Prediction of the Microstructure Morphology and Tensile Properties in Titanium Alloy

Marion Bessagnet, Christophe Daffos, Stephane Hollard, Philippe Heritier

Auhert & Duval , 09100 Pamiers , F ranee

Aubert&.Duval produces parts for Aircraft Companies. Today the dimensioning methods of the parts change and the mechanical requirements in­

crease more and more in order to be able to control the mechanical performances and the microstructure everywhere into the parts. Also, the de­velopment of predictive model is necessary.

The thermomechanical path into the part is calculated by standard numerical software as Forge 30 but the relationship between the mechanical performances, the microstructure and the temperature and deformation used during the process are not clearly established.

From its background on titanium parts and from a specific experimentation plan,Aubert&.Duval has developed a phenomenological model to pre­

dict the mechanical performances into the parts in Ti64 or in Til023.

Long campaigns of metallographic and SEM investigations and image analysis were performed to measure and count the distribution and the sizes of the a-laths and nodules to correlate them to the associated tensile properties. So the thermomechanical path, the microstructure and the tensile

performances are together known.

This paper presents one possible application of this model in order to predict the mechanical characteristics into the part versus the technique of

forging for parts in Ti64 and Til023.

Keywords: Ti64 ( TA6V), Ti1023, microstructure ,tensile properties, fracture toughness, crack propagation rates

1. Introduction

The use of Titanium for aircraft applications is constantly increasing, enforced by the necessity for weight reductions for improving fuel economy and higher payload capacity. For structural parts of the air­frame, both the high-strength and the low fatigue crack growth rate (FCGR) are required. Fracture toughness and FCGR properties are very important parameters in the design of structural components, because the peri-

. odic visual inspection to detect some eventual defect on

the surface of the part could become less frequent. The extreme value of the crack propagation curve versus

the stress concentration factor is the fracture tough­ness. Also these characteristics of the alloys are stud­iedH> because the requirements of the company are more and more exigent5>.

The T A6 V is classically used in the aero structur­

al parts from a long time. The understanding of the me­chanical behavior of the T A6V is well known. It is gen­erally accepted that the mechanical performances come

mainly from the a laths thickness and the ~ grain size. The famous Hall-Petch relationship between the yield strength and the grain size fits well and the thickness of the laths clearly shows a big effect on the strength6>.

The Ti-10V-2Fe-3Cr is another grade for very high performances aircraft part as landing gear parts.

The fracture toughness as well as the ultimate tensile strength ( UTS) are high and difficult to obtain. In fact, a very specific microstructure is needed by using

an adapted thermomechanical path. One of the most challenges of the aircraft compa­

ny is to reduce the cost of the plane. The reduction of the quantity of alloy used to produce a part should allow

such a cost reduction. By making more near net shape die

forging parts, the cost could be strongly reduced. In the same time the thermomechanical path following by this thinner part could modify the mechanical performances. In fact, the change of the cooling rate for example implies some variations of the secondary crrnicrostructuren.

In other plan, with the new aircraft program as the

A380 of Airbus, the dimensions of the parts have strongly increased. The biggest parts have local ruling section higher than 200mm with strong variation of ge­ometry. It could be often difficult to guarantee the me­chanical performances everywhere in these parts.

The aim of this paper is to show that the mechani­cal properties could be predicted taking into account the main parameters of the thermomechanical path whatev­er the type of parts ( the bigger ones as the smaller).

The morphology of the microstructure ~ or a~ could be estimated too. As example, the ultimate tensile strength on Ti-6Al-4 V parts and the tenacity on Ti-

10V-2Fe-3Cr parts will be predicted from three ther­momechanica 1 parameters:

(1) the cooling rate in some area of the part (2) the solution treating temperature (3) the level of deformation. These models are consistent with the classical be­

havior of the titanium grades. Indeed, a bi-modal micro­

structure has advantages in terms of tensile and fatigue properties whereas a fully lamellar structure is charac­terized by an elevated fatigue crack propagation resist­ance and a good fracture toughness8>.

This article proposes a comparison between the measured performances and the calculations by taking into account the main microstructural indicators as

grain size, a-laths thickness and a-nodules morphology. The model will allow to Aubert &. Duval to propose

the best die forging process versus the requirements of

9. Aerospace Applications • 1953 •

our customers. The last paragraph of thi s article shows the application of these models onto a par t produced both by a conventional way and a near net shape tech­niques that generate specific thermomechanical path.

2. Experimental Procedure

2. 1 Material and Process Two alloys have been considered : the Ti-6 Al-4 V

and the Ti-10V-2Fe-3 AI which their chemical contents are given respectively in T able 1 and in T able 2. The heat treatment employed was a solution treating above the beta transus plus an ageing at 730°C for the Ti-64 part and a solution trea ting at 760°C plus an ageing at

505 °C fo r the second.

Table l. Chemical content % weight of T A6V

V% Fe% AI% C% 0% % H%

4 o. 25 6 o. 1 0. 15 0. 05 0. 013

Table 2. Chemical content % weight of Ti l 023

V% Fe% AI % C% 0% % H%

10 2 3 0. 05 o. 13 0. 05 0.015

2. 2 Produced Part A modeling of the produced part is shown in Fig­

ure 1. It is a typica l structural part similar to the door

frame structures. The using of near net shape die fo rg­ing CNNS) for this kind of parts with deep pocket and ribs is relevant. By NNS techniques the dimensions of the rib are reduced and the thermomechanical path is particular. The dimensions of the two parts are given in Figure 2.

Figure 1. The produced part

195mm

Figure 2. The near net shaped part (a) and the conventional

produced part ( b) and their dimensions

(a)

(b)

The knowledge of the fracture toughness and the

tensile characteristic is really useful for design.

In order to be able to predict the fracture toughness and the tensile properties , several investigations were performed by Aubert&Duval. The following model is based on tensile testing and fracture toughness testing.

2. 3 Test Process

In order to see the mechanical properties , some tests were performed as tensile , crack propagation rate and fracture toughness tests on a lot of parts produced by Aubert& Duval. T ensile te ts were carried out on standard T7 A specimens and both fracture toughness and crack propagation rate tests on standard compact tension CCT ) specimens. T he specimens were mechani­ca ll y deformed at room temperature CRT) according to the standard NF E ISO 6892-1 , ASTM E64 7 and ASTM E399 for tensile , crack propagation rate and

fracture toughness tests respectively. The specimens for metallographic analysis were polished mechanica lly and by an electrolytic process and then attacked wi th a specific reagent. Microscopic examinations were carried out using a Leica optica l microscope and a scanning electron microscope ( SEM ) . Finally , analysis micro­

graphs of TilO. 2. 3 were carried out thanks to an im­age analysis software.

3. Model of Prediction

The ultimate tensile strength of the Ti-64 is relat­ed to the morphology of the grains ( named mesostruc­ture) and to dimension of the alpha laths ( named mi­crostructure) . The fracture toughness of the Til023 is related to the morphology of the a-nodules in the ~ ma­trix. In order to predict mechanical properties , models were established following few test campaigns conduc­ted within Aubert& Duva l.

3. 1 Tensile Properties for the Ti-64

The model used in order to predict the UTS is giv­

en in ( 1) as an adjustment of the famous H all Petch re­lationship taking into account the thickness of the a­

laths. It considers different parameters like the grain di­ameter ( d) and the size of laths 0 .) . A , B and n are ma­

terial constants and depend only on the considered alloy.

UTS = (A + Jr) * r " Cl ) Aubert& Duval has tested parts till 200mm of di­

ameter. The optical microstructure showing the influence

of the cooling rate on the thickness of the a- laths and the relationship between them is shown in Figure 3. Slower are the cooling rates, thicker are the a- laths9

' .

These microstructures were extracted from a classical

industrial part in Ti-64. Exactly the same trend could

be obtained whatever the parts.

In Figure 4 , it is illustrated the comparison be­

tween experimental and numerical data for the UTS.

• 1954 • Proceedings of the 12'h World Conference on Titanium

0

0

" I§ --ell"' c: -.....

·- U g ~ u 0

Figure 3. Effect of cooling rate on thickness of laths and micrographs associated

Few dispersion between the ca lculation and the measuring are noticed, that revea ls the coherence of our model. Also the relationship between the laths size and the UTS is shown as previously founded in literature : thinner are the laths, higher is the UTS6>. Moreover , a

fast cooling reduces the thickness of the laths.

IOOO 5 ~ 0

"' 4 ~ c.. 950 :2 E :::!_

VJ 3 ,_,

900 "' f- N

::> 2 ·~ o; ..<::: "C 850 Oi 0 ti Model/Mea ure I ...J :2

o Laths size 0

800 850 900 950 IOOO Experimental UTS ( MPa )

Figure 4. Compa ri son between experimental and numeri cal data

3. 2 Fracture Toughness The fracture toughness is dependent on the yield

strength as it is illustrated in Figure 5. Higher is the yield strength, lower is the fracture toughness.

Furthermore, the crack front geometry is an im­

portant parameter affecting the fracture toughness8·1

D .

When a crack is propagating, it can meet some nodules

that could have two main effects. Indeed, according to the dimensions of these nodules, it could be circled or

sheared by the crack. In the model, the a-nodules are described through

their size ( d), circularity ( p) and orientation ( 8) that

could change the crack propagation path ( Figure 6) .

Longer is the crack path, higher is the fracture

toughness.

120 - 100 ~ 80 c.. 60 ~ 40 u 20 ~

0 950

~

1000

b. ~

1050 UTS ( MPa )

A

11 00

Figure S. Fracture toughness versus UTS

~---- -- --~--- -~ Figure 6. Description of the model parameters

11 50

For a given UTS, the crack path length D was de­termined by the equation ( 2) and it takes in account

several parameters as : • N, number of nodules • o, inter-nodu les distance • F (8 ,d, p) ,a term depending on the size, ci rcu­

lari ty and orientation of the nodules D = N X o X FCO,d,p) (2)

In Figure 7 (a) the relationship between the frac­

ture toughness and the crack path length is shown. It

can be noticed that longer is the crack path length, higher is the fracture toughness.

Optical micrographs corresponding to different fracture toughness are given in Figure 7(b) , (c) , (d) .

The fracture toughness is higher when the a-nod­

u les are elongated that reveals low deformation.

It has been demonstrated by the model that if the crack path increases, the fracture toughness increases too.

9. Aerospace Applications • 1955 •

-E .;

Q..

~

2-~

70

65

60

55

50

45

40

35

30 0.5

"'-~

r

I v: LJ

~y \

\ Y• ...,....., ~ r~

1.5 2 2.5 3 3.5 Crack path lenght C mm )

( a) Comparison between experimental and numerical data

Cb) Optical micrograph of the sample A

Cc) Optical micrograph of the sample B

Cd ) Optical micrograph of the sample C

Figure 7. Comparison between experimental and numerical data (a)

and optical micrographs (b) (c) (d)

4. Applications of the Models

4. 1 Nmnerical Simulation of the Thennomechanical Path With the software FORGE 3D 2007, some numeri­

cal simulations have been done in order to calculate the strain and the temperature maps within the parts.

The area which was the most deformed has an equivalent strain of 2, 5 whereas the area the least de­formed has an equivalent strain of 0, 27.

On Figure 8, it is shown the map of temperature during the cooling 540s after the heat treatment at 1030"C for two parts, one produced by NNS process than in the part obtained by conventional process. Due to different thickness, the parts saw different cooling rates. For a same point, the cooling rate is faster in the part produced by NNS process than in the part obtained by conventional process. For example, at a particular node, in 540s after the heat treatment, the NNS part is at 560°C and the conventional produced part at 760°C.

TEMPERATURE[ node] Unit: Celsius Frin

800 760 720 680 640 600 560 520 480 440 400

Figure 8. Map of temperature 540s after heat treatment at 1030'C

4. 2 Mechanical Properties Results 4. 2. 1 Tensile Test

The calculated UTS is higher for the NNS pro­cessing as illustrated in Figure 9. This can be explained by the cooling rate that is higher after the die forging process. In particular in the node of the structure that is the heaviest area of the part, the ultimate tensile strength is reduced (Figure lO(a) (b) ).

UTSCMPa)

1040

IOOO

960

920

880

Figure 9. UTS map calculated in the two parts produced with

the conventional way and the near net shape process

4. 2. 2 Fracture T~ A part produced with a NNS process was more

deformed in order to obtain a lower thickness than a

• 1956 • Proceedings of the 12'h World Conference on Titanium

UTS ( MPa ) 1040

1000

960

920

880

( a ) UTS map calculated in the node of the structure for the near net shaped part

UTS <MPa )

1040

1000

960

920

880

( b ) UTS map calculated in the node of the structure for the conventional produced part

Figure 10. UTS map calculated in the node of the structure for the

near net shaped part (a) and for the conventional produced part (b)

part obtained with a conventional process. If we apply the model of fracture toughness prediction, we can pre­dict the higher fracture toughness (Figure 11 and Fig­ure 12(a) ( b)) .

K1cC MPa· m )

70

60

50

40

30

Figure 11. Klc map calculated in the two parts produced

with the conventional way and the near net shape process

5. Conclusions

The model presented in the article will permit to estimate the local mechanical characteristics. While our customers dimensions its parts, it is asked to Aubert&.Duval to locate specimen ( both fracture toughness and tensile test) in order to validate the entire

30

(a) Fracture toughness map calculated in the node of the structure for the near net shaped part

KIC( MPa·m ) 70

60

50

40

30

(b) Fracture toughness map calculated in the node of the structure for the conventional produced part

Figure 12. Fracture toughness map calculated in the node of the

st ructure for the near net shaped part (a) and fo r the conventional

produced part ( b)

part. The balance between the mechanical properties could be better adjusted.

The implementation of the new forging techniques as the NNS process should be used to reduce the weight of the die forging parts and to improve locally the mechanical properties and their balance.

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