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On the Machinability of Ductile and Strain Hardening Materials - Models and Methods for Analyzing Machinability

Schultheiss, Fredrik LU (2013)
Abstract (Swedish)
Popular Abstract in Swedish

Allteftersom kraven på kvalité och prestanda hos dagens produkter ökar används alltmer avancerade material som en del av modern produktion. Problemet som uppstår är dock att dessa material ställer nya krav på tillverkningsprocessen och då inte minst vid skärande bearbetning. Trots att en betydande mängd forskning redan har publicerats vad gäller skärande bearbetning av dessa material saknas fortfarande genomgripande kunskap inom flera viktiga delområden. Ett problem med forskning inom skärande bearbetning är att den ofta förlitar sig i huvudsak på kvantitativ data, primärt erhållen genom experimentella försök. Detta gör att de erhållna resultaten kan vara svåra att generalisera till andra... (More)
Popular Abstract in Swedish

Allteftersom kraven på kvalité och prestanda hos dagens produkter ökar används alltmer avancerade material som en del av modern produktion. Problemet som uppstår är dock att dessa material ställer nya krav på tillverkningsprocessen och då inte minst vid skärande bearbetning. Trots att en betydande mängd forskning redan har publicerats vad gäller skärande bearbetning av dessa material saknas fortfarande genomgripande kunskap inom flera viktiga delområden. Ett problem med forskning inom skärande bearbetning är att den ofta förlitar sig i huvudsak på kvantitativ data, primärt erhållen genom experimentella försök. Detta gör att de erhållna resultaten kan vara svåra att generalisera till andra bearbetningsfall. I denna avhandling har en ansats gjorts att tolka de erhållna resultaten med hjälp av fundamentala fysikaliska principer. Även om detta potentiellt resulterar i ett större modellfel så ser författaren stora fördelar med att kunna ge en fysikalisk tolkning av de erhållna resultaten.

Målet med denna avhandling har varit att öka kunskapen om, samt i viss utsträckning även förutsäga skärbarheten hos några vanligt förekommande duktila och deformationshårdnande material. Forskningen har primärt fokuserat på studier av skärbarheten hos duplexa rostfria stål, Ti6Al4V samt Alloy 718. Inflytandet från stagnationspunkten vid skärande bearbetning samt det närbesläktade fenomenet med en minsta teoretisk spåntjocklek har varit av central betydelse för denna forskning. Denna faktor påverkar alla bearbetningsprocesser och har en betydande inverkan på skärbarheten, inte minst för duktila och deformationshårdnande material. Som en del av denna forskning upptäcktes att den minsta teoretiska spåntjockleken har ett signifikant och i viss utsträckning förutsägbart inflytande på den bearbetade ytans topografi för samtliga undersökta material. Det visade sig även att storleken hos den minsta teoretiska spåntjockleken är relaterad till flera olika processparametrar så som använd skärdata samt även arbetsmaterialet duktilitet och deformationshårdnande. Det konstaterades även att skärverktygets ytfinhet har en mätbar inverkan på bearbetningsprocessen.

Som ett led av denna forskning iakttogs hur svårt det är att förutsäga skärverktygens livslängd vid bearbetning av dessa material primärt på grund av deras höga hållfasthet även vid förhöjd temperatur, adhesiva uppträdande under skärprocessen samt även deras förhållandevis låga termiska konduktivitet. Således är deformation och annan nedbrytning av skärverktyget alltid ett potentiellt problem vid bearbetning av dessa material. Genom att använda den presenterade metoden för att bestämma den potentiella skärbarheten hos ett givet arbetsmaterial kan denna problematik potentiellt reduceras genom användning av rimliga processparametrar redan under initieringen av produktionen.

Även praktiska metoder för att förbättra bearbetningsprocessen i form av till exempel lägre detaljkostnad eller ökad hållbarhet från ett miljö- och samhällsperspektiv har presenterats som del av denna forskning. Trotts att de föreslagna metoderna enbart förbättrar en liten del av produktionsprocessen bör de inte försummas då alla delar av en tillverkningsprocess måste optimeras för att erhålla en reellt hållbar och kostnadseffektiv produktionsprocess. (Less)
Abstract
As quality and performance demands on today’s products increases, more and more advanced materials are being used during modern production. The problem is however that this in turn place new demands on the machining processes utilized. Even though a significant amount of research has been published on the machining of these materials knowledge is still limited in several crucial areas. A problem with machining research is that it often relies heavily on quantitative data primarily obtained through experimental investigations. Due to the substantial amount of potentially different machining cases it could be difficult to generalize the obtained results to other scenarios. In this dissertation it has been attempted to model the investigated... (More)
As quality and performance demands on today’s products increases, more and more advanced materials are being used during modern production. The problem is however that this in turn place new demands on the machining processes utilized. Even though a significant amount of research has been published on the machining of these materials knowledge is still limited in several crucial areas. A problem with machining research is that it often relies heavily on quantitative data primarily obtained through experimental investigations. Due to the substantial amount of potentially different machining cases it could be difficult to generalize the obtained results to other scenarios. In this dissertation it has been attempted to model the investigated phenomena through using universal physical relationships. Even though this might result in a larger modeling error for the specific case investigated the author sees a great advantage of being able to have a physical explanation to the obtained results.

The aim of this dissertation has been to increase the knowledge on, and to a certain extent predict, the machinability of some common ductile and strain hardening materials. The research has focused on evaluating duplex stainless steel, Ti6Al4V and Alloy 718. However, the proposed models have been constructed in a way as to aid future implementation for other workpiece materials. A central pillar of the research has been the influence of the stagnation point and the related minimum chip thickness. This aspect influences all machining operations and could potentially have a significant impact on the machinability, not least for ductile and strain hardening materials. During this research it was found that even though cutting conditions have a major influence on the value of the minimum chip thickness, material factors such as ductility and strain hardening should not be neglected as these also influence the obtained value. In turn, it was found that the minimum chip thickness could to a certain extent be used to explain the obtained workpiece surface roughness. Also, the tool surface roughness was found to have a determinate influence on the mechanics of the machining process.

During the present research it was also found that it is difficult to predict the tool life using conventional models for the investigated materials, essentially due to their high strength at elevated temperatures, adhesive behavior during machining, and low thermal conductivity. The influence of these properties commonly results in rapid and unpredictable wear of the cutting tool. Plastic deformation of the cutting tool is always a concern when machining these materials and a first step towards establishing a method for measuring the initiation of plastic deformation by using the measured cutting force has been proposed. Also, through using a proposed method for determining the potential machinability of a specific workpiece material these effects could be reduced through the use of reasonable process parameters before commencing production.

Methods for improving the machining process in terms of for example part cost or sustainability has been developed as part of this research. Even though each of these methods only improves a small part of the whole production process these improvements should not be neglected as all parts of the process should be optimized in order to achieve a truly sustainable and cost efficient machining process. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Dr. Beno, Tomas, Department of Engineering Science, University West, Trollhättan, Sweden
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Alloy 718, Ti6Al4V, Duplex stainless steel, Machinability, Minimum chip thickness, Contact condition, Surface topography, Tool wear, Part cost, Sustainable machining, Polar diagram.
pages
152 pages
publisher
Division of Production and Materials Engineering
defense location
Lecture hall M:B, M-building, Ole Römers väg 1, Lund University Faculty of Engineering
defense date
2013-12-20 10:00
ISBN
978-91-7473-729-5
language
English
LU publication?
yes
id
894dced6-29e8-4c80-8675-f28dd396284f (old id 4172833)
date added to LUP
2013-11-25 14:14:16
date last changed
2016-09-19 08:45:15
@phdthesis{894dced6-29e8-4c80-8675-f28dd396284f,
  abstract     = {As quality and performance demands on today’s products increases, more and more advanced materials are being used during modern production. The problem is however that this in turn place new demands on the machining processes utilized. Even though a significant amount of research has been published on the machining of these materials knowledge is still limited in several crucial areas. A problem with machining research is that it often relies heavily on quantitative data primarily obtained through experimental investigations. Due to the substantial amount of potentially different machining cases it could be difficult to generalize the obtained results to other scenarios. In this dissertation it has been attempted to model the investigated phenomena through using universal physical relationships. Even though this might result in a larger modeling error for the specific case investigated the author sees a great advantage of being able to have a physical explanation to the obtained results.<br/><br>
The aim of this dissertation has been to increase the knowledge on, and to a certain extent predict, the machinability of some common ductile and strain hardening materials. The research has focused on evaluating duplex stainless steel, Ti6Al4V and Alloy 718. However, the proposed models have been constructed in a way as to aid future implementation for other workpiece materials. A central pillar of the research has been the influence of the stagnation point and the related minimum chip thickness. This aspect influences all machining operations and could potentially have a significant impact on the machinability, not least for ductile and strain hardening materials. During this research it was found that even though cutting conditions have a major influence on the value of the minimum chip thickness, material factors such as ductility and strain hardening should not be neglected as these also influence the obtained value. In turn, it was found that the minimum chip thickness could to a certain extent be used to explain the obtained workpiece surface roughness. Also, the tool surface roughness was found to have a determinate influence on the mechanics of the machining process.<br/><br>
During the present research it was also found that it is difficult to predict the tool life using conventional models for the investigated materials, essentially due to their high strength at elevated temperatures, adhesive behavior during machining, and low thermal conductivity. The influence of these properties commonly results in rapid and unpredictable wear of the cutting tool. Plastic deformation of the cutting tool is always a concern when machining these materials and a first step towards establishing a method for measuring the initiation of plastic deformation by using the measured cutting force has been proposed. Also, through using a proposed method for determining the potential machinability of a specific workpiece material these effects could be reduced through the use of reasonable process parameters before commencing production.<br/><br>
Methods for improving the machining process in terms of for example part cost or sustainability has been developed as part of this research. Even though each of these methods only improves a small part of the whole production process these improvements should not be neglected as all parts of the process should be optimized in order to achieve a truly sustainable and cost efficient machining process.},
  author       = {Schultheiss, Fredrik},
  isbn         = {978-91-7473-729-5},
  keyword      = {Alloy 718,Ti6Al4V,Duplex stainless steel,Machinability,Minimum chip thickness,Contact condition,Surface topography,Tool wear,Part cost,Sustainable machining,Polar diagram.},
  language     = {eng},
  pages        = {152},
  publisher    = {Division of Production and Materials Engineering},
  school       = {Lund University},
  title        = {On the Machinability of Ductile and Strain Hardening Materials - Models and Methods for Analyzing Machinability},
  year         = {2013},
}