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Machinability variations in Alloy 718 —With focus on machining of turbine components

Avdovic, Pajazit LU (2011) In LUTMDN(TMMV-1062)/1-108(2011)
Abstract
In gas turbine engine, nickel base alloy, such as Alloy 718, accounts for almost half of the total material requirement. Nickel-base superalloys are used for gas turbine components mainly because of their outstanding strength and resistance to oxidation at high temperatures (> 550°C). The massive cost involved with the machining of nickel alloys has driven continued research and development of cutting tool materials as well as of cutting techniques that ensure higher metal removal rate with minimum surface and sub-surface damage on the machined components. In machining of Alloy 718, one of major issues is the machinability variations as the result of variation in the mechanical properties and microstructures from workpiece. Sometimes it... (More)
In gas turbine engine, nickel base alloy, such as Alloy 718, accounts for almost half of the total material requirement. Nickel-base superalloys are used for gas turbine components mainly because of their outstanding strength and resistance to oxidation at high temperatures (> 550°C). The massive cost involved with the machining of nickel alloys has driven continued research and development of cutting tool materials as well as of cutting techniques that ensure higher metal removal rate with minimum surface and sub-surface damage on the machined components. In machining of Alloy 718, one of major issues is the machinability variations as the result of variation in the mechanical properties and microstructures from workpiece. Sometimes it was difficult to machine different regions of the same component and when that happened, machining issues arose with chip entanglement and tool failures.



The aim of the research addressed in this thesis is to explain machinability variation in Alloy 718 turbine discs with respect to material parameters such a grain size, hardness, deformation and work hardening; all of which are believed to have great influence on tool wear. The investigation was based on the large amount of production data retrieved from the production scene and the experimental data from laboratory. With this work it is expected to refine and adjust the gap between the theory and the production practice to optimize the manufacturing process in general and machining process in particular.



For Alloy 718 machined with Al2O3-SiCW ceramic tools the main types of wear are mainly notch wear, flank wear and flaking wear. From retrieving used round C670 tools in production, it was found that an optimum effective cutting angle range existed in terms of insert wear. This means that if the effective cutting edge length is controlled through programming for each feed rate lower wear rates should be obtained. A polar diagram method for describing and evaluating the machinability of Alloy 718 was developed. Five key parameters of the work material, representing the mechanical and physical properties which have strongest influence on its machinability, were employed in the construction of polar diagrams. Work materials of Alloy 718 in which the polar diagrams of machinability were similar in size and shape exhibited very similar behavior during the cutting process. A tool life model, named “ShortCut-Wear-Model”, was developed with consideration of work-hardening effect in the machining of Alloy 718 for tool life prediction. In addition, the statistic based model, Weibull model, was also used for tool life predication makes it possible to derive an optimal replacement strategy which will minimize the unit production cost and other costs associated with machining of Alloy 718. (Less)
Abstract (Swedish)
Popular Abstract in English

You may have never heard of gas turbine engines, but they are used in all kinds of un¬expected places. For example, many of the helicopters you see, a lot of s¬maller power plants and even the military vehicle use gas turbines.

A gas turbine consists of a compressor, a combustion chamber and a turbine. Air is taken into the compressor and mechanically compressed. It is then mixed with fuel in the combustion chamber and ignited. The exhaust gases expand and are either used to produce thrust (in aircraft engines) or to produce mechanical power (in industrial gas turbines). In the first case, the gas stream is used to propel an aircraft, that is, the purpose of the turbine is only to... (More)
Popular Abstract in English

You may have never heard of gas turbine engines, but they are used in all kinds of un¬expected places. For example, many of the helicopters you see, a lot of s¬maller power plants and even the military vehicle use gas turbines.

A gas turbine consists of a compressor, a combustion chamber and a turbine. Air is taken into the compressor and mechanically compressed. It is then mixed with fuel in the combustion chamber and ignited. The exhaust gases expand and are either used to produce thrust (in aircraft engines) or to produce mechanical power (in industrial gas turbines). In the first case, the gas stream is used to propel an aircraft, that is, the purpose of the turbine is only to drive the compressor. In the latter case, the desired output is mechanical power which means that more energy is taken out of the exhaust gases due to more turbine stages. The mechanical power is provided as shaft power and can for instance be used to drive a generator to produce electrical power. The efficiency of such an industrial gas turbine can be further increased by using the exhaust gases to produce steam in a heat exchanger. The steam can then be used for heating purposes or to produce more electricity in the steam turbine.



To increase the efficiency of gas turbines, one is aiming at high temperatures at the turbine entrance, which has led to a massive development of new materials (for example, superalloys and thermal barrier coatings) and also material processing techniques and technology (directionally solidified and single crystal blades, for instance) for turbine blades, discs, and combustion chambers. This is the fundamental reason for the development and application of the superalloys, such as Alloy 718, and their continued improvement.



The production of turbine was studied through-out this work, which includes three different turbine discs made of Alloy 718. This material is suitable for using in this section of turbine which operates at working temperature of about 800 ˚C. Alloy 718 fully retains its strength up to 700º C, yet it maintains high enough strength, for this turbine operation, up to 800 ˚C. That is main reason for using of Alloy 718 for turbine discs. The mechanical properties of this material are very suitable for high efficiency but at the same time this material is very difficult-to-machine because of these properties.



The turbine discs are amongst the most critical components in the turbine. The primary function of the turbine discs is to provide a fixture for the turbine blades located in the gas stream, from which mechanical energy is extracted. The complete assembly of discs and blades is then capable of transmitting power to the fan and compressor section, via the shafts which run along almost the complete length of the turbine/engine. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Docent Beno, Tomas, Högskolan väst, Trollhättan
organization
publishing date
type
Thesis
publication status
published
subject
keywords
polar diagram, flank wear, ceramic tool, variation in material, tool wear, tool life, machinability, Inconel, Alloy 718, Machining, notch wear
in
LUTMDN(TMMV-1062)/1-108(2011)
pages
203 pages
publisher
Lund University
defense location
Room M:E, M-building, Ole Römers väg 1, Lund University Faculty of Engineering
defense date
2011-04-28 13:15:00
ISBN
978-91-7473-109-5
language
English
LU publication?
yes
id
d5cc1b25-411e-4f1b-aff4-6504ba280f74 (old id 1887847)
date added to LUP
2016-04-04 11:54:24
date last changed
2019-05-22 07:26:50
@phdthesis{d5cc1b25-411e-4f1b-aff4-6504ba280f74,
  abstract     = {{In gas turbine engine, nickel base alloy, such as Alloy 718, accounts for almost half of the total material requirement. Nickel-base superalloys are used for gas turbine components mainly because of their outstanding strength and resistance to oxidation at high temperatures (&gt; 550°C). The massive cost involved with the machining of nickel alloys has driven continued research and development of cutting tool materials as well as of cutting techniques that ensure higher metal removal rate with minimum surface and sub-surface damage on the machined components. In machining of Alloy 718, one of major issues is the machinability variations as the result of variation in the mechanical properties and microstructures from workpiece. Sometimes it was difficult to machine different regions of the same component and when that happened, machining issues arose with chip entanglement and tool failures.<br/><br>
<br/><br>
The aim of the research addressed in this thesis is to explain machinability variation in Alloy 718 turbine discs with respect to material parameters such a grain size, hardness, deformation and work hardening; all of which are believed to have great influence on tool wear. The investigation was based on the large amount of production data retrieved from the production scene and the experimental data from laboratory. With this work it is expected to refine and adjust the gap between the theory and the production practice to optimize the manufacturing process in general and machining process in particular. <br/><br>
<br/><br>
For Alloy 718 machined with Al2O3-SiCW ceramic tools the main types of wear are mainly notch wear, flank wear and flaking wear. From retrieving used round C670 tools in production, it was found that an optimum effective cutting angle range existed in terms of insert wear. This means that if the effective cutting edge length is controlled through programming for each feed rate lower wear rates should be obtained. A polar diagram method for describing and evaluating the machinability of Alloy 718 was developed. Five key parameters of the work material, representing the mechanical and physical properties which have strongest influence on its machinability, were employed in the construction of polar diagrams. Work materials of Alloy 718 in which the polar diagrams of machinability were similar in size and shape exhibited very similar behavior during the cutting process. A tool life model, named “ShortCut-Wear-Model”, was developed with consideration of work-hardening effect in the machining of Alloy 718 for tool life prediction. In addition, the statistic based model, Weibull model, was also used for tool life predication makes it possible to derive an optimal replacement strategy which will minimize the unit production cost and other costs associated with machining of Alloy 718.}},
  author       = {{Avdovic, Pajazit}},
  isbn         = {{978-91-7473-109-5}},
  keywords     = {{polar diagram; flank wear; ceramic tool; variation in material; tool wear; tool life; machinability; Inconel; Alloy 718; Machining; notch wear}},
  language     = {{eng}},
  publisher    = {{Lund University}},
  school       = {{Lund University}},
  series       = {{LUTMDN(TMMV-1062)/1-108(2011)}},
  title        = {{Machinability variations in Alloy 718 —With focus on machining of turbine components}},
  year         = {{2011}},
}