Recent Progress on SPD Processes Empowered by Hydrostatic Pressure

Mahdi Zohrevand; Ali Reza Rezaei; Mohammad Reza Sabour; Erfan Taherkhani; Ghader Faraji

doi:10.2320/matertrans.MT-MF2022007

Abstract

Processing ultrafine-grained (UFG) and nanostructured (NS) materials in industrial size with simultaneous strength and ductility are the main challenges in severe plastic deformation (SPD) approaches. Hydrostatic SPD methods are a modified version of conventional SPD and forming methods with the difference of conducting fluid between the die and the sample to reduce surface contact or metal-to-metal contact. Recent studies show that hydrostatic SPD methods could process relatively long samples, making them usable in industrial applications. Also, a good combination of high strength and low ductility loss and more homogeneous properties could be achieved due to higher hydrostatic pressure. This article reviews hydrostatic SPD methods, their unique properties, and their capabilities.

1. Introduction

Over the past three decades, massive progress has been achieved in the field of SPD for producing UFG and NS materials with excellent properties. Several SPD methods have been developed for processing samples with different shape and size from laboratory to industrial scale. Among all the SPD methods, high-pressure torsion (HPT) is the most powerful method for substantial grain refinement.¹^–⁴⁾

The first investigations of the fundamental principles of HPT were done in 1930s.⁵^,⁶⁾ Two main characteristics of this method are the capability of processing hard to deform materials due to applying high hydrostatic pressure⁷⁾ and introducing severe shear strain.⁷^,⁸⁾ Accumulative von mises strain in the HPT could be achieved from eq. (1):⁹^,¹⁰⁾

\begin{equation} \varepsilon = \cfrac{\cfrac{2}{\sqrt{3}}(\pi Nr)}{h} \end{equation}

(1)

Where, r, N, and h are the radial distance from the center of the sample, the number of revolutions, and the thickness of the disk, respectively. As it is obvious one of the effective parameters of imposed strain is the radial distance (r) and increasing it leads to higher applied strain.¹¹⁾ In other words, lateral areas experience higher strain compared to central regions with small strain.¹²⁾ Thus, due to the non-uniform distribution of strain, the material processed by HPT has heterogeneous properties, which is reported by many researchers.¹³^,¹⁴⁾ As a case in point, Zhilyaev et al.¹⁵⁾ subjected commercial pure aluminum (99.7%) to HPT under a pressure of 1 GPa and studied the microstructural evaluation and mechanical anisotropy of the HPTed sample after various revolutions. They reported that after one revolution the amount of microhardness increased ∼100% in the outer areas while it increases only 16% in the center of the disk. Interestingly, increasing the number of revolutions does not have a noticeable effect on the microhardness of outer areas, however, this matter increases the microhardness in the central area, and after 4 revolutions a homogeneous microhardness could be achieved. Therefore, materials with uniform properties could be produced by HPT under certain circumstances that one of which is the number of revolutions. To clarify this manner Zhilyaev et al.¹⁶⁾ investigated the effective parameter on properties of pure nickel after the HPT under several applied pressures at room temperature. According to their work homogeneous properties (in this case microhardness) could be achieved under a high amount of pressure and a sufficient number of revolutions.

Despite the capability of grain refinement in HPT, the very short length of the sample⁷^,¹⁷⁾ is the most important limitation, which curbs the usage of this process in industrial applications. To overcome some limitations of conventional HPT, some other methods have been represented, such as incremental HPT,¹⁸⁾ SIHPT,¹⁹⁾ and High-Pressure Torsion Extrusion (HPTE).²⁰⁾ Additionally, a variety of techniques have been proposed for the processing of tubular specimens utilizing HPT principles, such as high-pressure tube twisting (HPTT)²¹^,²²⁾ and tube high-pressure shearing (THPS).²²⁾

Equal channel angular pressing (ECAP) is another important method, which was introduced by Segal²³⁾ in 1977. ECAP method suffers from various disadvantages such as limited imposed strain compared to HPT, limited sample length,²⁴⁾ inhomogeneous strain distribution, and time consumption. These limitations could be solved by conducting several modified methods, including ECAP with rotary-die,²⁵⁾ Side Extrusion ECAP,²⁶⁾ and ECAP with backpressure. In 2002 Beygelzimer et al.²⁷⁾ proposed Twist Extrusion (TE), in which the deformation mode is simple shear in the transversal plane, the same as HPT.²⁸^,²⁹⁾ Another ECAP-based method is tubular channel angular pressing (TCAP), which was presented by Faraji et al.³⁰⁾ in 2011 and successfully performed on Al-based³¹^,³²⁾ and Mg-based³⁰^,³³^,³⁴⁾ alloys. It is reported that applying ultrasonic vibrations reduces the processing load as a result of a lower coefficient of friction between surfaces.³⁵⁾ Like every other SPD process, TCAP cannot process long samples. To resolve the length problem and reduce the processing load, parallel tubular channel angular pressing (PTCAP) was developed based on TCAP,²²⁾ but the sample length problem was not completely solved. PTCAP was successfully accomplished on various types of materials, such as Al-based,³⁶^–³⁸⁾ Cu-based,³⁹^–⁴¹⁾ and Mg-based⁴²⁾ alloys.

Cyclic extrusion compression (CEC) which was invented in 1986⁴³⁾ is a proper method to process hard-to-deform materials as it imposes both high hydrostatic pressure and a large amount of strain simultaneously.⁴⁴⁾ Pardis et al. presented cyclic expansion extrusion (CEE) without an external back pressure system,⁴⁵⁾ which makes it an easy-to-perform process compared to the CEC method. However, both these methods still suffer from length limitation. Due to the high possibility of buckling or yielding of the punch and the limited travel distance of the plunger, processing long samples is the main drawback of this pressing-based method⁴⁶⁾ in which high friction between the die and sample curbs the processable length like most SPD methods. Increasing the sample length increases the possibility of yielding and buckling of the punch and reduces the hydrostatic pressure in some areas, which limits the processable length of the sample.²²⁾

As mentioned earlier, SPD methods produce samples with exceptional properties. However, they cannot benefit industrial purposes due to the length limitation,⁵⁾ which is the main challenge in SPD methods. This limitation arises from two main reasons. First, longer samples suffer from higher surface contact between the die and the sample, which increases the friction force that includes more than 80% of the processing load.⁴⁷⁾ Second, as a result of a higher processing load applied to a small cross-section of the punch, the possibility of yielding and buckling increases. One of the most effective solutions to overcome this issue is to use fluid to fill the gap between the die and the sample. This causes the reduction of the surface contact between the die and the sample and consequently reduces the friction force substantially. Another advantage of this solution is the remarkable hydrostatic pressure, which closes the microcracks and microvoids; so, more passes could be performed due to the postponement of the fracture of the sample.²²^,⁴⁸^,⁴⁹⁾ In other words, applying higher hydrostatic pressure increases workability and imposable plastic strain on the sample which results in smaller grain size and better mechanical properties.²²^,⁵⁰⁾

The current paper aims to review hydrostatic SPD methods, their fundamentals, and the properties that could be achieved by conducting these methods.

2. Methodology

In this part, the fundamentals, principles, and procedure of hydrostatic SPD methods, such as hydrostatic extrusion (HE), equal channel angular hydroextrusion (ECAH), Hydrostatic Extrusion integrated with Circular Equal Channel Angular Pressing (HECCAP), hydrostatic cyclic extrusion compression (HCEC), hydrostatic cyclic extrusion expansion (HCEE), hydrostatic twist extrusion (HTE), hydrostatic tube cyclic extrusion compression (HTCEC) and hydrostatic tube cyclic extrusion expansion (HTCEE) are described.

2.1 Hydrostatic extrusion (HE)

This method was developed in the 1960s to enhance cold workability due to the effect of hydrostatic pressure on ductility.⁵¹⁾ Lewandowska et al.⁵²⁾ showed that HE can produce UFG and nano-grained (NG) materials. No metal-to-metal contact between the die and the sample is observed unlike common extrusion, which causes plastic deformation in the sample under high fliud pressure. It has been reported that owing to the high efficiency-to-cost ratio of the HE process, it is a very favorable method from the perspective of industrial usage;⁵³⁾ therefore this method is the basis of all hydrostatic SPD methods. In other words, every hydrostatic SPD method combines hydrostatic extrusion (HE) and common SPD methods. Figure 1(a) illustrates the die structure of the HE method. At first, the billet is placed into the container, and the fluid fills around the billet. Pressing the ram increases the pressure of the fluid, and further movement of the ram forces the billet to pass through the die opening, which introduces severe strain to the billet. It is worth mentioning that replacing the direct extrusion die with a backward extrusion die gives an opportunity to produce high-strength tubular samples from the rod samples using hydrostatic backward extrusion (HBE) process.⁵⁴⁾

Fig. 1

(a) Die structures of the HE process, (b) the ECAH process and (c) the HECCAP process.

2.2 Equal channel angular hydroextrusion (ECAH)

Spuskanyuk et al.⁵⁵⁾ introduced ECAH based on the ECAP to process longer billets due to the noticeable reduction of billet-die contact friction. Figure 1(b) shows the die structure of the ECAH method. First, the sample is inserted into the inlet channel; then, around the sample is filled with fluid to eliminate the contact between the die and the sample. The high-pressure block is the main part of this structure as it experiences a high amount of pressure up to 1 GPa. Then, to prevent fluid leakage, the upper channel is sealed by a sliding plunger which showed in Fig. 1(b). By pressing the plunger downward, the required pressure is created, and the sample is pressed through the angular channel to experience strain. Intending to perform more passes without additional operation, the outlet channel (D_o) is designed to be slightly larger than the inlet channel (D_i).

2.3 Hydrostatic extrusion integrated with circular equal channel angular pressing (HECCAP)

This method is suitable for producing high-strength tubes using a combination of the hydrostatic extrusion (HE) and ECAP processes.⁵⁶⁾ Figure 1(c) depicts the schematics of the HECCAP method. This method begins by inserting the billet into the extrusion container, and then castor oil fills the container. After sealing the container with the ram to avoid fluid leakage, the ram moves down and pressurizes the oil, forcing the billet to pass through the deformation zone. As seen in Fig. 1(c), the deformation zone comprises two consecutive ECAP channels with a channel angle of φ, which makes the sample experience two passes of ECAP. AZ80 Mg billet was processed by this method at an extrusion ratio of 2.77 and a tube was produced with 335 MPa and 308 MPa as ultimate tensile strength (UTS) and yield strength (YS), respectively.⁵⁶⁾ Additionally, hydrostatic radial forward tube extrusion (HRFTE) was invented by Jamali et al.⁵⁷⁾ to produce long seamless tubes based on radial forward tube extrusion. This technique uses hollow samples as the primary specimen, in contrast to the HECCAP which utilizes small-diameter samples.

2.4 Hydrostatic cyclic extrusion compression (HCEC)

The CEC method shows the capability to use in industrial applications, but the short length of the sample limits the usage of this method. Siahsarani and Faraji⁵⁸⁾ proposed hydrostatic CEC to solve this restriction in 2020. The schematics and the die structure of HCEC are depicted in Fig. 2. According to Fig. 2, first, the sample is positioned in the inlet channel, and the fluid fills the space between the die and the sample. To prevent fluid leakage, a polytetrafluoroethylene (PTFE) with a diameter larger than the inlet channel was conducted.⁵⁸⁾ Then, Ram A (F_A) moves down to pressurize the fluid, and by further movement of Ram A, the sample passes through the deformation zone and experiences plastic deformation. Ram B exerts F_B as a back pressure load during the process to achieve a constant sample size for performing more passes. In the first cycle, F_A must be larger than F_B to press the sample through the deformation zone. Then, in the second cycle, F_B should be larger than F_A. It must be noted that like CEC conducting an external back pressure system is considered a primary drawback of the HCEC.

Fig. 2

Schematics of the HCEC process.⁵⁸⁾

2.5 Hydrostatic cyclic expansion extrusion (HCEE)

Hydrostatic cyclic expansion extrusion (HCEE) as shown in Fig. 3 was developed by Samadpour et al.⁵⁹⁾ based on CEE, which is a counterpart of the CEC method without an external backpressure system. This method solved the limited processable sample length problem of the CEE method. At first, the sample is inserted into the die, and like HCEC, the fluid fills the gap between the die and the sample; then, to pressurize the fluid, the punch press down. Like the HCEC method, PTFE polymeric is used to seal the die (Fig. 3(a)). In order to fill the deformation zone by the sample, the bottom punch is used in the outlet channel. It is removed when the sample reaches the bottom punch (Fig. 3(b)). More cycles could be achieved by rotating the die by 180°, as be seen in Fig. 3(d). HCEE and CEE differ in using fluid which causes a frictionless condition. It should be noted that even though HCEE has the advantage of performing without an external backpressure system over HCEC, the HCEC method shows higher hydrostatic pressure.

Fig. 3

Procedure of HCEE method (a) initial phase, (b) expansion phase, (c) throughout the first cycle, and (d) throughout the second cycle.⁵⁹⁾

2.6 Hydrostatic twist extrusion (HTE)

This method was presented by Haghpanah et al.⁶⁰⁾ as a counterpart of twist extrusion (TE) to produce relatively long samples with a square cross-section in 2022. Figure 4 shows the procedure of the HTE method. In the beginning, the sample is positioned into the inlet channel, and around the sample is surrounded by fluid. Then, to prevent the leakage of the fluid, it is sealed, and the fluid pressure increases by pressing the punch (Fig. 4(a)). It is worth mentioning that back pressure is not required during the procedure. However, to avoid the exitance of the corner gap between the die and the sample, it is better to conduct a backward punch (as seen in Fig. 4(b)) in the outlet channel. The punch moves down to increase the pressure of the fluid then, due to further movement of the punch, the sample is twisted by passing through the deformation zone. The sample should be removed and inserted into the inlet channel to perform more passes.

Fig. 4

Schematic of procedure of HTE method (a) die structure, (b) during the first cycle, and (c) after the first cycle.⁶⁰⁾

2.7 Hydrostatic tube cyclic extrusion compression (HTCEC)

HTCEC process shown schematically in Fig. 5, is a modified version of the TCEC method, which makes it suitable for the production of relatively long UFG tubes.⁶¹⁾ According to Fig. 5, after placing the tube into the die with a moving mandrel inside it, a back pressure mandrel is positioned in the outlet channel to produce back pressure. Then, hydraulic fluid surrounds the tube sample. To prevent the leakage of the fluid, a PTFE part with a diameter slightly larger than the channel diameter is used. The upper punch is pressed down to pass the tube material through the deformation zone (Fig. 5(b)). It is worth mentioning that the required hydrostatic pressure decreases significantly by using a moving mandrel placed inside the tube.⁶¹⁾ When the deformation zone is filled by the deformed material, the tube reaches the back pressure mandrel, and the hydraulic fluid fills the outlet channel and seals like the inlet channel (Fig. 5(c)). In order to pressurize the fluid in the outlet channel, a separate hydraulic jack system presses lower punch upward. To finish the first cycle, the movable mandrel, the back pressure mandrel, and the tube move downward simultaneously. More strain could be imposed on the sample by rotating the die by 180° and performing more cycles.

Fig. 5

Schematics of die components and procedure of HTCEC method (a) before the first cycle, (b) during phase one, (c) phase two, and (d) during the first cycle.⁶¹⁾

2.8 Hydrostatic tube cyclic expansion extrusion (HTCEE)

The method was developed based on tube cyclic expansion extrusion (TCEE).⁶²⁾ Figure 6 illustrates the die component of the HTCEE process. As seen in Fig. 6(a), in the beginning, the tube and the moving mandrel inside it are placed into the inlet channel, and then like other hydrostatic SPD methods, fluid fills the gap between the die and the sample. A PTFE piece with a larger diameter than the channel is used to seal the die. The tube experiences plastic deformation by the downward movement of the punch.

Fig. 6

Schematic of procedure of HTCEE method (a) before the first cycle, (b) during the first cycle, (c) after the first cycle, and (d) during the second cycle.⁶²⁾

The mandrel (back pressure system) is used to fill the deformation zone by tube (Fig. 6(b)). After the tube reaches the mandrel, the mandrel is removed from the die (Fig. 6(c)); therefore, unlike the HTCEC method, the HTCEE method does not need back pressure during the process, which is considered as the main advantage of this method compared to the HTCEC. If more cycles are needed, the die should rotate by 180° (Fig. 6(d)).

3. Discussion

As mentioned earlier, the main barrier to the way of industrialization of the SPD methods is the short length of processing samples. The immediate solution to overcome this matter is conducting continuous SPD methods.⁴⁷⁾ Another way is the utilization of hydrostatic SPD methods in which the length limitation is solved besides several other brilliant advantages. As mentioned earlier, when processing longer samples, the probability of buckling and yielding of the pressing punch increases. Therefore, hydrostatic SPD methods as shown in Table 1 were developed for reducing the required load, to process longer samples. According to eq. (2), total processing load (F_T) comprises three components:

\begin{equation} F_{T} = F_{D} + F_{f} + F_{R} \end{equation}

(2)

F_D, F_f, and F_R are the load required for the deformation, friction, and redundant work, respectively. F_D is almost equal between conventional and hydrostatic SPD methods, and F_R results from shear deformation. Nevertheless, F_f includes a high fraction of the processing load and is mainly affected by the amount of surface contact between the sample and the die. In hydrostatic SPD methods, by using pressurized fluid between the die wall and sample (before the deformation zone), the friction force is almost eliminated (Fig. 7). In other words, for a certain cross-section area, a longer sample could be processed by hydrostatic SPD methods compared to conventional ones, which is described in Table 1. Furthermore, hydrostatic SPD methods provide the possibility of processing hard-to-deform and brittle materials due to increasing the material workability via increasing the hydrostatic pressure. To express the workability as a measurable parameter, the workability parameter β is defined as:

\begin{equation} \beta = \frac{3\sigma_{h}}{\bar{\sigma}} \end{equation}

(3)

Where σ_h (hydrostatic component of stress) and $\bar{\sigma }$ (effective stress) are:

\begin{equation} \sigma_{h} = \frac{\sigma_{1} + \sigma_{2} + \sigma_{3}}{3} \end{equation}

(4)

\begin{equation} \bar{\sigma} = \sqrt{\frac{(\sigma_{1} - \sigma_{2})^{2} + (\sigma_{2} - \sigma_{3})^{2} + (\sigma_{1} - \sigma_{3})^{2}}{2}} \end{equation}

(5)

Where σ₁, σ₂, and σ₃ are the principal stresses. Increasing the β value with a negative sign leads to higher material workability. The more compressive hydrostatic component of stress, the more β value with a negative sign will be achieved, and finally, higher fracture strain (ε_f) could be imposed;²²^,⁶³⁾ therefore, more passes could be performed without any cracks, and failure. This manner could be well shown in ECAP with back pressure (BP) which applies higher hydrostatic pressure compared to conventional ECAP. As proof, Xu et al.⁶⁴⁾ studied the ECAP under various BPs on CP Al (99.65% purity) and compared the results with conventional ECAP. The results indicated that more strain could be achieved with higher BP, which comes from remarkable hydrostatic pressure in this method. It should be noted that, as a result of the nature of the HPT method, it has the highest β value with a negative sign among all SPD methods.⁶³⁾ This matter powers this method to process brittle material like semiconductors, which was discussed by Ikoma.⁶⁵⁾ Despite other SPD methods, which obey the Hall-Petch equation for strength, samples produced by the HPT have shown higher strength than the predicted one by Hall-Petch; it seems that this is the result of the high β in this process and the nature of this process. As a case in point, Valive et al.⁶⁶⁾ studied the effect of the HPT process on Al 1xxx and reported that the achieved UTS was ∼910 MPa, which is greater than the amount predicted by the Hall-Petch equation.

Table 1 Comparison of properties between conventional and hydrostatic SPD methods.

Fig. 7

(a) Processing load of HCEC and CEC of samples with various lengths,⁸⁰⁾ (b) force-displacement diagram of samples with different length during HCEE and CEE on AM60,⁸¹⁾ (c) required force to process samples with different length by the TE and HTE methods,⁶⁰⁾ (d) and (e) processing load versus displacement in HTCEE and TCEE methods for 100 mm and 200 mm length sample respectively.⁸²⁾

Table 2 shows various materials processed by different hydrostatic SPD methods. As can be seen, a comprehensive range of materials, including hard-to-deform samples, are processable due to imposing high hydrostatic pressure by hydrostatic SPD methods. For instance, hydrostatic extrusion could be mentioned as an outstanding method, which is the base of all hydrostatic SPD methods. The study by Skiba et al.⁶⁷⁾ is a marvelous example, which shows the capability of processing fully brittle materials at room temperature. It is reported that for GJL250 grey cast iron with a back pressure of 540 MPa, a crack-free sample with extrusion ratio R = 1.57 was achieved, while achieved R for the same material without back pressure was 1.35 and the later sample suffered from developing cracks. Pure bismuth is another example of fully brittle material, which only was extruded with back pressure without cracking. It seems that in both cases applying back pressure led to a positive effect, which enables processing materials with almost no elongation at room temperature. The same group reported that ZW3 magnesium alloy was processed at room temperature with R = 1.36 without back pressure; Whereas, conducting back pressure of 700 MPa resulted in R = 2.66, which shows a significant increase; Even though Skiba et al.⁶⁷⁾ performed HE at room temperature it is worth mentioning that almost every Mg alloy is processed at an elevated temperature so the deformation mechanism could be changed to dislocations movement.⁶⁸^,⁶⁹⁾

Table 2 Materials conducted in defferent hydrostatic SPD methods.

Industrial applications like aerospace, transportation, biomedical, and sports require materials with superior properties. Material properties are the function of microstructure,⁷⁰⁾ which intensely depend on the minimum saturated grain size. Minimum saturated grain size is the smallest grain size in which the generation and annihilation rate of dislocations is balanced.⁶³^,⁷¹⁾ According to reports, minimum saturated grain size is mostly related to hydrostatic pressure of the deformation process, deformation rate, level of impurity, initial microstructure before SPD, staking fault energy (SFE) and process temperature.⁷²^–⁷⁴⁾ Concerning Table 1, a smaller grain size is achieved after hydrostatic SPD methods, which is in total agreement with other reports.⁷²^,⁷⁵⁾ In other words, materials with higher strength and micro-hardness could be produced compared to conventional SPD methods. It is interesting that ductility loss is remarkably lower in hydrostatic SPD processes. In the case of Mg alloys, the unexpected ductility increase in conventional and hydrostatic SPD methods happens due to the fragmentation⁷⁶^,⁷⁷⁾ and uniform distribution⁷⁸⁾ of the brittle phase in the grain boundaries, which is initially formed by the fast-cooling rate in casting stage.⁷⁹⁾ According to Table 1, a more significant improvement in elongation was achieved through hydrostatic SPD methods, which could be caused by a more uniform distribution of the broken brittle phase. Another interesting result is that although imposed strain in HTE is almost smaller than TE, better grain refinement are achieved by HTE (Table 1), which is indicated in Fig. 8(a)–(c). In some cases, the homogeneity of material properties is vital, which depends on microstructure distribution that is influenced by strain distribution. As shown in Fig. 8(d)–(f), the applied strain on the sample surface in conventional SPD methods is higher than in the hydrostatic SPD method. In other words, high existence of metal-to-metal contact and therefore higher friction force in conventional methods creates more strain gradient, which results in strong non-uniform properties.

Fig. 8

(a) and (b) Transmission electron microscopy (TEM) microstructure of HTEed sample in the surface area,⁶⁰⁾ (c) the optical microscope (OM) micrograph of the TEed sample after one pass,⁸³⁾ (d) strain distribution along the sample thickness after HTCEE and TCEE,⁸⁴⁾ (e) the equivalent strain for CEC and HCEC along path A to B⁸⁰⁾ and (f) Strain homogenity in HCEC and CEC methods.⁸⁰⁾

4. Conclusion

This study focuses on the properties and procedures of hydrostatic SPD methods. A comparison was conducted between the conventional and hydrostatic SPD methods, revealing that due to high friction forces, conventional SPD methods suffer from the length limitation of processing samples. Hydrostatic SPD methods are introduced to overcome this issue to eliminate friction force that causes a significant decrease in required load. So, the longer samples suitable for industrial applications can be processed without the risk of buckling or yielding the pressing punch. Higher hydrostatic pressure enhances the material’s workability via the prevention of crack initiation and propagation during SPD. This matter also leads to more homogeneous mechanical properties and microstructure while reducing the minimum saturated grain size. Due to this imposed hydrostatic pressure, hard-to-deform and low-ductility materials are producible even in cold operations according to the improvement of workability. Thus, a more comprehensive range of materials may be processed by conducting hydrostatic SPD methods.

Acknowledgements

This work is based upon research funded by Iran National Science Foundation (INSF).

REFERENCES

Corresponding author

Register with J-STAGE for free!