to refine the microstructure of materials. Therefore, according to the Hall-Petch relationship [56], VCCAE process has the potential to improve the comprehensive mechanical properties of components through the microstructure refinement. In other words, the VCCAE process could ensure the formation of components and improve their performance at the same time.

Experimental procedures
The plate with ribs as a load-bearing component of the aircraft cabin has a complex geometric shape like thin wall and ribs, and its service conditions are very harsh (high pressure). The plate with ribs is a typical thin-walled component with ribweb structure. The shape and key dimensions of the scaling part are shown in Fig. 3, which was focused in our research. The wall of the scaling part is thin of 15 mm, and its ribs are thin of 10 mm and high of 27 mm; that is, the ratio of height and thickness of rib is 2.7. The complicated shape and structure and harsh service requirements put forward high requirements on its forming process and performance; in this paper, we utilize the VCCAE process to study the high-performance precision forming of the scaling plate with ribs.
The VCCAE experiment was performed on the 20 MN hydraulic press. And the billet of pre-extruded AZ80 magnesium alloy was processed into a cylindrical shape with height of 202 mm and diameter of 59 mm. The experimental setup for VCCAE process of scaling cover plated is shown in Fig. 4. The diameter of the billet container is 60 mm. The experimental setup is equipped with isothermal heating system and data acquisition system, which can ensure that the temperature of the die remains basically constant during the experiment. Before loading the billet, the cavity of the combined die is lubricated with molybdenum disulfide and heated to 380 °C for heat preservation. The billet was heated up to 400 °C and held isothermally in heating furnace for 2 h to eliminate thermal gradient. The hydraulic press can drive punch to axially extrude cylindrical billet with speed of 1 mm/s.
After extruding, the sample was cooled to room temperature in the air. As shown in Fig. 5a, the sample of cover plate was cut into two same part from the central axis position, and a 3-mm thin slice is cut out on the central axis surface as shown in Fig. 5b. The thin slice was divided into five specimens for analysis of microstructure evolution; the specific locations for the microstructure characterization at different region are marked with triangle in Fig. 5c. After normal grinding and polishing processes, the central axis surface of specimens was subsequently etched with a solution of   The specimens for c OM observation was cut from a scaling part at b the symmetry plane 1.5 ml picric acid saturated solution, 1 ml distilled water, 12 ml ethyl alcohol, and 1 ml acetic acid. The microstructure characterization in the center region of each specimen was carried out using DM4000M optical microscope.

FE modelling
The material of plate with ribs is selected as AZ80 magnesium alloy, which shows strain rate sensitivity behavior, temperature sensitivity behavior, and work hardening behavior. The process parameters in simulation are determined from the hot processing map [57]. As shown in Fig. 6, the FE model of forming the scaling plate with ribs is established). The constitutive equations with a correlation coefficient of 0.985 of AZ80 magnesium alloy [12] is given as follow: where A, Q, n, and α are material parameters; σ represents the flow stress and its unit is MPa; ε denotes the strain; and T is the absolute temperature. R is the universal gas constant (8.314 J·mol −1 ·K −1 ).
Considering the VCCAE hot forming process of magnesium alloy is a rigid-visco-plastic thermal deformation process, and in order to promote the convergence of the numerical simulation, the FE model was established by used of the DEFORM-3D software, which adopts explicit iterative algorithm. The FE model of the VCCAE consists of upper die, bottom die, punch, and billet, respectively. Considering that the width of ribs in simulation is 10 mm, so the size of meshes is limited in the range of 1 ~ 2 mm to avoid the severe distortion of meshes around ribs. On the other hand, the absolute meshes were adopted, and adaptive mesh criterion is applicable to automatically conduct the mesh reproduction when meshes are seriously distorted, and the type of mesh is tetrahedral. The cylindrical specimen with the height of 202 mm and diameter of 59 mm is set as to be plastic body, and all other tools are regarded as rigid bodies. The initial temperatures of AZ80 magnesium alloy billet and rigid bodies are set as to be 400 °C and 380 °C, respectively. The material of die is H-13 steel. The boundary conditions for the heat transfer by convection and radiation are took into account. Heat transfer coefficient between die and workpiece is 11 N/s/mm/C, and the heat dissipation coefficient between die/billet and air is 0.02 N/s/ mm/C. The shear friction coefficient of 0.2 between the workpiece and rigid bodies in the process of VCCAE was consistently adopted.

Feasibility analysis of VCCAE process
In order to reveal the feasibility of VCCAE process, the experimental and simulated forming force in different stages was shown in Fig. 7 Fig. 7 Forming force variation in VCCAE process of scaling plate with ribs force, while the solid line denotes simulated one. The experimental force is slightly greater than the simulated force, but the trend is basically same as the simulation result. It can be attributed to the temperature decrease of the billet during the transfer process between resistance furnace and mold. According to the simulated results, the process of VCCAE could be divided into three stages according to the variation trend of forming force. In the upsetting stage when the displacement of punch Δl is 4.5 mm, the forming force sharply ascends to about 250 kN, which is caused by that the working hardening plays a dominant role in upsetting process. In the stable forming stage, the increasing rate of forming force obviously descends, which can be attributed to temperature rise and the occurrence of dynamic recrystallization (DRX). The severe plastic deformation caused severe deformation heat effect, which maintains or even increases the material temperature by thermal diffusion. On the other hand, for magnesium alloys with low stacking fault energy metal materials during hot deformation process, massive dislocation multiply and accumulate caused by working hardening; although the dynamic recovery is accompanied by strain hardening, the dynamic recovery process is often slow and hard to simultaneously offset the dislocation proliferate and accumulation [58]. When dislocations accumulated to a certain extent, i.e., the dislocation density reaches the critical recrystallization dislocation density, the mainly softening mechanism is DRX [59]. The grain boundaries of a large number of recrystallized grains could coordinate the deformation; besides, the DRX would eliminate a large number of dislocations. Thus, softening is greatly enhanced and basically offset the working hardening. In particular, cavity corner changes will lead to increasing deformation resistance, so the forming force slightly increases when Δl = 25.1 mm and Δl = 98.7 mm, respectively. In the last filling stage, the forming force significantly increases from 503 to 2023 kN when the displacement improves from 159 to 184.4 mm, because in this stage, the deformation only occurs at the corner position of cavities where the metal flowability is pretty poor.
The cavity of the integrated die is intricate, so the metal flow behavior is complicated in VCCAE process of plate with ribs, which inevitably causes inhomogeneous deformation. In view of this, the metal flow velocity vector and contours are shown in Fig. 8 to investigate the mechanism of VCCAE process. As described in Fig. 8a, b, the metal mainly flows along the radial direction (RD) to fill the crown part. According the law of least resistance, the metal flows to fill the waist part, and there are bulges in the position of the ribs with the motion of punch. As shown in Fig. 8c and its enlarged view, the metal continues to flow in the radial direction and material flow velocity decreases from 0.413 mm/s (line F) to 0.100 mm/s (line A) along the towards the edge of the web. In addition, the metal flow velocity around the top of the ribs is inhomogeneous due to the inhomogeneous deformation here. And then, the metal continues flows to gradually fill the web part and finally fill the ribs. It can be seen in the Fig. 8d, the metal gradually flows to ribs rather than the RD direction, and it is demonstrated that the growth of ribs is slower than the other parts.
The formed part of VCCAE process of plate with ribs is shown in Fig. 9. The deviation in shape between experimental and simulation results could be attributed to two aspects. On the one hand, the friction coefficient between the billet and die is regarded as a constant in the simulation, but the actual friction coefficient is variable and affected by temperature and stress. On the other hand, the shape of die could slightly change during the extruding process and extracting the extruded parts because of the slight elastic deformation, but the mold is always regarded as rigid in simulation. In summary, the experimental results demonstrate that the shape of the experimental results is in good accordance with that of the simulation. The growth laws of crown, waist, web, and ribs is in agreement with the principle of the VCCAE process. Thus, the feasibility of VCCAE process forming plate with ribs is validated.

Distribution laws of stress and strain
In order to figure out the deformation mechanism of billet in the VCCAE process, the effective stress distribution in different stage is investigated. The distribution laws of effective stress in the scaling plate with ribs under VCCAE process are displayed in Fig. 10. As shown in Fig. 10a, after the upsetting stage of VCCAE process, i.e., when the stroke is 4.5 mm, although there is slightly bulge at the outer surface of billet, the effective stress has reached to 50 MPa in the center of crown part. When the punch stroke reaches 25.1 mm, metal has filled the whole crown part. And the maximum value 47 MPa of effective stress distributed in region I which shown in Fig. 2b. That is to say, deformation mainly appears at this position. Similar distribution phenomenon could be found that the deformation mainly distributed in region I and region II, resulting in the severe plastic deformation, and metal could fill the crown part and waist part in this stage. Furthermore, the web, crown, and waist all almost satisfy the requirement of shape when the stroke reaches 159 mm, so the effective stress in these positions sharply descends. On the other hand, with the increment of punch stroke, there are obvious bulge at the rib part of cavities, and the effective stress distribution in longitudinal section is extremely inhomogeneous. In addition, the part of ribs connected with the waist deformed before the part connected with the web.
The stress states in the process of VCCAE process of plate with ribs is complicated, so inevitably caused the inhomogeneous deformation. The effective strain distribution in symmetry longitudinal section was selected to systematically reflect the deformation degree of web, waist, crown and ribs. In addition, three typical cross sections were also selected to display the effective strain distribution around ribs. Figure 11 shows the effective strain distribution in the longitudinal section and cross sections, respectively. The effective strain distribution along the path from A to F in longitudinal section is shown in Fig. 11a. The abscissa represents the actual distance on the path from A to F. The effective strain at web from A to B ranges from 3.2 to 3.5. On the path B to F, the maximum effective strain at junction (as shown in Fig. 2a) is 5.5, and the effective strain shows a trend of gradual decrement from the junctional part between waist and crown to web, which indicates that deformation of the junctional part is the largest. The effective strain gradually increases from the center of the lower surface of the crown to the radial direction of the upper surface. Particularly, the strain in the shear direction (as shown in Fig. 2b) is larger than that at the point D. The reason caused the effective strain distributions at junction and shear direction may attributed to the severe  shear deformation at the angular of channel (as discussed in Sect. 2).
In order to further investigate the distribution laws of effective strain, the effective strain distribution in the three cross sections of the upper section, medium section, and lower section are depicted in Fig. 11b-d. The effective strain gradually increases from the lower section to the upper section, because the shear deformation of material in the upper section is larger than that in lower. Moreover, the effective strain distributions in the three cross sections show a similar trend that strain gradually descends from the outer edge to the center in the radial direction. On the other hand, the effective strain distribution perpendicular to the radial direction is uniform. In particular, the effective strain in the upper cross section was affected by the severe deformation at the junctional part, so the effective strain is high, and the fluctuations is very small in rib part.

Stress state analysis
For the VCCAE process, the shearing deformation induced by the variable channel angular plays a critical role in refining the microstructure. In order to better reveal the forming mechanism of scaling plate with ribs in VCCAE process, points B, C, and D are selected as typical points to illustrate the severe shear deformation on plastic forming. The contribution of shear deformation could be measured by the Lode parameter which can be expressed as [60] where μ b is the Lode parameter and σ 1 , σ 2 , and σ 3 denote the three principal stress respectively. μ b = 0 means pure shear stress state, while μ b = -1 or μ b = + 1 represent the uniaxial compressive or the uniaxial tensile stress state. In other words, the smaller absolute value of Lode parameter implies the more shear deformation [61]. In addition, the hydrostatic pressure μ m also plays important role in plastic deformation, and it can be described as The Lode parameter and hydrostatic pressure at points B, C, and D are calculated from the simulated stress states according to Eqs. (3) and (4). Figure 12a, b display the stress state parameters variation at typical points during the VCCAE process. The results show that the hydrostatic pressure at point D is the highest, and the Lode parameter here is always approximately 0. It means that the stress state in point D is uniaxial tensile stress state. Compared with point D, the hydrostatic pressure of point B and point C decreases sequentially, and the absolute value of Lode parameter is between 0 and 1, which means that the stress state of points B and C is the simple shear stress state. As shown in Figs. 11 and 12, high hydrostatic is helpful for the realization of metal plasticity, while the stress state also plays important role in the metal deformation. The reason could attributed Fig. 11 The effective strain distribution in a longitudinal section and cross section at b upper section, c medium section, and d lower section that the severe shear deformation and optimized stress state could not only promote the formation of high density of high angle grain boundaries (HAGBs) but also provide enough energy for DRX nucleation and DRX grain growth [62].

Microstructure analysis
According to Sect. 5.3, the effective strain of the outer edge of crown part is obviously higher than the other part, and it gradually descends from the upper cross section to the lower cross section in waist part and rib. Therefore, in order to further investigate the effect of the VCCAE process on the grain size of AZ80 alloy plate with ribs, several regions are selected in longitudinal section (see in Fig. 5).
The microstructure evolution in all regions can be obtained from the OM observation in Fig. 13. Previous researches have indicated that DRX is the main deformation mechanism of Mg-Al alloys at the temperature ranging from 200 to 400 °C, which usually caused the new generated grains and growth of new DRXed grains around the deformed grains [63].
In can be observed from Fig. 13e that the microstructure in the center of crown part (region IV) is mainly composed of incomplete DRXed grains and few fined grains. Meanwhile, it can be also seen in region I (shown in Fig. 13b) and region VII (shown in Fig. 13h) that the microstructures in these regions are consisted of coarse deformed grains and fine DRXed grains around them. Furthermore, compared with the region II (shown in Fig. 12c) and region VI (shown in Fig. 13g), the average grain size is finer (~ 10 μm) and microstructure is more uniform in region III (shown in Fig. 13d) and region V (shown in  Fig. 13f). Combined with the variation of the effective strain in longitudinal section (shown in Fig. 12a), microstructure in regions III and V with equiaxed fine grains could result from that the severe strain caused by the angular channel can significantly improve the dislocation density in these regions, which promote the nucleation and growth of the DRXed grains. Therefore, it can be inferred that the deformation uniformity and the grain refinement ability can be increased by the angular channel in the VCCAE process. It is worth noting that this work only involved variable circular channel angular extrusion process of scaling plate with rib-web structure of magnesium alloy. In fact, many other plate shape parts with ribs could be also formed by this novel process. In the future, more intensive studies focusing on the optimization of cavity shape and process parameter will be executed for the other typical components.

Conclusions
In this paper, a novel variable circular channel angular extrusion process is proposed, and the novel process in forming typical component is investigated by FE simulation and experiment. The following conclusions are drawn: 1. The novel variable circular channel angular extrusion process is put forward to manufacture plate component with rib-web structure of magnesium alloys. The proposed process innovatively combines conventional extrusion and equal channel angular extrusion process, and the material will undergo severe shear deformation for three times induced by the integrated die consist of variable angular channels. 2. Metal flow behaviors and the shape of simulated part are in good agreement with that in experiment, which indicated the feasibility of the proposed process and the reliability of the FE model. In addition, the effective stress distribution in longitudinal section is extremely inhomogeneous, and obvious stress concentration at the shear plane of the variable channel angular is founded. Furthermore, the high hydrostatic pressure is helpful for deformation, while the simple shear stress state can also promote the severe plastic deformation. 3. From the microstructure analysis, the microstructure is more uniform and finer (~ 10 μm) in angular region, which indicate that the severe shear deformation induced by the angular channel in the novel process could promote the deformation uniformity and the grain refinement ability.