Chemical milling effect on the low cycle fatigue properties in cast Ti-6Al-2Sn-4Zr-2Mo alloy: a fractographic study

Chemical milling effect on the low cycle fatigue properties in cast Ti-6Al-2Sn-4Zr-2Mo alloy: A fractographic study

Birhan Sefer^1,2,*, Antonio Mateo², Joan Josep Roa², Raghuveer Gaddam³, Robert Pederson¹, Marta-Lena Antti¹

¹Division of Materials Science, Luleå University of Technology, S-97187 Luleå, Sweden

²Department of Materials Science, Universitat Politècnica de Catalunya, Avda. Diagonal 647, Barcelona, Spain

³Sandvik Materials Technology, Sandviken, S-81181, Sweden
* Corresponding author, e-mail: XXX

Abstract

One of the final steps in manufacturing titanium alloy aero-engine components is chemical milling, i.e. a treatment in solutions containing a mixture of concentrated fluorhidric and nitric HF and HNO₃ acids in relevant molar ratios. Chemical milling removes the brittle alpha-case layer formed during thermo-mechanical manufacturing processes. Although chemical milling shows excellent advantage in handling components with complex net geometries, it may have detrimental effects on the surface, such as pitting corrosion, which affect the mechanical properties of aero-engine components. Particularly, the formation of pits might decrease the fatigue properties of titanium alloys. The current work presents the results of a low cycle fatigue (LCF) study carried out with specimens of cast Ti-6Al-2Sn-4Zr-2Mo alloy. A first series of specimens were tested in the as-received state, whereas two more series were, prior to fatigue testing, subjected to a short and a long chemical milling treatment, respectively. Fatigue lives were substantially shortened for the chemically treated specimens. A complete fractographic analysis was performed on all the LCF tested specimens by using scanning electron microscopy (SEM) and also focused ion beam (FIB), in order to elucidate the influence of the chemical milling treatment on the nucleation and growth of fatigue cracks.

Keywords: Titanium alloy;, Aero-engine components, ; Chemical milling, ; Pitting corrosion, ; Low cycle fatigue;, Scanning electron microscopy, Fractography.

1. Introduction

Titanium and its alloys are widely used for manufacturing aero-engine components, mainly because of their high strength/weight ratio and excellent corrosion resistance [1-3]. Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) is a near-alpha alloy that was designed for long-term applications at temperatures up to 450 ºC. Ti-6242 has good tensile, creep, fatigue and toughness properties and therefore is used for manufacturing compressor blades, discs and impellers of aero-engine turbines. Investment casting is a cost efficient technology which finds broad application for manufacturing titanium alloys aero-engine components compared with other forming methods because of the possibility for obtaining near to the net complex shape of the components. However, investment casting has one drawback commonly known as “alpha-case”. Alpha-case is a continuous hard and brittle layer which develops as result of the dissolution of interstitials such as carbon, nitrogen and especially oxygen into α-Ti from the mould materials during the casting process [4,5]. The formed alpha-case layer has a detrimental effect on important mechanical properties of titanium alloys such as ductility, fracture toughness and fatigue life [5-8] by facilitating crack initiation and leading to faster failure of the aero-engine component under dynamic loading conditions. Therefore, in the industrial practice this brittle layer is removed by chemical milling treatment [1,2,9] or prevented by using high temperature coatings [1,2,9-14].

The chemical milling treatment involves soaking of the components into a mixture of concentrated HF and HNO₃ acids in relevant molar ratios under strict and well controlled conditions [15]. However, this mixture of acids may have strong side effects on the surface, and consequently on the mechanical properties, of the etched components [16]. A common surface effect due to chemical milling is localized corrosion phenomena. In general, titanium and its alloys demonstrate excellent corrosion resistance in different corrosive environments due to spontaneous formation of a strongly adherent, insoluble and passive thin film of TiO₂ oxide onto the metal surface. However, in presence of aggressive ions, such as F^–, the passive TiO₂ film loses its protective nature and localized corrosion in form of pitting and/or intergranular corrosion may occur.

In the present work, the main focus was to explore the effect of pitting and intergranular corrosion on the LCF properties of a cast Ti-6242 alloy. In doing so, a systematic and detailed fractographic analysis of all the tested LCF specimens was performed by using SEM and FIB.

2. Experimental part

2.1 Material

The studied material was a near-alpha Ti-6Al-2Sn-4Zr-2Mo alloy in cast condition. Figure 1 is an optical micrograph showing the microstructure of the material in as-received condition. The material has a Widmanstätten microstructure consisting of alpha colonies, i.e. alpha laths (or lamellas) packages that are oriented randomly within the prior beta grains.

2.2 Mechanical testing

A total of nine cylindrical LCF specimens were cut from the as-received material using electric discharged machining (EDM). Figure 2 shows the geometry and dimensions. All those specimens were grinded and polished to a final surface roughness to about 0.2 μm.

LCF tests were performed under total strain control by uniaxial loading of the specimens at a strain ratio R=0 and a frequency f=0.5 Hz, with triangular waveform, according the ASTM E606-04 [17]. The gauge length of the extensometer was 12.7 mm and all LCF tests were performed at room temperature using closed loop servo controlled hydraulic system.

2.3 Chemical milling treatment

Three different values of total strain range were considered for the LCF testing, Δε_t = 0.5%, 0.8% and 1.2%. Three specimens were tested immediately after the fabrication, whereas the other six were subjected to chemical milling treatment prior the LCF testing. The milling bath contained HF (55%) and HNO₃ (70%) acids in molar ratio 1:11. Three specimens were milled for 5 minutes and the other three for 60 minutes. The 60 minutes milling treatment corresponds to physical removing, i.e. etching, of 30 μm material in industrial application conditions.

2.4 Microscopy

Scanning electron microscopes (SEM), JEOL JSM-6460LV and FE-SEM JEOL JSM-7001F, were used to analyze the fracture and lateral surfaces of the LCF specimens. Accelerating voltage of 15-25 kV and secondary electrons were used.

A dual beam Focused Ion Beam FIB-SEM Microscope, Zeiss Neon 40 Crossbeam, was used to mill the surface of the LCF specimens in order to observe the corrosion pits in a cross-section view. Prior to the ion milling, a thin platinum layer (~500 nm) was deposited onto the surface with 200 pA FIB current. Ga⁺ ion source was used to remove material from the surface in the following order: coarse milling with 10 nA, medium polishing with 2 nA and final polishing with 500 pA FIB current.

3. Results and discussion

3.1 Surface properties and localized corrosion

Figure 3 shows representative digital images of the specimens after fatigue testing and chemical milling treatment. It can be seen that with increasing the duration of the chemical milling the specimens surface becomes significantly affected. The change of the surface aspect is due to the chemical reaction between the Ti-6242 alloy and the HF+HNO₃ acid mixture that causes physical removing of material. Chemical milling is a complex process that is function of a variety of parameters, such as the chemical composition of the milling solution (molar ratio of HF and HNO₃), its temperature, the stirring rate and the duration of the milling process. Maintaining a precise control of all these parameters, a uniform and constant removal rate of material can be achieved. The chemical reaction of titanium and HF acid could be represented by the following equation [18]:
Ti + 6HF = H₂TiF₆ + 2H₂ (g) (1)
The reaction (1) results in dissolution of Ti metal and leads to formation of soluble titanium fluoride complexes and gaseous hydrogen. The hydrogen formed during the reaction may have a detrimental effect on the mechanical properties, since it can induce hydrogen embrittlement and hydride formation. A strong oxidizing agent, such as HNO₃ acid, is used in combination with HF in order to hinder the formation of hydrogen gas.

The overall chemical reaction of Ti metal with the mixture of HF + HNO₃ acids could be represented by the following chemical equation [18]:

Ti + 6HF + 4HNO₃ = 3H₂TiF₆ + 4H₂O + 4NO₂ (2)
In practice, it is very difficult to maintain ideal conditions for the chemical milling treatment and therefore undesired consequences often occur. One of those consequences is an uneven removal rate due to exhausting of the chemical milling solution. The exhausting of the solution is due to the formation of titanium fluoride complexes that lower the activity of fluoride ions towards titanium and thereby the removal rate is decreased. In order to maintain the activity of the chemical milling solution, refreshment is required by adding new portions of HF acid to the mixture.

The surfaces of the LCF specimens shown in Figure 3 were inspected by SEM. Figure 4 shows SEM micrographs of the lateral surfaces of tested specimens without chemical milling (Figure 3a) and after acid treatment for 5 (Figure 3b) and 60 minutes (Figure 3c), respectively. Characteristic marks were observed on the non-etched specimen resulting from the machining and polishing process (Figure 4a). Similar features were observed on the surface of the 5 minutes etched specimen too, but accompanied by a moderate etch and corrosion products (Figure 4b). However, the SEM inspection on the surface of the LCF specimen etched for 60 minutes (Figure 4c) revealed a severely etched surface, combined with pitting corrosion and intense attack along the prior beta grain boundaries.

FIB was used to perform trenches perpendicular to the surface, in order to examine the morphology of the subsurface of the etched LCF specimens in cross-section. Figure 5 shows a trench at two different magnifications. It corresponds to a specimen subjected to 60 minutes of chemical milling. From Figure 5b, it can be seen that this prolonged milling treatment resulted in formation of pits, i.e. holes with a depth in the range of 1-3 μm and width around 3-5 μm. Cracks propagating from the pits towards the metal bulk were not detected. However, deep cracks along the prior beta grain boundary were observed (see right side part of Figure 5a), which implies that those grain boundaries are more prone to cracking than the alpha lath colonies.

3.2 Low cycle fatigue life

Figure 6 plots the Coffin-Manson diagram (ε vs. N_f, where ε is the total applied strain range and N_f is the number of cycles to fracture) obtained from the LCF tests of Ti-6242 in the different chemical milling conditions. As it was expected, fatigue life is function of the applied strain amplitude and depends also on the duration of the chemical milling. The ε-N_f diagram shows a remarkable reduction of the fatigue life as a result of the performed chemical milling treatment.

Table 2 summarizes the percentage of reduction of the LCF life for the specimens that were subjected to chemical milling with respect to the non-etched ones. From this table, the severe detrimental effect of milling treatment on the fatigue response can be quantified.

Fatigue life reduction is particularly remarkable for the two lowest imposed strain ranges: the short chemical treatment led to a fatigue life reduction close to 60%, whereas for the long acid milling the reduction reached 80%. For the highest strain range, i.e. Δε _t = 1.2%, the influence of the chemical milling is strong but not so dramatic. Specimens subjected to 60 minutes of soaking in the acid mixture had half the life of non-etched ones, while for those exposed during 5 minutes the reduction was of 29%.

3.3 Fractography

Figure 7 shows SEM micrographs of the fracture surfaces of all nine LCF specimens tested. It can be observed that all the fracture surfaces have typical fatigue features, including crack initiation site or sites (marked with black circles), main cracks propagation (whose direction is indicated with white arrows) and final fracture area (marked with dashed lines).

In addition, the comparison of the different fractographic images of Figure 7 shows a difference in the number of crack initiation sites, which is in close correlation with the duration of the chemical milling treatment. For non-etched specimens, only one main crack initiation site was noticed, whereas for specimens that were subjected to chemical milling multiple crack initiation sites were detected (see Figure 7d-i).

Since the SEM overview on the fracture surfaces of all LCF specimens shown in Figure 7 indicated that the main difference between them is the number of crack initiation sites, a thorough SEM analysis was performed on those nucleation sites of each specimen. Figure 8 shows SEM micrographs of the crack nucleation areas of the three non-etched specimens. It can be observed that in all cases the crack was initiated either on the surface (see Figure 8b and c) or in the subsurface (see Figure 8a), which is typical of fatigue cracks. The specimen tested at the lowest strain exhibited subsurface crack nucleation and the SEM inspection revealed that it was due to the presence of a inclusion (see Figure 8a). In the other non-etched specimens, surface crack initiation was probably related to small surface defects formed during the fabrication process of the LCF specimens.

Figure 9 shows SEM micrographs of the crack initiation sites for the specimens that prior to the LCF testing were chemically milled for 5 minutes. Figure 9a shows a magnified view of the area pointed with A in Figure 7d. Straight lines with sharp angles are observed at the crack surface edge, mainly following the prior beta grain boundaries. Those boundaries were affected by the chemical etch and then they act as stress raisers where fatigue cracking is initiated. The LCF specimens tested at higher strain amplitudes showed similar crack initiation scenario (see Figure 9b-crack initiation site pointed with B in Figure 7e and Figure 9c-crack initiated site pointed with A in Figure 7f). In addition, it was observed that, for all three tested strain amplitudes, the crack propagates readily along alpha lath colonies with the most favourable orientation, leaving flat transgranular areas clearly visible in the fracture surface.

Figure 10 shows SEM micrographs of the six crack initiation sites found in the specimen that was chemically milled for 60 minutes prior to LCF testing at _t = 0.5%. From those micrographs, it can be seen that cracking was initiated on the surface, similarly than in most of the other fatigue specimens both not-etched and treated in the acid mixture for 5 minutes. However, in this case the surface of the specimen was severely affected by the long chemical milling in the HF+HNO₃ mixture. This long acid treatment resulted in the formation of large grooves along the prior beta grain boundaries of the Widmanstätten microstructure of the cast Ti-6242 alloy (see Figure 10).

SEM inspection of the grooves revealed that their depth and width was in the range of 10-15μm. Moreover, the 60 minutes chemical milling treatment induced the formation of pitting corrosion, with pits up to 100 μm in depth, in the triple joint points of the chemically attacked prior beta grain boundaries. Cracking was probably initiated in those pits and propagated along the grooves, i.e. etched prior beta grain boundaries.

For the 60 minutes chemically milled LCF specimens tested at higher strain amplitudes, similar crack nucleation scenario was met, involving crack initiation from the pits formed in the triple joint points of the prior beta grain boundaries and crack propagation along the etched prior beta grain boundaries.

All those fractographic observations are in total agreement with the LCF results, i.e. the dramatic decrement in the fatigue life measured for Ti-6242 specimens subjected to chemical milling. Fatigue lifetime is composed for two main stages: microcrack initiation (the number of cycles required to develop a small crack of some specific size) and crack propagation (corresponding to the portion of the total cyclic life which involves growth of that crack up to the critical size that brings the final fracture) [19]. The percentage of the fatigue life necessary for nucleation is usually much higher than the portion for its propagation, and it is well-known that when the imposed strain is low the microcrack nucleation stage becomes more prolonged. This is because the nucleation process during cyclic straining is usually the result of localized plastic deformation at the surface, either at an extrusion/intrusion within a persistent slip band or at stress concentrations such as inclusions, pores or grain boundaries. In the present situation, the surface defects produced by chemical milling were the stress raisers that assist the nucleation of surface microcracks. The clearest example of that is the specimen subjected to the lowest strain range and the long chemical treatment. In that case, six microcrack initiation points were detected, demonstrating the readily nucleation. This fact, together with the subsequent coalescence of those microcracks, led to the huge fatigue life reduction (80%) as compared to the non-etched specimen subjected to the same cyclic strain, where only one microcrack initiation was observed.

4. Conclusions

From the present work, the main experimental issues concerning the influence of chemical milling treatments on the fatigue life of components made on Ti-6242 are:

• 
  Fatigue lives of cast Ti-6242 alloy in the low cycle fatigue regime were substantially shortened for the chemical milling treatment.
  
• 
  The chemical milling treatment for sixty minutes in the HF+HNO₃ acid mixture had a detrimental effect on the surface state of fatigue specimens. Corrosion pits with depths up to 100 μm and severe etching of prior beta grain boundaries generating grooves with depth and width in the range of 10-15 μm were produced. The shortest milling treatment, i.e. for 5 minutes, did not produce pitting but chemical attack on prior beta grain boundaries was observed.
  
• 
  Fractographic analysis of the LCF specimens tested under different scenarios revealed one main crack initiation site for the non-etched specimens whereas multiple crack initiation sites were discerned for the chemically milled specimens. These crack nucleation sites were clearly connected with pitting corrosion and grooves following prior beta grain boundaries.
  

Taking into account all these experimental observed facts, the main inferred conclusion is that the remarkable reduction of the fatigue life of Ti-6242 specimens subjected to chemical milling, as compared with non-etched specimens, is due to the easier crack nucleation from the pits and grooves produced by the acid treatment. Even the short milling for only 5 minutes induced a significant surface damage and the corresponding reduction in fatigue resistance.

Acknowledgments

The authors would like to acknowledge Erasmus Mundus Programe through the European Joint Doctoral Programme in Materials Science and Engineering Programme (DocMASE, Grant number 512225-1-2010-1-DE-ERA MUNDUS-EMJD) and the Swedish Foundation for Strategic Research (SSF) for providing the financial assistance. GKN Aerospace Engine Systems, Sweden is acknowledged for its support during the research work. We would like to acknowledge Raghuveer Gaddam from AB Sandviks Materials Technolgy, Sandviken, Sweden and Isaac López Insa from Department of Materials Science at the Universitat Politècnica de Catalunya, Barcelona, Spain for performing the SEM fractography. We would also like to acknowledge Trifon Trifonov from the Centre for Research in Nano-Engineering (CRNE) at at the Universitat Politècnica de Catalunya, Barcelona, Spain for performing the Focus Ion Beam (FIB).

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Table captions

Table 1. Reduction of LCF life in percentage for the LCF specimens subjected to different chemical milling time with respect to non-etched LCF specimens at all three tested strain amplitudes.

Figure captions

Figure 1. Widmanstätten microstructure of as-received Ti-6Al-2Sn-4Zr-2Mo alloy.
Figure 2. Geometry and dimensions of the EDM fabricated LCF specimens.

Figure 3. Representative digital images of the LCF tested specimens after performing chemical milling for: a) 0, b) 5 and c) 60 minutes.
Figure 4. SEM micrographs of the lateral surface of LCF tested specimens after chemical milling for: a) 0, b) 5 and c) 60 minutes.
Figure 5. Focused Ion Beam milling of the surface of prior beta grain close and perpendicular to the fracture surface of LCF specimen subjected to chemical milling for 60 minutes.
Figure 6. ε-N_f diagram for Ti-6242 subjected to different chemical milling treatments.
Figure 7. SEM micrographs of the fracture surfaces of the LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes and after chemical milling for 0, 5 and 60 minutes.
Figure 8. SEM micrographs of the crack initiation sites of the non-etched LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes.
Figure 9. SEM micrographs of the crack initiation sites of the 5 minutes etched LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes.
Figure 10. SEM micrographs of the crack initiation sites of the 60 minutes etched LCF specimen tested at 0.5 % strain amplitude.

Table 1

Reduction of LCF life in percentage for the LCF specimens subjected to different chemical milling time with respect to non-etched LCF specimens at all three tested strain amplitudes.

Total strain range, t (%)	Life reduction (%)
	5 minutes	60 minutes
0.5	58	80
0.8	57	72
1.2	29	50

Figure 1

Widmanstätten microstructure of as-received Ti-6Al-2Sn-4Zr-2Mo alloy.

Figure 2

Geometry and dimensions of the EDM fabricated LCF specimens.

Figure 3

Representative digital images of the LCF tested specimens after performing chemical milling for: a) 0, b) 5 and c) 60 minutes.

Figure 4

SEM micrographs of the lateral surface of LCF tested specimens after chemical milling for: a) 0, b) 5 and c) 60 minutes.

Figure 5

Focused Ion Beam milling of the surface of prior beta grain close and perpendicular to the fracture surface of LCF specimen subjected to chemical milling for 60 minutes.

Figure 6

ε-N_f diagram for Ti-6242 subjected to different chemical milling treatments.

Figure 7

SEM micrographs of the fracture surfaces of the LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes and after chemical milling for 0, 5 and 60 minutes.

Figure 8

SEM micrographs of the crack initiation sites of the non-etched LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes.

Figure 9

SEM micrographs of the crack initiation sites of the 5 minutes etched LCF specimens tested at 0.5 %, 0.8 % and 1.2 % strain amplitudes.

Figure 10

SEM micrographs of the crack initiation sites of the 60 minutes etched LCF specimen tested at 0.5 % strain amplitude.