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Effect of Heat Treatment on Additively Manufactured Ti-6Al-4V Alloy

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: The key objective of this work is to evaluate the mechanical properties and microstructural characteristics of post-process heat treatments of Additively Manufactured (AM) Ti-6Al-4V alloy including process-structure-property relationships. Tensile testing, smooth bar high cycle fatigue testing and microstructural analyses are to be performed on Laser Powder Bed Fusion (L-PBF) manufactured near net shape Ti-6Al-4V specimens having four heat treatment types after Hot Isostatic Pressing (HIP). These heat treatments are Mill-Anneal (MA), ANNeal (ANN), Solution Treat and Age (STA) and Beta Solution Treat and Overaged (BSOA). The resulting mechanical properties and microstructures will be compared to the traditional Ti-6Al-4V alloys bars, forgings and castings. The quantitative process-structure-property relationships will be determined with computational modeling with respect to build orientation. 

DESCRIPTION: Additive Manufacturing (AM) is a new production technology that enables reduced manufacturing steps, part consolidation and production of near net shape parts from 3-D model data. Current applications mainly focus on secondary structures or other non-critical applications. Recent developments in AM technology and AM Standards offer great potential to implement AM produced part in the US Army. In the past few decades, there is an increasing interest to produce metallic AM parts for structural and non-structural applications, as these materials show acceptable performances compared to the traditional materials with shorter lead times, less material usage and near net-shape parts [1-8]. The use of AM titanium alloy replacement of a currently used traditional titanium alloy in the US Army helicopters with a same traditional alloy heat treatment types may not provide an increased utilization of the AM titanium alloy and may cause additional performance risk since heat treatment for AM titanium alloy is not optimized. The current standard ASTM F2924 [9] of the AM Ti-6Al-4V alloy mechanical properties in all direction requires minimum tensile properties (130 ksi UTS/120 ksi YS/10% Elong) regardless of heat treatment types and part thicknesses. The thickness of the part affects the grain size of the part during solidification as such the grain sizes are smaller for the small thickness compared to the thick part. Therefore, tensile properties are higher for a part with small grains even though both part had same heat treatment types. On the other hand, the tensile properties are also depends on heat treatment types. For example, annealing heat treatment provides lower tensile properties compared to the solution heat and aged heat treatment. Therefore, the effects of heat treatment types coupled with part thickness and resulting mechanical properties need to be thoroughly investigated. In this study, the L-PBF AM process will be used to manufacture near net shape AM Ti-6Al-4V test specimens, and evaluate the effect of post-processing heat treatment types on tensile and fatigue properties, compared to the baseline Ti-6Al-4V alloy bar, forging and casting material properties. To understand the process-structure-property relationships, computational modeling is to be utilized to predict the quantitative mechanical properties such as tensile, yield, elongation and fatigue strength. The project will be conducted in three phases. Phase I will focus on assessment, design and selection of parameters, computational model, manufacturing options and procurement of AM Ti-6Al-4V alloy powder. Phase II will focus on demonstrating the ability to manufacture near net-shape of tensile and fatigue specimens, perform tensile, fatigue tests and evaluate tensile, fatigue test results of four heat treatment types (MA, ANN, STA and BSTOA) and quantitative prediction of process-structure-property relationships along different directions. Ten tests (10) at room temperature per heat treatment types will be evaluated for both tensile and high cycle fatigue (0.1 R-ratio) tests. Beta transus temperature of the two specimens will be determined for heat treating guidance. The chemical composition and density of the each lot will be determined. The chemical composition and physical properties of the Ti-6Al-4V powder will be verified. Only one batch of virgin powder and no recycled powder will be used. A few trial manufacturing is to be made to verify specimen sizes, quality and tensile properties. Phase III will focus introduction of AM Ti-6Al-4V alloy into a broad range of defense and civilian applications. This technology has been demonstrated in a laboratory research scale and prototype parts. The current effort would use existing technology to develop an optimized heat treatment types for AM Ti-6Al-4V alloy process utilizing simple shaped tensile and fatigue specimens. It is noted that the listed references [9-20] are to be used for general guidance on materials, manufacturing, heat treatment, testing and reporting to accomplish the objective of this project. The project’s three major phases are described below. 

PHASE I: Phase One evaluates the engineering merit and feasibility of the proposed technology. It identifies and builds team with industrial partners, design and select AM Ti-6Al-4V powder type, size and amount of experimental test specimens, AM manufacturing and, assesses application and manufacturing options, addresses producibility and inspectibility using these test specimens, selects predictive computer modeling and investigates the overall benefits of the project. The interrelations among AM processed Ti-6Al-4V alloy heat treating conditions and resulting microstructure parameters (alpha layer thickness, alpha and beta phase content, grain size, density, etc.) and mechanical properties are still not fully understood. To understand the process-structure-property relationships computational modeling need to be utilized to predict the quantitative mechanical properties. The objective of the process-structure-property relationships between the heat treating conditions and microstructural features is to be able to predict the microstructure and resulting mechanical properties for a given part geometry, size, and feature orientations for a given heat treating conditions. Such a model would be the basis for improving first part yield and enabling rapid part qualification. In order to verify the process-structure-property relationships, experimentally measured microstructural features and tensile properties are required in x, y and z directions. A generic computational model or a modified one to predict properties could be used for predicting process-structure-property relationships. To predict a complex part process-structure-property relationships, selected complex shaped parts will be modeled to determine properties. These representative complex shaped parts will be manufactured in Phase II and mechanical properties and microstuctural features will be measured with respect specimen orientations for modeling verification. Recommended computational modeling is to be demonstrated with open source microstructure and mechanical data for the AM Ti-6Al-4V alloy. Further ideas beyond described are welcome. An appropriate process modeling could be used to minimize process defects and maximize the mechanical properties for optimum producibility if needed and funding are available. Required Phase I deliverables include monthly progress reports, final technical report including specimen sizes, testing specimens locations, tests, powder specification and amount, AM build layout and manufacturing plans, predicted computational model examples demonstrating the process-structure-property relationships including complex shapes. 

PHASE II: Phase Two will manufacture the specimens and evaluate tensile and fatigue test results, predictive computational modeling compared to the traditional Ti-6Al-4V alloy bars, forgings and castings. The process will utilize an L-PBF process. The shapes of AM specimens will be simple-shaped L-PBF manufacturing. The overall dimension in length could be 8.0 inches with three wall thickness ranges as 0.25, 0.50, 1.00 and 2.00 inches with appropriate machining stocks. Any required radiuses could be 0.02 inches. All specimens will undergo HIP prior to the following heat treatments: 1) mill-anneal, 2) anneal, 3) solution treated and age and finally 4) beta solution treated and overaged. Tensile, fatigue, hardness, density, optical microscopy, scanning electron microscopy and computer tomography (CT) analyses are to be utilized to generate and analyze the resulting data during the Phase II effort. Ten (10) tests will be performed at room temperature per heat treatment types. Additionally, ten (10) tests will be performed at room temperature as AM manufactured and as HIPed for baseline comparison. Tensile, fatigue, hardness, density, optical microscopy, scanning electron microscopy and computer tomography (CT) analyses are to be utilized to generate and analyze the resulting data during the Phase II tensile evaluation. The specimens will undergo both tensile and high cycle fatigue (0.1 R-ratio) tests. Beta transus temperature of the two specimens as well as the chemical composition of each lot will be determined for heat treating guidance. Additionally, the chemical composition and physical properties of the Ti-6Al-4V powder will also be verified. Only one batch of virgin powder (no recycled powder) will be used, and all specimens will come from the same AM build feedstock. Trial printed specimens will be made to verify specimen sizes, quality and tensile properties. The interrelations among AM processed Ti-6Al-4V alloy heat treating conditions and resulting microstructure parameters (alpha layer thickness, alpha and beta phase content, grain size, density, etc.) and mechanical properties are to be determined. To understand the process-structure-property relationships, computational modeling is to be utilized to predict the quantitative mechanical properties. Such a model would be the basis for improving first part yield and enabling rapid part qualification. In order to verify the model, experimentally measured microstructural features and tensile properties are required in x, y and z directions. The resulting data is to be used to validate the computational modeling. Required Phase II deliverables include bi-monthly progress reports, test reports, computational predictive mechanical property evaluation and a final technical report including powder chemical and physical properties, AM Ti-6Al-4V chemical analysis, CT and dimensional inspections, tensile, hardness, fatigue testing, microstructure, fractography analysis, computational model inputs to predict properties, verification and example of the model predictions. 

PHASE III: Phase Three will address the transition path of this technology resulting from Phase II effort to various US Army components with industrial partners and Original Equipment Manufacturers (OEMs). This technology has been demonstrated in a laboratory research scale and on prototype parts. This program effort would use existing technology to develop an optimized heat treatment process for AM Ti-6Al-4V alloys, quantitative process-structure-property relationships utilizing simple shaped tensile and fatigue specimens. It is noted that the listed references [9-20] are to be used for general guidance on materials, manufacturing, and heat treatment, testing and reporting to accomplish the objective of this project. The implementation targets are defense applications. The expected benefit of the resulting project data could become a heat treatment guide for AM Ti-6Al-4V alloy components used in US Army applications requiring tensile strength, fatigue strength and combination of both tensile and fatigue strength for performance requirements. The relevant technical data generated and experience gained in this project are expected to positively impact application of additively manufactured titanium components in a broad range of defense applications where light weight and reduced lead time are needed for very complex parts that use titanium components. All these project benefits will results in improved U. S. readiness and capability. 

REFERENCES: 

1: Seifi, M.

2:  Salem, A.

3:  Beuth, J.

4:  Harrysson, O.

5:  Lewandowski, J.J. Overview of Materials Qualification Needs for Metal Additive Manufacturing. JOM, 2016, 68, 747–764.

6:  Song, B.

7:  Dong, S.

8:  Zhang, B.

9:  Liao, H.

10:  Coddet, C. Effects of processing parameters on microstructure and mechanical property of selective laser melted Ti6Al4V. Mater. Des. 2012, 35, 120–125.

11:  Kranz, J. Herzog, D. and Emmelmann, C., Design Guidelines for Laser Additive Manufacturing of Lightweight Structures in Ti6Al4V, Journal of Laser Applications, 2015,Vol. 27, S1400-1-S14001-16.

12:  Vilaro, T.

13:  Colin, C.

14:  Bartout, J.D. As-Fabricated and Heat-Treated Microstructures of the Ti-6Al-4V Alloy Processed by Selective Laser Melting. Metall. Mater. Trans. A, 2011, 42, 3190–3199.

15:  Kong, C.J.

16:  Tuck, C.J.

17:  Ashcroft, A.I.

18:  Wildman, R.D.

19:  Hague, R. High density Ti-6Al-4V via SLM processing: Microstructure and mechanical properties. In Proceeding of the 22nd Annual International Solid Freeform Fabrication Symposium, Austin, TX, USA, 8–10 August 2011, 475–483.

20:  Quintana, O. A. Alvarez, J. McMillan, W. R. and Tomonto, C., Effects of Reusing Ti-6Al-4V Powder in a Selective Laser Melting Additive System Operated in an Industrial Setting, JOM, 2018,Vol.70, No. 9 1863-1869.

21:  Leuders, S.

22:  Thöne, M.

23:  Riemer, A.

24:  Niendorf, T.

25:  Tröster, T.

26:  Richard, H.A.

27:  Maier, H.J. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: Fatigue resistance and crack growth performance. Int. J. Fatigue, 2013, 48, 300–307.

28:  Cao, F., Zhang, T., Ryder, M. A., and Lados, D. A., A Review of the Fatigue Properties of Additively Manufactured Ti-6AL-4V, JOM, 2018, Vol. 70, No. 3, 349-357.

29:  ASTM F2924-14, "Standard Specification for Additive Manufacturing Titanium-6 Aluminum-4 Vanadium with Powder Bed Fusion," ASTM International, West Conshohocken, PA, 19428.

30:  AMS2801B, "Heat Treatment of Titanium and Titanium Alloy," Issued 2003-03, Society of Automotive Engineers, Warrendale, PA 15096.

31:  ASTM F3049-14, "StandardGuide for Characterization Properties of Metal Powders for Additive Manufacturing Processes," ASTM International, West Conshohocken, PA, 19428.

32:  AMS7003, "Laser Powder Bed Fusion Process," Issued 2018-06, Society of Automotive Engineers, Warrendale, PA 15096.

33:  AMS7002, "Process Requirements for Production of Metal Powder Feedstock for Use in Additive Manufacturing of Aerospace Parts," Issued 2018-06, Society of Automotive Engineers, Warrendale, PA 15096.

34:  ASTM F3301-18, "Standard for Additive Manufacturing – Post Processing Methods –Standard Specification for Thermal Post-Processing Metal Parts Made Via Powder Bed Fusion," ASTM International, West Conshohocken, PA, 19428.

35:  ASTM F3122, "Standard Guide for Evaluation Mechanical Properties of Metal Materials Made via Additive Manufacturing Processes," ASTM International, West Conshohocken, PA, 19428.

36:  ASTM F2971, "Standard Practice for Reporting Data for Test Specimens Prepared by Additive Manufacturing," ASTM International, West Conshohocken, PA, 19428.

37:  ASTM B311, "Standard Test Method for Density of Powder Metallurgy (PM) Materials Containing Less Than Two Percent Porosity," ASTM International, West Conshohocken, PA, 19428.

38:  ASTM E1441, "Standard Guide for Computed Tomography (CT) Imaging," ASTM International, West Conshohocken, PA, 19428.

39:  ASTM F3303, "Additive Manufacturing – Process Characteristics and Performance: Practice for Metal Powder Bed Fusion Process to Meet Critical Applications," ASTM International, West Conshohocken, PA, 19428.

40:  AS1814, "Terminology for Titanium Microstructures." Society of Automotive Engineers, Warrendale, PA 15096.

KEYWORDS: Additive Manufacturing, Heat Treatment, Ti-6Al-4V Alloy, Tensile Properties, Microstructure, Laser, Powder Bed Fusion, Process Structure Property Relationships 

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