Introduction to Additive Manufacturing: Part Six

Additive Manufacturing Material Extrusion processes have actually been in existence since the 1980’s and offer a rapid prototyping method to reduce the cost of an otherwise expensive field.
AM-ME can also be known by a number of other names including Direct Ink Writing or DIW, Fused Filament Fabrication or FFF, Extrusion Freeform Fabrication or EFF to name but a few.
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Twist Extrusion Processes of Non-Ferrous Alloys

Focus on grain refinement has continued to be a hot topic of material technology and as a known severe plastic deformation (SPD) technique, twist extrusion offers good potential.
Some key benefits of twist extrusion include being able to obtain ultra-fine grain crystalline and nano-crystalline structures and increased plasticity in the alloy.

In the recent years much attention has been paid to the development of ultra-fine grained and nanostructured materials due to their superior properties. Several severe plastic deformation (SPD) techniques have emerged in the recent years for producing ultra fine grained materials in bulk metals and alloys. Among the various SPD techniques, the Twist Extrusion process is used to produce grain refinement in bulk forms.

In comparison with ECAE, TE provides some benefits, such as the ability to extrude hollow parts and rectangular cross-sections. In addition, it is possible to produce more isotropic and homogeneous deformation, by turning the samples through 90° in each consecutive deformation or alternatively, make use of consecutive clockwise-anticlockwise twists. This feature is very important for electronic and magnetic materials. Therefore, the present work has been undertaken to develop fine grained aluminum alloys by twist extrusion process and to examine the microstructure and mechanical properties of twist extruded samples. TE can be applied for a wide range of materials such as Cu, Ti alloys, commercial purity aluminum, Al-Mg alloys, Al-Mg-Si alloys, and Al-Mg-Zn alloys.

This process uses extensive hydrostatic pressure to impose very high strain on bulk solids, producing exceptional grain refinement without introducing any significant change in overall dimensions of the sample. As the specimen is processed, it undergoes severe plastic deformation while maintaining its original cross section. The schematic sketch of twist extrusion process is shown in Figure X.

The twist extrusion principle consists in initiating intensive shear deformation by extruding a billet with rectangular cross section through a die with a twist channel. The channel shape and cross section does not change along the axis of extrusion, while the channel is twisted along this axis. The work-piece shape and cross section does not change as well, which allows repeated extrusion and thus an accumulation of plastic deformation.

The principle of twist extrusion (TE) is shown in Figure 1, in a special closed mold cavity with a helix angle β and a cross-sectional rotation angle α (the cross-section of the spiral channel is always orthogonally with the central axis and remains the unchanged same), shear plastic deformation in the metal billet is generated after twist deformation process, and a “cumulative” strain can be obtained after multi-passes twist extrusion deformation, so a new organization and performance enhancement can be acquired, too. Obviously, the twist rotation angle β and the cross-sectional rotation angle α determine the strength of the strong deformation caused by the twist extrusion (TE), and when the spiral channel length is constant, the twist angle α affects indirectly the helix angle β. Therefore, it is very important to determine and design the optimum twist angle α.

Figure 1: Schematic sketch of Twist Extrusion Process

Features of Twist Extrusion

  • The size of the terminating areas of the specimen, that is, the head and rear parts, of the billet, is much smaller under TE than under ECAE, which is especially important when doing repeated runs.
  • TE can handle profile billets including those with an axial channel.
  • TE can easily be installed on any standard extrusion equipment, by replacing a standard reduction die with a twist die.
  • TE (unlike ECAE) does not change the direction of a billet’s movement, which allows TE to be easily embedded into existing industrial lines.

Benefits of Twist Extrusion

There are currently three main benefits of Twist Extrusion:

  • Obtaining ultra-fine grain crystalline and nano-crystalline structures in bulk specimens
  • Increasing the plasticity of secondary non-ferrous metals and alloys, which allows one to significantly broaden the range of production
  • Obtaining bulk specimens by consolidating porous materials which allows one to create substantially different new compositions with unique characteristics

Applications of Twist Extrusion

  • Aerospace – Engine components (blades, discs, rings and engine cases)
  • Airframe components (tail sections, landing gear, wing supports and fasteners)
  • Automotive applications – Clamps in locking devices, fasteners in racing bikes
  • Medical devices-joint replacement (hip balls and sockets), surgical instruments, wheel chairs, etc.
  • Sport products-weight sensitive products, such as high-performance mountain bycicles, tennis rackets.
  • Food and chemical industries -Heat exchangers, tanks, process vessels, etc.


References 

1. C. Sakthivel, V. S. Senthil kumar: Determination of hardness and microstructure during cross plastic flow evaluation on twist extrusion processes, IJESMR, International Journal of Engineering Sciences & Management Research, ICAMS: March 2017, ISSN 2349-6193, Accessed April 2018;

2. B. Srinivas, Ch. Srinivasu, Banda Mahesh, Md Aqheel: A Review on Severe Plastic Deformation, Advanced Materials Manufacturing & Characterization Vol 3 Issue 1 (2013), p.291-296, Accessed April 2018;

3. M.Greger: Advanced Forming Technologies, Subject number: 633-0807, VŠB – Technical University of Ostrava Faculty of Metallurgy and Materials Engineering Department of Materials Forming, Ostrava 2016, Accessed April 2018;

4. Y. Li, Y-zhi Li: Densification Optimized Design for CuZnAl Sintered Powders by Different Twist Angle During Twist Extrusion Process by Numerical Method, 2017 3rd International Conference on Electronic Information Technology and Intellectualization (ICEITI 2017), p.459-464, ISBN: 978-1-60595-512-4;

5. Twist Extrusion, Accesseed April 2018.

Date Published: Feb-2019

Iron Spark Plasma Sintering: Part Two

Spark Plasma Sintering (SPS) is a sintering technique which is well matched to mechanically milled materials such as tool steels due to its low temperature and short cycle time.
One of the key objectives using SPS is to increase the density of the sinter in order to attain better overall property profiles of the finished part. Continue reading

Dispersion Strengthened Copper Alloys: Part Two

Coppers range of advantageous characteristics a quite well known including high electrical and thermal conductivity, excellent corrosion resistance to name but a few.
Dispersion strengthened coppers add the advantage of higher strengths which means they can be used for a range or applications such as welding consumables. Continue reading

Cu-MgO Composites

Metal matrix composites reinforced with ceramic particles can be interesting for a range of applications due to their strength performance at high temperatures and a relatively low thermal expansion potential.
Manufacturing of Cu-MgO composites critically requires that the raw materials have a very high level of purity (99.5-99.9%) to achieve the desired manufacturing results. Continue reading

Iron Spark Plasma Sintering (SPS): Part One

Spark Plasma Sintering (SPS) is a sintering technique which is well matched to mechanically milled materials such as tool steels due to its low temperature and short cycle time.
Tool steels have been specifically manufactured to exhibit exceptionally high strain hardening, a characteristic which can be undone by high temperature sintering processes such as hot isostatic pressing. Continue reading

Semi-Solid Rheocasting of Alumina Alloys: Part One

Aluminum is well established at the front of the pack with regards to providing the technological answer to the increasing challenges of light weighting whilst maintaining integrity of the material for the desired applications.
Semi solid rheocasting is a development within the casting sector which enables improved quality in die casting without increasing cost. Continue reading

Forging

Forging was the first of the indirect compression-type process and it is probably the oldest method of metal forming. It involves the application of a compressive stress, which exceeds the flow stress of the metal. The stress can either be applied quickly or slowly. The process can be carried out hot or cold, choice of temperature being decided by such factors as whether ease and cheapness of deformation, production of certain mechanical properties or surface finish is the overriding factor.
There are two kinds of forging process, impact forging and press forging. In the former, the load is applied by impact, and deformation takes place over a very short time. Press forging, on the other hand, involves the gradual build up of pressure to cause the metal to yield. The time of application is relatively long. Over 90% of forging processes are hot.

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High Temperature Materials

The subject of high temperature materials is a very broad topic indeed. When a material is used at elevated temperatures, its strength, as reflected in tensile strength, stress rupture life, or fatigue life, is of prime importance.
Currently, there are three main categories of superalloys that include iron (iron nickel)-based, nickel-based, and cobalt-based alloys.

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