A critical evaluation of the microstructural gradient along the build direction in electron beam melted Ti-6Al-4V alloy

Hitesh Sharma, David Parfitt, Abdul Syed, David Wimpenny, Emmanuel Muzangaza, Gavin Baxter, Bo Chen

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Abstract

It is generally recognised that electron beam melted (EBM) Ti-6Al-4V alloys exhibit a microstructural gradient along the build direction, but there have been some inconsistent experimental observations and debate as to the origin and magnitude of this effect. Here we present an unambiguous evaluation of this microstructural gradient and associated mechanical property along the EBM build direction on purpose-built round bar RB samples with build height of 380 mm and rectangular plate RP samples with build height of 120 mm. Columnar prior β grain width was found to increase (from 86±38 to 154±56 μm in RB and from 79±34 to 122±56 μm in RP samples) with the build height and the similar increase was also observed for α lath width (from 0.58±24 to 0.87±33 μm in RB and from 1.50±45 to 1.80±49 μm in RP samples). These observations can be attributed to the thermal gradient in the powder bed that produced a cooling rate gradient along the build height. The measured α lath width variation along the build height followed a lognormal distribution. The graded microstructure resulted in a decrease in micro-hardness which correlated very well with the mean α lath width by following a Hall-Petch relation.
LanguageEnglish
Pages182-194
Number of pages13
JournalMaterials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing
Volume744
Early online date5 Dec 2018
DOIs
Publication statusPublished - 29 Jan 2019

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Electron beams
electron beams
gradients
evaluation
Powders
Thermal gradients
Microhardness
Cooling
Mechanical properties
Microstructure
rectangular plates
microhardness
beds
titanium alloy (TiAl6V4)
Direction compound
mechanical properties
cooling
microstructure

Keywords

  • Electron beam melting
  • additive manufacturing
  • Titanium
  • graded microstructure
  • mechanical property

Cite this

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title = "A critical evaluation of the microstructural gradient along the build direction in electron beam melted Ti-6Al-4V alloy",
abstract = "It is generally recognised that electron beam melted (EBM) Ti-6Al-4V alloys exhibit a microstructural gradient along the build direction, but there have been some inconsistent experimental observations and debate as to the origin and magnitude of this effect. Here we present an unambiguous evaluation of this microstructural gradient and associated mechanical property along the EBM build direction on purpose-built round bar RB samples with build height of 380 mm and rectangular plate RP samples with build height of 120 mm. Columnar prior β grain width was found to increase (from 86±38 to 154±56 μm in RB and from 79±34 to 122±56 μm in RP samples) with the build height and the similar increase was also observed for α lath width (from 0.58±24 to 0.87±33 μm in RB and from 1.50±45 to 1.80±49 μm in RP samples). These observations can be attributed to the thermal gradient in the powder bed that produced a cooling rate gradient along the build height. The measured α lath width variation along the build height followed a lognormal distribution. The graded microstructure resulted in a decrease in micro-hardness which correlated very well with the mean α lath width by following a Hall-Petch relation.",
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AU - Sharma, Hitesh

AU - Parfitt, David

AU - Syed, Abdul

AU - Wimpenny, David

AU - Muzangaza, Emmanuel

AU - Baxter, Gavin

AU - Chen, Bo

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AB - It is generally recognised that electron beam melted (EBM) Ti-6Al-4V alloys exhibit a microstructural gradient along the build direction, but there have been some inconsistent experimental observations and debate as to the origin and magnitude of this effect. Here we present an unambiguous evaluation of this microstructural gradient and associated mechanical property along the EBM build direction on purpose-built round bar RB samples with build height of 380 mm and rectangular plate RP samples with build height of 120 mm. Columnar prior β grain width was found to increase (from 86±38 to 154±56 μm in RB and from 79±34 to 122±56 μm in RP samples) with the build height and the similar increase was also observed for α lath width (from 0.58±24 to 0.87±33 μm in RB and from 1.50±45 to 1.80±49 μm in RP samples). These observations can be attributed to the thermal gradient in the powder bed that produced a cooling rate gradient along the build height. The measured α lath width variation along the build height followed a lognormal distribution. The graded microstructure resulted in a decrease in micro-hardness which correlated very well with the mean α lath width by following a Hall-Petch relation.

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