ultrasonic inspection of titanium …ultrasonic inspection of titanium airframe components 719 that...

ULTRASONIC INSPECTION OF TITANIUM AIRFRAME COMPONENTS

K. L. Kremer R. J. Lord

R. J. Roehrs

McDonnell Aircraft Company St. Louis, Missouri

INTRODUCTION

'Tne performance demands praced on iil:i..litary aircraft are becoming more and more stringent with the further development of advanced military weapon systems. Continuing demands for higher performance and lighter weight aircraft structures along with the trend for longer life with improved reliability have re-emphasized the need for maximum efficient use of materials. These demands can be met with the use of titanium alloys of higher strength and/or operation at higher operating stresses. However, the tolerance for imperfections in titanium decreases as the strength or operating stresses increase.

From this situation has evolved a need for improvement in existing nondestructive testing (NDT) methods, such as ultrasonics, to provide better assurance of the soundness of components used in high performance aircraft. With the advent of more complex titanium airframe components, improvements in the ultrasonic techniques are required because of such considerations as the relative sound transmission characteristics of titanium, the surface curvature of die forgings, and the near surface resolution limitations of present day ultrasonic equipment.

Today, the ultrasonic method is of great importance in ensuring the integrity of titanium aircraft components. The present confidence in the ultrasonic method, particularly for the detection of internal defects, is related to improvements in the basic techniques and effective implementation of those improvements.

717

718 K. L. KREMER, R. J. LORD, AND R. J. ROEHRS

SOURCES OF DEFECTS

The sources of defects in components manufactured from titanium alloys include many of those which are also common to other metals. These defects may originate from either the raw material, the ingot melting practice, or the subsequent processing of the ingot or mill products.

Raw Material and Melting Defects

Several defect types can occur during the production of raw material and ingots. Alpha stabilized areas can occur from burned, contaminated sponge which is high in nitrogen, or from gas leaks during the melting process. Alloy segregation may be present as a result of power interruption in melting, or may be caused by faulty hot topping practices. High density inclusions sometimes appear from portions of tungsten carbide tool bits in scrap material that is added to the titanium consumable electrode. If the titanium comsumable electrode is fabricated by tungsten inert gas (TIG) welding, portions of the tungsten welding electrode may be deposited in the weld and subsequently form high density inclusions in the ingot. Voids and porosity can occur as a result of unhealed pipe. Faulty hot topping or power interruption can result in voids and porosity in addition to the alloy segregation type defects. Porosity can also occur when argon is entrapped during melting. Titanium borides have been formed because of excessive boron levels in the master alloy.

Processing Defects

A second source of defects in titanium is related to thermomechanical processing such as ingot conversion, rolling, component forging, machining, forming and heat treatment. The defects produced during these processes can be categorized as either cracks or porosity and voids. Bursts, laps, and seams may occur during working of the material. Surface connected cracks can be produced during machining or forming. Voids and porosity have occurred because of excessive hot working forces causing strain induced porosity.

DESIGN/STRUCTURESJNDT/INTERFACE

Effective use of ultrasonics demands selection of the appropriate techniques, and the definition of inspection criteria

ULTRASONIC INSPECTION OF TITANIUM AIRFRAME COMPONENTS 719

that is based on engineering requirements. This can be provided by close coordination between design, structures, and NDT personnel. Limitations and advantages of each ultrasonic technique and the product form variables that affect the test results must be considered. Techniques for ensurin£ adequate inspection coverage should then be discussed.

To provide a more meaningful ultrasonic inspection, the following items should be defined on each part drawing where ultrasonic testing is required:

(a) Loading direction (b) Critical area(s) (zones) of each part (c) Acceptance class(es) for each zone

In the past, material and forging suppliers inspected parts to find the discontinuities most likely to be present; little or no attention was given to engineering needs, i.e., flaw size or orientation. With the above infoniation, the vendors have lmowledge relative to the orientation of flaws that can be most detrimental in relation to the applied stress direction in addition to their Jmowledge of the probable orientation and location of processing defects.

Establi~linent of Acceptance Criteria

An appropriate engineering approach is to base the ultrasonic acceptance criteria for a particular component upon the particular loads that the component must withstand and the quantitative defect resolution capability of the ultrasonic techniques chosen. Investigations need to be conducted to establish the minimum detectable flaw size for ultrasonic inspection. In addition, ultrasonic inspection techniques must be applied to various structural test progra.~s in order to develop further quantitative information.

The appropriate ultrasonic acceptance criteria can then be established through discussion with strength and design personnel as to vlhat sizes of discontinuities are structurally significant. In many cases, the component should be zoned for several acceptance classes to reflect the varying stress levels that are present in different areas of the component. The critical orientation of flaws with respect to the loading direction should be established to aid in ultrasonic inspection. The establishment of critical orientation is important since this orientation may differ from the probable orientation of processing defects.


IMPLEMENTATION

After the basic engineering requirements have been established, three areas must be controlled to ensure valid ultrasonic inspection results - qualified procedures, qualified equipment, and qualified personnel. The engineering requirements for control and application of ultrasonic inspection can be translated and transmitted into vendor and in-house production operations by means of process specifications. Each process specification should be coordinated with and approved by engineering, manufacturing, and quality assurance prior to issuance, and concurrence should also be obtained from the customer engineering representative. This ensures that all requirements are compatible with engineering needs and can be met by the affected departments.

Detailed Procedures

The process specifications cover the general requirements for ultrasonic inspection; however, detailed inspection procedures must be developed for each specific component. Consequently, it is a requirement that all ultrasonic inspections be performed to a detailed written procedure. For subcontract work, these procedures should be submitted to the prime contractor for evaluation and approval.

Preliminary approval may be granted based on the acceptability of the written procedure, but many of the procedures should be verified in-house by inspection of specific cor:i.ponents to the same procedure used by the vendor. If the procedure is satisfactory, the vendor can then be given final approval.

Facilities and Equipment

To ensure that the vendor's facilities and equipment are acceptable for the intended use, they should be subjected to an initial survey and evaluation. After becoming an approved source, the vendor's facilities must be re-surveyed periodically to assure continued compliance.

Qualified Personnel

The effectiveness of ultrasonic inspection depends upon the capabilities of the persons who are responsible for carrying out the testing. Consequently, only qualified personnel should perform ultrasonic inspection. To provide a single standard for training and qualification of personnel, ASNT Recommended Practice No. SNT-TC-lA can be used as a common basis for personnel qualification. SNT-TC-lA was prepared by the American Society for


Nondestructive Testing and is gaining industry-wide recognition as a recommended practice for qualification of NDT personnel. The recor.nnended practice consists of three parts; personnel are required to have a certain educational background, a certain amount of training, and an acceptable grade on an examination.

GENERAL ULTRASONIC CONSIDERATIONS

Ultrasonic testing has been performed on airframe components for 4lBl1Y years; however, some of the important variables that affect the results of a test have not been well understood. In fact, most specifications do not cover the control of these variables. Since it is impossible to evaluate all of the parameters of an ultrasonic test and establish controls, several of the more significant parameters have been investigated. These include the effect of variations in sound transmission characteristics of the reference standards vs. those of the material to be tested, the effect of various surface curvatures, the effect of test parameters on the detection of discontinuities located near the surface of the test article, and the effect of other geometric limitations.

Sound Transmission Characteristics

In ultrasonic testing, corrections should be made for response differences between the reference standard and the part to be inspected because of differences in sound transmission characteristics. This correction is accomplished by comparing the response from reflectors which are common to the reference standard and the part followed by an adjustment of the ultrasonic instrument gain to compensate for the response differences.

Some of the variables that may affect sound transmission include fine porosity, surface roughness variations (within allowable limits), constituent variations, grain direction, and macro and/or micro structural conditions. Because the ultrasonic response is related to the internal structure of the test material, it is possible to detect material having abnormal internal structure. Limited effort has been performed in this area and additional quantitative work is required.

Surface Curvature Compensation

Many titanium forgings encountered in the aircraft industry which must be ultrasonically tested possess curved surfaces. Therefore, if the commonly used flat surface reference standards are used, compensation· must be made for surface curvature or


erroneous test results may be obtained. Without correction, a discontinuity in a production part may appear to be smaller than it actually is.

Concave surfaces may or may not focus the ultrasonic beam depending upon such variables as radius of curvature, depth of discontinuity below the sound entry surface, water path, search unit size and frequency. When the beam becomes focused, the reflected energy from discontinuities may produce signal amplitudes greater than expected in reference to the same size reflector tested from a flat surface. Most conditions encountered when testing from either concave or convex surfaces tend to spread the ultrasonic beam and produce lower amplitude responses than obtained from the same size reflector that has been evaluated from a flat surface. Consequently, data need to be developed to provide compensation for surface curvature.

Near Surface Resolution

Defects at material depths greater than 1/4 inch or 1/10 the section thickness (whichever is greater) in titanium forgings and mill products are usually resolvable provided the test is performed in conformance with conventional immersion pulse echo methods used by the oajor wrought metal suppliers. Generally, a nominal frequency of 5 .o MHz and a water path of 3 inches are considered to be normal straight beam test parameters. However, resolution of defects nearer to the surface than 1/4 inch is difficult because of the size of the ultrasonic response from the water/metal interface. ~:here machining offal and trim is to include this first 1/4 inch, no problem exists, but for many parts, this is not the case. For these parts, either an additional ultrasonic inspection of the final machined part, a supplementary inspection from the opposite surface, or an inspection at higher test frequencies is necessary.

FORGING STOCK

\·7hen usin£ titanium forgings, it is advisable to have the billet material that is to be used as forging stock ultrasonically inspected to minimize the possibility of defects being revealed in the final product. Engine manufacturers feel that the ultrasonic inspection of billets must be performed on latheturned, ground (or equivalent) round stock to be meaningful. The ground surface allows for better transmission of sound into the part, the round geometry permits more adequate coverage of the billet, and both the surface and the geometry afford the use of automated scanning as well as maintenance of a back reflection to monitor the adequacy of the energy transmission.

K. L. KREMER, R. J. LORD, AND R. J. ROEHRS 723

The reasoning behind these requirements is basically good. However, although £rOund round starting stock may be acceptable for engine parts, round stock doesn't lend itself to the production of many airframe col'llponents.

The surface finish of billets for airframe components has a profound effect on the ultrasonic inspection. Surface finishes rougher than 125 RHR have resulted in ultrasonic reflections which appear to be from discontinuities located as deep as 1 inch from the billet surf ace. The responses are prevalent ~lhen calibration is made on a flat bottom hole 3/64 inch or less in diameter. These type indications have been shown to be spurious by machining the surface to a 32 RHR finish removing only a few mils of material and re-inspecting. The responses are no longer observable from the 32 RHR finish surface.

DIE FORGJJ.JGS AND PREFORMS

Each type or shape of forging in reality sets its own parameters with respect to the reliability of the inspection. It is extremely difficult to describe an adequate surface preparation for an inspection short of actual machining with a stated surface tolerance because 11flatness 11 of the surface is as important as 11 roughness 11 • Also, the degree of curvature of the part and the nuraber and relative location of ribs, fillets, conical surfaces, etc., have an important bearing as to the validity of the ultrasonic inspection. However, most suppliers will inspect forgings and guarantee the print requirements as well as the specification requirements when parts are machined to proper geometric configurations and reinspected.

Recognizing the necessity for a more effective ultrasonic inspection, several parts were selected for possible ultrasonic inspection after rough machinine. Cost studies, however, indicated that intermediate machining to facilitate ultrasonic inspection would be prohibitively costly; therefore, techniques were developed to perf or:;i the ultrasonic inspection after final machining.

Theoretically, vendors should not be performing tests just for the sake of meeting a customer specification requirement. The basis for performing any test must be predicated on the assumption that beneficial data and results will be derived. In reality, the ultrasonic inspection of complex die forgings such as bulkheads, will only yield infor:r.iation significant to warrant continued machining to a stage where the configuration and surface conditions are more amenable to inspection (such as the final machined part). Recently, some vendors have felt so strongly about the lack of confidence when inspecting die forged surf aces


and contours that they will not guarantee most of their forgings when reinspected as a machined part by the prime contractor Wlless the original inspection is performed on preform shapes that have been properly conditioned.

When it is still necessary to inspect the die forgings, correction should be made for the contoured surfaces encoWltered in the forgings versus the flat surfaces foWld on reference standards. In order to develop data to allow for surface curvature compensation, test procrar.is have been conducted using equivalent size flat bottom holes as reference reflectors in both flat surface and contoured surface reference standards. The testing was performed using a variety of search Wlit sizes, test frequencies, metal travel distances, and radii of curvature and the ultrasonic responses from the curved surface standards were compared with the responses from the flat surface standards. An ultrasonic response from a reflector in a curved surface test piece can be as much as 40 times less than the response from the same size reflector in a flat surface test piece. The correction factor was foWld to vary with search Wlit size, metal travel distance and test frequency, depending upon the radius of curvature.

The curved surface problem can also be attacked by machining reference standards that contain radii of curvature similar to the curvatures found on the die forging. Another approach is to make standards from an actual forging by cutting out sections that are representative of the contours to be inspected followed by the oachining of back surfaces and the drilling of holes to produce the proper metal travel distances and flat bottom hole sizes.

When ultrasonic inspection of titanium is performed, particularly on die forgings and preforms, difficulty with high backgroWld 11noise 11 levels may be encoWltered because of the internal structural characteristics of some titaniur:i components. This condition makes it imperative that corrections be made for the differences in SOWld trans~.ission characteristics from the calibration standard to the material to be inspected, and from material to ~.aterial. The importance of performing this correction can be illustrated by reviewing the data obtained when testing a 6 inch thick Ti-6Al-4V hand forging which had been forged and then annealed at 1300°F for 2 hours.

The hand forgin[ was to be ultrasonically inspected to a 3/64 inch diameter level, but excessive "noisen prevented an effective inspection. Figure 1 shows the indication from a 3/64 inch diameter flat bottom hole in a 7i-6Al-4V calibration standard. FiQ.ll'e 2 illustrates the responses obtained fron the same size reference hole in the hand forging both before and after a beta annealing operation. The difference in instrument gain required


~.·· .. ~-,,,~· 1,·

"''11:111 ?l

FIGURE 1 RESPONSES FROM 3/64 INCH DIAMETER FLAT BOTTOM HOLE Ti-6Al-4V

CALIBRATION STANDARD

725


to present the indications shown in Figure 1 and Figure 2 (before annealing) was 23 dB, while the differences in gain (reduction) observed in Figure 2 after beta annealing versus the as-received material was 20 dB.

The original rn.a.crostructure of the hand forging was examined and found to be uniform. The subsequent beta anneal resulted in a significant lowering of the ultrasonic noise level. However, lowering of the noise level could not be attributed to an improvement in the t18.crostructure of the material since there was little observable chan[e in the macrostructure as a result of the beta annealing.

ULTRASONIC INSPECTION CHANGES CAUSED BY FRACTURE HECHANICS

Fracture nechanics analysis nay be used to establish maximum flaw sizes which will not grow to failure durinc the design life of the product. Inspection requirements can then be selected to ensure that production hardware will not contain flaws which are larger than the critical initial sizes. In some cases, this approach will reduce the total area of a given part that requires ultrasonic inspection. For exar.iple, if the r:iarimum allowable flaw size is i::reater than the section thickness, an existing flaw would be surf ace connected and could be detectable by liquid penetrant methods. The net result of this approach is more effective utilization of ultrasonic inspection.

1'iith the ability to define acceptance criteria (flaw size) in the final rn.a.chine part by fracture mechanics, the zoning concept for ultrasonic inspection can be implemented. By utilizing selective ultrasonic inspection along with complete penetrant inspection, it becomes economically feasible to inspect the final ma.chined part in lieu of performing the undesirable intermediate die forging inspection provided a reliable and ~eaningful starting stock or preform inspection is performed.

Even though requirements can be modified to renove ultrasonic inspection from titanium die forgings, the forging suppliers must guarantee that they will replace the forging if, when the part is inspected as a machined part, defects are found in excess of the acceptance classes noted on the forging drawing regardless of which inspection technique reveals the defect. Since a cuarantee is still required, some forging suppliers insist on inspecting the parts for their °''in protection although their assumed cost saving r.J8Y not be realized in such cases.

As Received Beta Annealed

FIGURE 2 RESPONSES FROM 6 INCH THICK Ti-6Al-4V HAND FORGING (3/64 INCH DIAMETER FLAT BOTTOM HOLE SENSITIVITY)

c .--I

"" > (/)

0 z n z (/) .,, m n -t

0 z 0 -n -t =i > z c ~

~ "" -n

"" > ~ m n 0 ~ .,, 0 z m z -t (/)


HACHrnED PART INSPECTION

Ultrasonic inspection of finish machined parts has proven to be highly effective. For example, Ti-6Al-6V-2Sn bar was procured to fabricate a particular airfra..~e conponent. The bar was ordered to a Class A ultrasonic acceptance level (single discontinuities Greater than a 5/64 flat botto~ hole and multiple discontinuities greater than a 3/64 flat bottom hole are rejectable). After final machining was cor!Ipleted, the parts were re-inspected to a Class Al ultrasonic acceptance level (single discontinuities greater than a 3/64 flat botton hole and nultiple discontinuities greater than a 2/64 flat bottom hole are rejectable) and were rejected. A subsequent netallographic eY.aznination and electron microprobe analysis revealed rod-like titanium boride inclusions. The inclusions varied in length from 0.001 to 0.007 inch and in some cases were aligned to produce a string .030 inch long. Hand forgings were procured as replacement material and were inspected by the supplier to a Class A ultrasonic level. After raa.chining to 2.7 inch thickness, half of the parts would not meet the acceptance level for nrultiple indications for Class Al. A subsequent metallographic examination revealed inclusions 0.0005 to 0.0015 inch long and two strint;ers 0.020 inch long were found. Because of the a'i!B.ll size of the individual stringers it is speculated that the ultrasonic indications are due to a response froM several strincers located in close proximity to each other.

In order to alleviate some of the near-surface resolution problems on final machined parts, additional ultrasonic inspections are sometimes required. On I:lailY parts, pitch-catch and surface wave techniques are used to detect discontinuities at or near the surface of tte finished machined part.

TUBING

Thin wall tubing is presently used for aircraft hydraulic systems. In general, the finished tubing is produced from extruded hollows which are ultrasonically inspected to detect gross discontinuities prior to the extrusion operation. The smallest detectable discontinuities in the hollow are considerably larger than the snallest detectable discontinuities in the finished tubing. The ultrasonic inspection of the finished tubini:; must be capable of detecting cracks, laps, etc., with responses as small as those from an electrical discharge machined (EDH) reference notch 0.002 inch deep and 0.06o inch long. To reliably detect such defects, an automated ultrasonic system 1-tiich provides controlled longitudinal and rotational motion (helical scan) is


usually a practical necessity. The finished tubing is inspected with 100% coverage.

During the fabrication of the tubing, the most col!lI!lonly produced defect is a surface connected crack or lap which fays into the inner or outer surface of the tube. The most likely orientation for these defects is parallel to the axis of the tube; however, they can be oriented in any direction. Since there are several possible orientations for these defects, the finisli.ed tubing should be ultrasonically inspected with the ultrasound transr.litted in two longitudinal and two transverse directions, 180° opposed, under conditions identical to those used for the equipment calibration (see Figure 3).

729

The najority of the discontinuities occurring in plate are oriented parallel to the surface and are elongated in the direction which received the maximum amount of rolling. Consequently, plate is normally ultrasonically tested using straight bea.~ methods vtlth the ultrasonic beam directed in the short transverse direction. Occasionally, however, discontinuities occur perpendicular to the rolling direction and for some airfra~e components, such as wing skins, this discontinuity orientation is the most detrL~ental. In order to locate such discontinuities in selected plate, an angle bea.~ ultrasonic inspection is performed in addition to the straight beam exa."llination.

The majority of the discontinuities occurring in forged and rolled bar are oriented parallel to the direction of predominant grain flow. But, as in plate, occasionally a discontinuity can be oriented perpendicular to the grain flow. ·solid rounds are ultrasonically inspected using straight beam methods with the ultrasonic beam directed toward the central axis of the bar, and an additional angle beam inspection should be performed to locate discontinuities oriented perpendicular to the central axis of the bar. Rectangular bar is ultrasonically inspected using straight beam methods with the ultrasonic beam directed in the short transverse grain direction and the long transverse grain direction.~ Hexar;onal bar is usually inspected using straight beam methods fron three adjacent longitudinal faces to locate discontinuities lying in several orientations.


FIGURE 3 FOUR DIRECTION SCANNING FOR ULTRASONIC INSPECTION OF TUBING


SUMMARY

Through the efforts described in this paper, confidence has increased in the effectiveness of the ultrasonic inspection method to ensure the soundness of titaniun airframe components. This confidence is a result of more effective definition of acceptance criteria, technical improvements in the ultrasonic techniques, and better L'!lpleraentation of the ultrasonic method.

ultrasonic inspection of titanium …ultrasonic inspection of titanium airframe components 719 that...

Documents