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

Cryo-rolling of Cu Alloys: Part One

Cryo-rolling is effectively deformation of materials at liquid nitrogen temperature (LNT) that assists in producing ultra-fine grained microstructures.
Resulting gains can be made in the mechanical properties of the material and in studied cases, it was found that with a 40% thickness reduction through conventional and cryo-rolling techniques and improvement in hardness can be made from 85HRB to 90.8HRB respectively. Continue reading

Medium Manganese Steels: Part One

Medium manganese steels should be considered in the same circles of many of the technologically focused and well known materials in the automotive industry such as AHSS, DP, CP as well as second generation steels.
Medium-Mn steels show a strong tendency for transformation-induced plasticity (TRIP) or twinning induced plasticity (TWIP) depending on the stability of the retained austenite. Continue reading

Gas Nitriding of Titanium Alloys: Part One

Titanium alloys have an extremely wide appeal due to a range of advantages including a very high strength to weight ratio, high fatigue resistance and biocompatibility.
Through gas nitriding, an established thermochemical surface treatment method, the surface hardness, corrosion resistance and friction coefficient can all be improved.
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Sintered Iron Based Materials

Generally there is a huge application of powder metallurgy produced parts, especially in the automotive industry.
The complexity of these parts however can mean that through the presence of pores crack propagation can be a very real problem which needs to be addressed through various alloying and process techniques. Continue reading

Dynamic Strain Aging in Steel: Part One

Dynamic and static strain aging are the two main methods by which a material is further aged either during or after a period of plastic deformation.
Dynamic strain aging is specifically characterized by a rapid aging process which occurs during the actual straining and is associated with subsequent strength property advancements of the material. Continue reading

Introduction to Additive Manufacturing: Part Three

Additive manufacturing is a relatively recent manufacturing method which has become a key area of interest in multiple industrial sectors.
Some of the most prominent benefits of AM include very low energy consumption, reduced waste, and a reduced time to market, to name but a few. Continue reading

Cryogenic Treatment of Stainless Steels

Cryogenic treatment of stainless steels is just one method that can be used to reduce commonly occurring microstructural defects in stainless steels.
With cryogenic treatment the general strength properties of stainless steels can be improved and with the additional heat treatment, plasticity can also be effectively preserved. Continue reading

High Boron Cast Iron: Part Two

Boride emerged as a good alloying option for iron based alloys to help improve toughness and specifically help in applications where wear resistance is important.
There are many studies available but here we discuss the effect of different tempering temperatures on the microstructure and mechanical properties of high boron white cast iron after air quenching. Continue reading