The Dissection of the Requirement for Noise Level on Ultrasonic Testing for Two

The Dissection of the Requirement for Noise Level on Ultrasonic Testing for Two-phase Titanium alloys

by Xia Jizhen and Qian Xinyuan

note: The article is published on "12th International Committee on Non-Destructive Testing(WCNDT)"(Amsterdam April 23/29,1989)

Summary

The aim of this article at explanation that the requirement for noise level is essential in ultrasonic testing(UT) for Titanium alloys by a lot of testing examples.It is not only necessary for assurance of signal-noise ratio(SNR) in testing,and also an important parameter that is characteristic of homogeneities and features of microstructure of Titanium alloys.The article also shows the author's point of view about the definition of noise level and its evaluating method.

Generality

There is requirement for the noise level which is an important part in most ultrasonic testing specification of Titanium alloy.Usually,that took notice of necessity for assurance the SNR in testing to avoid masking some indications from defect which is must rejecting.There are much examples in fact proven that some indications from defect were masked when the noise level was higher,for example,the inclusions with high density,micro-cracks,holes,and so on.But numerous literatures and quantity testing results indicated that is one hand alone,on the other hand,the noise level is an important parameter of characterization of the homogeneity and feature of material's microstructure [in addition,the ultrasonic attenuation is that also].If we don't clearly know the correlation between the noise level and microstructure of materials,we often may ignore some microstructure of disadvantageous for service performance,or needlessly rejecting some materials.Therefore,we ought to research and dissect the noise indications in UT for Titanium alloy.

Definition of the ultrasonic noise level and its evaluating method

In UT by A-scan,as a rule,called the grass-echo as the noise which is approximately uniform distribution[or bush] along the time-base line on the screen of the cathode-ray tube[CRT](fig.1).The noise mainly concerned in three hands:[1] the elactro-noise from ultrasonic instrument own;[2] the boundary-surface noise,which is ultrasonic diffuse scattering on the entry-surface caused by surface roughness;[3] the noise[background noise] connected with microstructure of materials.

fig.1 The noise indication of Titanium alloy forging

when it is certain combination between ultrasonic instrument and probe,and choose specific reference block with the same condition of surface to material tested,then two noise level will relatively fixed at the former,so that noise level evaluated will relevant to the material own.

The noise,that is interesting to us,is a result of the scattering reverberation caused by effect of particles,dispersioners,grain-boundaries and grain size,etc. in the microstructure on ultrasonic propagation in the meterial.It may also related to the anisotropy caused by orientation of lattice structures.For example,the α-Ti is close-over hexagonal lattice,β-Ti is body-centered cublic lattice,and there are different lattice in various metallic-compound.That is to say,it is related to features of microstructure,i.e. to the process of materials,for example,heating,forging and heat treatment,etc.

Up to the present,we have not yet unifying standard method for evaluating noise level,in practical measurement,we usually can choose the highest noise as evaluating aim on the screen's time-base line,and compare it with the echo-amplitude from flat-bottom-hole[FBH] with same depth and certain diameter in the specific reference block,then we can obtained noise level expressed by means of equivalent diameter of FBH.Some ultrasonic testing standards define that the noice level is expressed by means of percentage of full vertial scale on screen.This method is influenced by difference from vertial linearity and dynamic range of various instrument,and don't consider the sound distance,so its accuracy and reliability is suspectable.

when the noise level is evaluated,it must be careful about that evaluating noise level has difference for the identical material if the ultrasonic frequency[wavelength] and diameter of piezoelectric element[effective beam diameter] have difference,as the ultrasonic waveform and microstructure shown in fig.10~12.The noise indication is related to superimposing effect of scattering reverberation.In practical evaluation,the probe usually is used with 4~5MHz and the piezocrystal diameter from 12 to 14mm.

The scatter not only depended on material,frequency and probe's size,and on input power and the kind of beam wave,so we must be to choose suitable testing condition,wave-model,frequency range,transmitting power and receiving sensitivity and so on,during evaluting.

Testing examples and analysis of correlation between noise level and microstructure

The microstructures of Titanium alloys have much effect on the service performance,and the noise level is obviously related to microstructural features.In UT for Titanium alloys,noise level is higher due to following circumstances probably:

In the first place,the smelting process of Titanium alloy has a considerable difference from that of ordinary metals.It employ two or three times consumable electrode vaccum smelting technology,and easily bring on uneven structures.In addition,the Titanium alloy's thermal conductivity is not good,the adhesiveness is big,and the metal-flowability is not good also,so the deformation in within and outside part of the forging billet is easily led to the inhomogeneity,i.e. the structure of forging is in homogeneous,therefore,there are some local region with higher noise level in the forging,as shown in fig.8 and fig.11.

In the second place,the microstructural features of Titanium alloy is closely related to hot-working process[heat temperature,deforming amount,cooling velocity and heat treatment,etc.].Commonly,it has four types of structure:laminated,network,equiaxed and binary-state[equiaxed α particle plus β transformed structure].

In these structures,that features often have different extent and they are often interlace together each other.Various features have different effect on ultrasonic propagation and lead to different scattering reverberation,so that showing different noise level.

The feature of α particle [spherical,lamellar,colonial α and the thickness of α lamellar,the orirntation of α colony,etc.] have the most main action for ultrasonic scattering.And the size of initial β grain,the continuity of α particle on the grain-boundary have also important action.In general case,when the ultrasonic transmitting direction is nearly perpendicular to the α particle elongating direction,the noise level is higher,but that noise level is lower when both directions are almost parallel.It can be understanded that the former have more scattering faces.But the ultrasonic attenuating character is contrary to noise level,i.e. when ultrasonic transmit along the lines of grain-boundary of elongating grain,the sound energy is more absorbed in material.

As shown in fig.3,6 and 12,α particle is elongated and tend to be colony,so the noise level is higher and its strength property at room-temperature have a little reduction as compared with normal equiaxed α particle[fig.2].In fig.4,there are strip α and agglomerate α on grain-boundary,the size of α colony is bigger[approximated to juxtaposition] which cause higher noise level and its strength property at room-temperature and plasticity[percentage reduction in area] have all a little reducing also.The sample shown in fig.5,that is centre of punch when the ring billet was forged.It has coarse-grain and shear zone[banded structure] at the centre caused by high-speed stamping to bring on deforming heat effect too big.Its impact toughness at room-temperature have a little deterioration,and the strength value is higher in the instant elevated temperature tension test at 500℃,but its plasticity value is reduced.

The Widmanstatten structure,shown in fig.7,9 and 10,is caused by over temperature when forging.Its macrostructure indicated the distinct grains[coarse grain],and the microstructure is aciluar α-β structure which have some incompletely destroyied β grain-boundaries and flat juxtaposing α stripe,as well as the bigger α colony,and continuous α particle on grain-boundary.These structures often lead to the plasticity severely reduced and fatigue property deteriorated.

As stated above,we can say that the reason,which cause higher ultrasonic noise-level of Titanium alloys is various.At the same time,it is must be indicated that the ultrasonic scattering reverberation need turn back to the probe for indication on screen,so the noise level's height is closely related to transmitting direction of sound.Although some structures is disadvantageous for service performance from metallurgical analysis,but it not necessarily all indicated in form of ultrasonic noise level.In this case,it ought to consider otherwise way,for example the evaluation of ultrasonic attenuation[loss of backface echo].As the sample shown in fig.10,it not only has higher noise level,but its spectrum analysis for the first backface echo showing that the higher harmonic have bigger attenuation in it.Ultrasonic attenuation is a parameter also which is the most sensitive to change of microstructure,but we don't prepare to introduce herein.

On the other hand,although some microstructures indicated higher noise level,but it don't yet exceed the acceptance criterion of metallography,because the noise level's class in ultrasonic inspecting criterion of Titanium alloys is incompletely corresponding with the metallography evaluating class at the present.With the applying range of Titanium alloy is increasingly enlarging,for the various application,the requirement will be different for microstructure and service performance of Titanium alloys.For example,the equiaxed and network structure have unimpeachable superiority more than acicular structure on the thermal stability and fatigue strength,but the equiaxed structure is not good than latter two on the creep limit and long-time strength.In addition,the notch stress-breaking performance,fatigue cracking expansion rate and low-cycle fatigue performance,etc. is all important property for service performance,so that the requirement about noise level ought to be different for various application or request of various service performances.Of course,with processing level of smelting and forging at Titanium industry is incessantly improving,the structural inhomogeneity of Titanium alloys will can get more effective control.

Conclusion

The main disadvantage of Titanium alloys is big notch-sensitivity,its microstructural inhomogeneity have bigger change and considerably effect on mechanical property of materials.Consequently,for the Titanium alloy applying to importanter part,we ought to employ strict ultrasonic inspection standard.For the noise level's requirement in the standard can not only consider that is necessary to control SNR to avoid masking metallurgical defect which is must be rejected,and ought to consider that is response of metallurgical quality of Titanium alloys.It is a important parameter about microstructural features and homogeneities,so that we ought to pay great attention to the significance and rationality of requirement for noise level.

With the application of Titanium alloys is incessantly extending and going deep into,we have to pay great attention to research the corresponding quantitative relationship between the metallurgical structure's evaluation and ultrasonic response's indication.

Authors and acknowledge

This article was wrote by Mr.Xia Jizhen,NDT engineer[UT Level III],teaching research group of NDT in NANCHANG Institute of aeronautical Technology.The collaborater is Mr.Qian Xinyuan,NDT engineer[UT Level III],director of NDT department in Aeronautical Materials Institute.

The authors acknowledge mis.Lin zemin for her help on metallurgical analysis.

Reference

1.R.K.Granville and J.L.Taylor:High noise levels during the ultrasonic testing of Titanium alloys.《The British Journal of NDT》Vol.27，No.3，May 1985，p156

2.F.N.Billman and F.F.Nudolph:Effects of Ti-6Al-4V Alloy Metallurgical Structure on Ultrasonic response characteristics.《Titanium Science and Technology》Vol.1(1973)，p693~705

3.Xia Jizhen:Ultrasonic Inspection of Titanium Alloy Forgings 《Non-Destructive Testing》[NDT Institute of Chinese]Vol.7,1985,No.2,p20

Fig.2 Ti-6Al-4V Forging rod,dia.76mm,traverse microstructure(500x) at centre of rod section.Its noise level is Φ0.8mm-6dB FBH (α particle is elongated).
Tensile test at room-temperature (sample dia.5mm)

	σ_b MPa	σ_0.2 MPa	δ₅ %	ψ %
noise level≤Φ0.8mm-12dB	921	862	17	49.5
noise level≈Φ0.8mm-6dB	882	813	17	52

fig.3 Ti-6Al-4V Forging rod,dia.125mm,traverse microstructure(500x) at centre of rod section.Its noise level is Φ1.2mm-6dB FBH (α particle is elongated and tend to be colony).

Fig.4 Ti-6Al-2.5Mo-2Cr-0.5Fe-0.3Si Rolling rod,dia.70mm,
traverse microstructure(500x) at the place of noise level = Φ0.8mm FBH (α particle have striped and agglomerated,and tend to be colony,exist α strip on the grain-boundary).
Tensile test at room-temperature (sample dia.5mm)

	σ_b MPa	σ_s MPa	δ₅ %	ψ %
noise level≈Φ0.8mm	1078	1024	17.5	45
	1068	1024	17.5	43
	1078	1039	17.0	43
noise level≈Φ0.8mm-10dB	1117	1058	15.0	46
	1122	1058	16.5	49
	1117	1068	15.5	47

Fig.5 Ti-6Al-3Mo-Zr-Si,A centre of punch from ring billet,the shear zone[banded structure](100x) and ultrasonic response.

Impact toughness at room-temperature:A_k=5.25,5.0 KgM/cm
Instant elevated temperature tension test at 500℃(thermal retardation 20 min.):

σ_b MPa	δ₅ %	ψ %
789	16.0	67.0
804	12.0	65.5

Fig.6 Ti-6Al-3Mo-Zr-Si Ring billet,traverse microstructure(250x) at the noise level = Φ1.2mm-15~16dB FBH.This ring billet is forged by the forging rod(dia.180mm) that noise level = Φ1.2mm FBH (α particle is elongated and tend to be colony,exist α strip on the grain-boundary)

a)	b)
Fig.7 Ti-6Al-3Mo-Zr-Si Forging rod,dia.160mm,traverse microstructure(250x) at the noise level = Φ1.2mm-1~3dB FBH,it is Widmanstatten structure:a) At the centre of rod,it has complete initial β grain-boundary; b) At the edge of rod,it is acicular α-β structure and incompletely destroyed initial β grain-boundary.

Fig.8 Ti-6Al-3Mo-Zr-Si Die forging disk,traverse microstructure(200x) at the wheel rim,its noise level = Φ0.8mm-8dB FBH.In the middle zone with grey colour,the primary α particle is more,and the secondary α[transformed β] is smaller,the α particle's colour is different from that of the normal α particle in base.

a)		Fig.9 Ti-6Al-3Mo-Sn-Si forging billet: a) equiaxed structure(300x) and corresponding ultrasonic response with lower noise level; b) Widmanstatten structure(300x) and corresponding ultrasonic response with higher noise level.It has distinct initial β grain-boundary and α colony is bigger within the grain.
b)

Fig.10 Ti-6Al-4V Forging,it is Widmanstatten structure(juxtaposing α structure)(200x),its noise level is Φ0.8mm-6~9dB FBH(5MHz,Φ12.5mm straight probe).On the left that is ultrasonic spectrum analysis picture of the first backface echo(10MHz,Φ7mm straight probe).The higher harmonic about 12MHz is attenuated very much.

Fig.11 Ti-6Al-4V Forging rod,dia.65mm,traverse microstructure(100x) at the noise level = Φ1.2mm-6dB FBH(5MHz,Φ5mm straight probe).It is segregation of the pure titanium.On the right,that is ultrasonic response picture(5MHz, Φ14mm straight probe,the noise level is Φ0.8mm-8dB FBH).So the measurement is different with the different beam diameter.

Fig.12 Ti-6Al-4V Die forging disk: a) the traverse microstructure(2x) of bright lines at weel arm,it is along the flow line.Its reflecting equivalent is Φ0.8mm FBH(2.5MHz,7°,10mmx10mmx2,combining double-element probe) and Φ0.8mm-18dB FBH(5MHz, Φ7mm straight probe); b) the microstructure at place of the bright line(100x),this are elongated α strip and tend to concentrate; c) ultrasonic response(5MHz, Φ12.5mm straight probe,its reflecting equivalent is Φ0.8mm-3dB FBH).

Fig.13 Ti-6Al-3Mo-Sn-Si forging billet,network structure(200x) and ultrasonic spectrum analysis picture of the first backface echo(10MHz, Φ7mm straight probe).Its higher harmonic about 7~10MHz is attenuated greatly.