High Pressure Torsion Extrusion Research Articles

Corrosion Activity of Ultrafine-Grained Pure Magnesium and ZK60 Magnesium Alloy in Phosphate Buffered Saline Solution.

The magnesium alloy ZK60 is a promising candidate as a material for biodegradable implants. One of the most important factors for biodegradable implants is the modification of their corrosion behavior to match the requirements for the healing bone or tissue. The corrosion behavior can be influenced by different factors, among them the grain size, which can be changed by severe plastic deformation processes such as High Pressure Torsion Extrusion (HPTE). This study focuses on the corrosion behavior of samples of pure magnesium and ZK60 before and after HPTE, and the influence of the microstructure on the corrosion activity. The samples are subjected to immersion tests in phosphate buffered saline solution (PBS). The corrosion activity is defined by the emerging hydrogen volume from the corrosion process which is collected and by subsequently observing the resulting sample surfaces. The findings of this study suggest that pure magnesium shows lower corrosion activities than ZK60 and that HPTE processing leads to higher corrosion activities in PBS.

Processing of high-strength thermal-resistant Al-2.2% cerium-1.3% lanthanum alloy rods with high electric conductivity by High Pressure Torsion Extrusion

A novel severe plastic deformation (SPD) process of High Pressure Torsion Extrusion (HPTE) was applied to the rods of the Al-2.2 wt.% Ce-1.3 wt.% La (Al–3.5RE) alloy. The microstructure, microhardness, the mechanical strength, thermal stability, and electrical conductivity of the alloy after HPTE and subsequent annealing have been investigated. It was demonstrated that HPTE processing can simultaneously increase yield strength from 127 to 225 MPa and electrical conductivity from 54.7% IACS to 55.7% IACS in this alloy. Such a remarkable combination of properties was achieved thanks to significant refinement of microstructure constituents: grain size of Al matrix was reduced down to 0.9 µm and initially continuous network of Al11RE3 phase was broken to micrometer- and nanometer-sized particles. Furthermore, the yield strength of the HPTE-processed Al–3.5 RE alloy remains stable at 230 °C for 1 h due to micrometer- and nanometer-sized particles that pin the grain boundaries. Therefore, HPTE processing of Al–RE alloys has a high application potential in the electric power industry.Graphical abstract

Precipitate-mediated enhancement of mechanical and electrical properties in HPTE-processed Al–Mg–Si alloy

In this study the effect of high-pressure torsion extrusion (HPTE) and artificial aging (AA) on the strength and electrical conductivity of an Al–Mg–Si alloy was investigated. The resulting material shows remarkable improvement in ultimate strength reaching 380 MPa and in electrical conductivity with 53 % IACS (International Annealed Copper Standard), after post-processing aging at 160 °C for 10 h. X-ray diffraction analysis and electron backscatter diffraction were employed to analyze the structure of the alloy caused HPTE followed by AA. Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy were used to study the morphology, elemental composition, and crystal structure of the secondary phases. The study confirms that Guinier-Preston (GP) zones and nano-scale precipitates created during the deformation and subsequent heat treatment are evenly distributed inside the grains. Chemical composition analysis of the precipitates indicated the presence of one main phase based on the Mg/Si ratio coexisting with a low volume fraction of other phases in this Al grade. High-resolution TEM revealed precipitates either with a partially ordered structure or a fully crystalline structure. Strength and electrical conductivity modeling were carried out based on the microstructural characterization results. The findings of this study indicate that HPTE together with thermal treatment has the potential as new approach for processing of commercially significant aluminum alloys.

Design and Analysis of High-Pressure Torsion Process Using Finite Element Method

High Pressure Torsion (HPT) is a procedure that is frequently used to change the structure and characteristics of materials by using severe plastic deformation. It is widely believed that astraightforward shear causes deformation during HPT. In order to create nanocrystalline structures in metals and alloys as well as to cold weld powders, HPT is utilized. The flow kinematics and stress condition of a material under deformation dictate the HPT treatment’s outcome. Recent studies have revealed that the actual plastic flow during HPT might deviate greatly from the straightforward concept. Based on a modification of the traditional high pressure torsion technique of severe plastic deformation, a new approach called High Pressure Torsion Extrusion (HPTE) will be proposed. A specimen will be extruded via sectional containers that will be spinning in relation to one another during HPTE. The area where the containers converge on the specimen will be exposed to shear deformation. The findings of the FEM inquiry will be presented in this report

A novel approach in mechanical nanostructuring synthesis of metal hydride: Hydrogen sorption enhancement by High Pressure Torsion Extrusion

In this study, we evaluated the influence of a new mechanical nanostructuring technique called High Pressure Torsion Extrusion (HPTE) on the microstructural evolution of niobium and the subsequent effects on the mechanical properties and hydrogen storage behaviour. Two different regimes with the extrusion speeds of ν = 7 mm/min and ν = 10 mm/min were implemented in the experiments. A remarkable microstructural refinement and increase in hardness were achieved after one pass of HPTE. The initial grain size of 16.5 μm decreased to 600 nm and the initial hardness of 80 Hv increased to 284 Hv. Using a Sievert apparatus, it was found that the HPTE processed sample could absorb hydrogen to its full capacity within about 6 h while the as-received sample did not absorb even after one day of exposure to hydrogen gas. Rate limiting step modelling of the hydrogen absorption revealed that the absorption is a 3-dimensional diffusion-controlled reaction with a constant or decreasing interface velocity, depending on the HPTE regime.

Perspectives of Scaling up of Severe Plastic Deformation: A Case of High Pressure Torsion Extrusion

A demand for cheap and simple methods of processing of nanocrystalline materials attracted attention to metal forming techniques allowing introduction of large strains into metallic materials. Thanks to that a new scientific field, “Materials by severe plastic deformation”, had been born and became an important segment of material science. High pressure torsion (HPT) is the most effective SPD method in respect of the microstructure refinement and processing costs. Despite attractive mechanical and physical properties demonstrated by HPT-processed materials, this technique cannot be used in industry due to a limited size of processed samples. To overcome this problem, a plenty of SPD methods, allowing processing of large-scale billets, was developed. However, none of them has found any application in industry, except a few cases, which are rather exceptions that confirm the rule. This overview is an attempt to analyze the reasons why industrial players remain reluctant to use SPD in technological processes. For this purpose, the advantages and drawbacks of the High Pressure Torsion Extrusion method of SPD are regarded.

Equivalent strain distribution at high pressure torsion extrusion of pure copper: Finite element modeling and experimental validation

Equivalent strain distribution at high pressure torsion extrusion of pure copper: Finite element modeling and experimental validation

The effect of the die rotation during extrusion on the shape of embedded markers

The introduction of die rotation during extrusion changes the material flow pattern. An initially straight flow marker embedded into a billet to visualise plastic flow is twisted into a spiral during rotation-assisted extrusion in addition to the reduction in the cross-sectional area. Moreover, the shape of the marker as observed on two-dimensional sections perpendicular to the extrusion axis changes from a circle to a teardrop. A simple model was proposed to describe these changes. The method is based on two transformations to account for the grid pattern distortion during conventional extrusion without rotation and for the torsion due to the die rotation. A good agreement was obtained between calculations and experimentally observed marker shape changes during high-pressure torsion extrusion and previously reported results for friction stir extrusion.

Analysis of the reciprocal wear testing of Aluminum AA1050 processed by a novel mechanical nanostructuring technique

This research aims to investigate the impact of a novel technique in mechanical nanostructuring on the wear resistance of materials. This technique with the name of High Pressure Torsion Extrusion (HPTE) can produce bulk nanostructured materials with enhanced mechanical properties. Results of microstructural analysis and microhardness testing showed significant enhancement in materials after HPTE. Microstructural characterization by using Electron Back-Scattered Diffraction (EBSD) method illustrated the presence of Ultra-Fine Grained (UFG) materials in the specimens Analysis of the wear by implementing reciprocal wear testing revealed that the amount of displaced volume markedly decreased after processing. This change in the wear behavior can be explained by referring to the hardness increase and the reduction of plasticity in materials which confined the plastic shearing and diminished the built-up edge around the wear track.

Tailoring the microstructure and tribological properties in commercially pure aluminium processed by High Pressure Torsion Extrusion

Tailoring the microstructure and tribological properties in commercially pure aluminium processed by High Pressure Torsion Extrusion

Finite element modeling of Conform-HPTE process for a continuous severe plastic deformation path

A novel continuous severe plastic deformation (SPD) approach was proposed by combining continuous extrusion forming (Conform) with high pressure torsion extrusion (HPTE), which is called Conform-HPTE process. Finite element method (FEM) simulations were employed to investigate the severe plastic deformation behavior of aluminum alloys under various processing conditions. The results showed that the novel Conform-HPTE process effectively accumulated large plastic strain after a single deformation pass. During the Conform-HPTE process, the Conform machine not only provided driving power for the whole equipment, but also introduced shear deformation and heating in the specimen for subsequent HPTE process. The subsequent HPTE process further introduced severe plastic deformation in the specimen. And the severe gradient shear deformation condition could lead to the formation of refined and gradient microstructure. Therefore, the novel Conform-HPTE process could be a promising industrial SPD technique to continuously produce advanced engineering materials with the advantages of energy and cost saving and high process efficiency.

Structure and Tensile Strength of Pure Cu after High Pressure Torsion Extrusion

The microstructure and mechanical properties of rod-shaped samples (measuring 11.8 mm in diameter and 35 mm in length) of commercially pure (CP) copper were characterized after they were processed by high pressure torsion extrusion (HPTE). During HPTE, CP copper was subjected to extremely high strains, ranging from 5.2 at central area of the sample to 22.4 at its edge. This high but varying strain across the sample section resulted in HPTE copper displaying a gradient structure, consisting of fine grains in the central area and of ultrafine grains both in the middle-radius area and at the sample edge. A detailed analysis of the tensile characteristics showed that the strength of HPTE copper with its gradient structure is similar to that of copper after severe plastic deformation (SPD) techniques, typically displaying a homogeneous structure. Detailed analysis of the contributions of various strengthening mechanisms to the overall strength of HPTE coper revealed the following: The main contribution comes from Hall–Petch strengthening due to the presence of high and low angle grain boundaries in gradient structure, which act as effective obstacles to dislocation motion. Therefore, both types of boundaries should be taken into account in the Hall–Petch equation. This study on CP copper demonstrated the potential of using the HPTE method for producing high-strength metallic materials in bulk form for industrial use.

Evolution of microstructure and hardness in aluminum processed by High Pressure Torsion Extrusion

An investigation was conducted on aluminum samples (AA1050) to study the microstructure and hardness evolution during processing by High Pressure Torsion Extrusion (HPTE). The equivalent strain accumulated in the samples after one pass of HPTE varied in a wide range between 0.9 and 65.5, depending on the processing parameters. HPTE led to the formation of a gradient microstructure in which the grain size decreased by increasing the distance from the central axis of the billets. This gradient decreased significantly by increasing the ratio of rotational speed to extrusion speed. Most of the grain refinement and hardness-increase occurred up to the equivalent strain of ~5 but saturation level was achieved at the strain of ~20. Saturation levels in grain size and hardness were ~0.7 μm and ~67 HV, respectively. HPTE technique can provide similar saturation in microstructure and hardness as those obtained by HPT but in much larger samples. Therefore, HPTE can be a named as a suitable candidate for practical applications to produce bulk ultrafine-grained materials with considerable enhancement in hardness.

A Mathematical Model of Deformation under High Pressure Torsion Extrusion

High pressure torsion extrusion (HPTE) is a promising new mechanism for severe plastic deformation of metals and alloys. It enables the manufacture of long products with a radial gradient ultrafine-grained structure and of composite materials with a helical inner architecture at the meso and the macro scale. HPTE is very promising as a technique enabling light weighting, especially with magnesium, aluminium and titanium alloys. For the first time, this article presents an analytical model of the HPTE process that makes it possible to investigate the role of the various process parameters and calculate the distribution of the equivalent strain over the entire sample length. To verify the model, its predictions were compared with the numerical simulations by employing the finite element software QForm. It was shown that potential negative effects associated with the slippage of a sample relative to the container walls can be suppressed through appropriate die design and an efficient use of the friction forces.

Experimental and numerical analysis of HPTE on mechanical properties of materials and strain distribution

High Pressure Torsion Extrusion (HPTE) is a novel technique which has been recently introduced to the society of Nano-SPD researchers. HPTE exploits the deformation mechanics of HPT but in a larger scale within rod-shape samples and is capable of applying high values of strain to materials in one pass. This research aims to evaluate the effect of HPTE on mechanical properties of materials and also to study the effect of geometry of HPTE die on strain distribution in deformed samples by using Finite Element Method (FEM). Commercial pure Aluminium AA1050 was used for experimental work; and eccentric dies with parallel-misaligned channels were developed for evaluation by numerical modelling. Results of this research will help us better understand the effect of process parameters and also geometry of the die on materials.

High Pressure Torsion Extrusion as a new severe plastic deformation process

A new method named High Pressure Torsion Extrusion (HPTE) is proposed based on a modification of the conventional high pressure torsion technique of severe plastic deformation. During HPTE, a specimen is extruded through sectional containers rotating relative to each other. The specimen is subjected to shear deformation in the area where the containers meet each other. One of the main advantages of the HPTE process is that already after a single extrusion pass a high accumulated strain can be achieved in the specimen. Furthermore, the presence of a strong velocity gradient in the specimen cross-section during HPTE provides the possibility to process hybrid materials or composite parts with helical architecture of functional elements. The HPTE method is evaluated theoretically by using finite element methods (FEM) and experimentally by using HPTE for processing copper specimen and the results are presented and discussed.