NASA CONTRACTOR

REPORT

NASA CR-178745

ON THE DETERMINATION OF THE ORIGIN OF LINEAR ANOMALY

IN THE MACROSTRUCTURE OF VPPA WELDED 2219-T87

ALUMINUM ALLOY - PRELIMINARY REPORT

By W. A. Jemian

Professor, Mechanical Engineering

and Materials Engineering

Auburn University

Auburn, Alabama

NASA/ASEE Summer Faculty Research

Fellowship Program

Preliminary Report

February 1986 _

-CR-178745) ON THE DETERMINATIO N OF N86-22665

THE ORIGIN OF LINEAR- ANGHAL Y IN THE

MftCROSTRUCTORE OF VPPA WELDED 2219-T87

ALUHINUM ALLOY: PRELIMINARY REPORT {Auburn Onclas

Univ.) 29 p HC B03/BF. A01 . CSCL J IE G3/26 05875

Prepared for

NASA-Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812
TECHNICAL. REPORT STANDARD TITLE PAGE

1. REPORT NO.

NASA CR - 178745

2. GOVERNMENT ACCESSION NO. 3. RECIPIENT'S CATALOG NO.

4. TITLE AND SUBTITLE

On the Determination of the Origin of Linear Anomaly in the

Macrostructure of VPPA Welded 2219-T87 Aluminum AUoy —

Preliminary Report

5 REPORT DATE

February 1986

6. PERFORMING ORGANIZATION CODE

7. AUTHOR(S)

W. A. Jemian*

8.PERFORMING ORGANIZATION REPORT ft

9. PERFORMING ORGANIZATION NAME AND ADDRESS

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

10. WORK UNIT NO.

1 | CONTRACT OR GRANT NO.

NGT-01-008-021

12. SPONSORING AGENCY NAME AND ADDRESS

National Aeronautics and Space Administration

Washington, B.C. 20546

13. TYPE OF REPOR'i & PERIOD COVERED

Contractor Report

14. SPONSORING AGENCY CODE

15. SUPPLEMENTARY NOTES

Prepared by Materials and Processes Laboratory, Science and Engineering Directorate.

*Professor, Mechanical Engineering and Materials Engineering, Auburn University, AL.

16. ABSTRACT

The objective of this research was to determine the cause and significance of the

weld radiograph enigma, which is a linear anomaly in the features of the x-ray film.

By observing features on available radiographs and in studying published reports of

similar features, it was possible to conclude that there are many manifestations of the

enigma, and that they are all specific features of fine structure in radiographs due

to natural processes connected with welding and to specific x-ray absorption and

diffraction phenomena. These processes include the thermal distribution and liquid

metal flow in welding, the development of microstructure, morphology, second phase

particles and porosity due to the solidification process, and to the pattern of residual

stresses after the weld metal has cooled to the ambient temperature. Microdensitometer

traces were made across weld radiographs of standard and enigmatic types. Similar

patterns were produced by computer simulation. These show that the enigma is a

relatively low contrast feature compared to real weld defects, such as undercuts or

centerline cracks. The enigma can be distinguished from weld defects by these microdensitometer traces. The enigma effect on weld properties is not known but is

expected to be minor.

17. KEY WORDS

Variable Polarity Plasma Arc Welding

Enigma

Ghost

Radiography

Weld Defect

18. DISTRIBUTION STATEMENT

Unclassified — Unlimited

19. SECURITY CLASSIF. (of thU rap Off)

Unclassified

20. SECURITY CLASSIF. (of thl« page)

Unclassified

21. NO. OF PAGES

28

22. PRICE

NTIS

MSFC - Form 3292 (May 1969)

For sale by National Technical Information Service, Springfield, Virginia 22151
ACKNOWLEDGMENTS

This work was sponsored through the Summer Faculty Fellowship Program of

NASA in cooperation with the American Society for Engineering Education. It comprises the report on a summer of research carried out by a professor of mechanical

and materials engineering from Auburn University in the Materials and Processes

Laboratory of the Marshall Space Flight Center.

The work was primarily of a survey nature but is dependent on the experimental work of Mr. E. O. Bayless and his staff in experimentally simulating these

features. It also benefitted from the information and inspiration of Dr. Arthur C.

Nunes. Both are my NASA counterparts. Any success is directly due to their

guidance and cooperation.

It is my pleasure to acknowledge the assistance of several other individuals who

helped me directly. They are Mr. Carl Wood, who was totally sympathetic and supportive of my needs with respect to basic engineering information. Mr. Paul H.

Schuerer inherited me into his branch with good grace and was very encouraging in

his support. Messrs. Paul Munafo, Bobby Rowe, and Joe Montano offered constructive criticism. Mr. Claud Williamon and I worked together collecting x-ray data on

fluorescence effects and Mr. Jim Coston was very helpful in analogous studies using

the SEM. Dr. Walter Fountain provided the critical bit of experimental evidence to

confirm the basic ideas concerning the enigmas. Mr. C. Lovoy, USBI, retired NASA

staff member, has taken an interest in my activities and provided helpful background

and information on several occassions. Several individuals of the Martin Marietta

staff at Michoud were helpful in providing information. I am especially grateful to

Messrs. Monty McAndrews and W. Mattheessen and to Ms. Linda Johnston.
TABLE OF CONTENTS

Page

INTRODUCTION 1

EXPERIMENTAL 3

CONTRIBUTING FACTORS 5

Welding 5

Solidification 7

Radiography 10

RESULTS AND DISCUSSION 13

ENIGMA SIMULATION 17

CONCLUSIONS 19

REFERENCES 20

ill
LIST OF ILLUSTRATIONS

Figure Title Page

1. The typical appearance of enigmas and crack indications in a

weld radiograph 1

2. X-ray diffraction as a cause of the dark line weld radiograph

enigma 2

3. General shape of the keyhole mode weld pool 4

4. Shape of the weld after the root pass and the cover pass in

VPPA welding 5

5. Shrinkage distortion in a butt weld 6

6. The effect of heat sink configuration on the weld metal grain

structure 6

7. The effect of welding speed on shape of the fusion zone and

weld metal microstructure 7

8. Aluminum rich end of the Al-Cu phase diagram 8

9. The desired and misaligned shapes of the root pass weld metal 9

10. Representation of the general configuration for the preparation

of an x-ray weld radiograph 10

11. The spectrums of the initial and transmitted x-ray beams in

radiography 11

12. Typical absorption curve 11

13. Microdensitometer trace across radiograph of nearly symmetric root

pass in a VPPA weld 13

14. Microdensitometer trace across radiograph of root pass made

under conditions of severe torch misalignment 14

15. Microdensitometer trace of weld radiograph with cover pass

over a misaligned root pass 14

16. Microdensitometer trace across production weld radiograph 15

17. Microdensitometer trace across center line crack 15

18. Ratio of backscatter over Cu to backscatter over Pb as a

function of tube voltage 16

IV
LIST OF ILLUSTRATIONS (Concluded)

Figure Title Page

19. Computed film density for the sample shape shown 17

20. Film density variation across weld with both crown and root

reinforcements 18

21. Computer simulated radiograph density across a weld with both

reinforcements and a longitudinal band of eutectic mixture 18
CONTRACTOR REPORT

ON THE DETERMINATION OF THE ORIGIN OF LINEAR ANOMALY

IN THE MACROSTRUCTURE OF VPPA WELDED 2219-T87

ALUMINUM — PRELIMINARY STUDY

INTRODUCTION

There is an anomalous, generally linear feature, in line with the weld direction

that appears intermittently in weld x-ray radiographs. It bears an appearance similar

to that of a defect, but for which there is no defect or discontinuity in the structure

of the weld metal and no apparent effect on the weld mechanical properties. It is

commonly called an "enigma" or "ghost defect." Duren and Risch reported in 1970 [1]

that: "The enigma is the most difficult discontinuity to determine, often being

mistaken for incomplete fusion, and many times for a crack, or even a diffraction

pattern. Identification of an enigma demands long experience in interpreting radiographs. The interpreter should notice that the dark line is always accompanied by a

parallel light line. Many destructive tests for strength have been made of this type

of discontinuity, but so far there has been no noticeable effect on the weld's ultimate

strength." This represents the understanding 15 years ago and indicates that the

enigma must have been considered a problem for some time before that period. Welds

of that time were not plasma arc, therefore the phenomenon is general.

Present indications are that there are two or more distinct manifestations of the

enigma that occur separately. These are the dark form, frequently occurring along

the weld centerline, and the gray or white line at the side of the weld, just within

the fusion boundary. Real weld defects are not a manifestation of the enigma. Both

features, which have been observed separately, have the appearance of a serious

weld defect, but on close examination, by metallographic sectioning, have been found

to be free of any defect. The problem is that the condition is misleading and disconcerting. The appearance is similar to that shown in Figure 1.

white dark crack

enigma enigma defect

Figure 1. The typical appearance of enigmas and crack

indications in a weld radiograph.

Recently, L. Johnston [2] reported that "the.. .enigma occurrence was characterized by a grey line running along the toe side of the weld. The line ran continuously, only occurring on one side at a time. No violations of weld parameters
were found. The trim area was large enough for one tensile bar and one macro

specimen. The UTS of the tensile bar was greater than 40 KSI. Visual examination

of the weld revealed no cracks, undercut, suckback, open porosities or tunnels, or

other evidences of inconsistent welding processes. No anomalies were found in ultrasonic inspection. Post-proof x-ray showed no change of character, position, or

intensity." This report was corroborated by Kinchen and Brown [ 3] .

The suggestion was made that the phenomenon is a diffraction effect, similar

to Kikuchi lines in electron diffraction or Kossel lines in x-ray diffraction. Rummell

and Gregory [4] reported a similar occurrence in radiographs of TIG welded 2014

aluminum alloy. "Recent studies on welding defects have revealed a phenomenon

which appears as 'lack of fusion' or 'lack of penetration' in a weld radiograph, but

does not affect the performance of the weld.. .indications are generally not as sharp

as true lack of fusion, and are curved at the ends.. .Macro and micro examination of

the weld cross section shows a dendritic grain structure in the fusion zone... Grain

orientations are at right angles to each other...The unique grain orientation is due to

a particular cooling and freezing process in the weld...Now, if a series of crystals

are properly oriented with respect to an X-ray beam, a 'focusing' effect will be

observed on the radiograph, in the form of a dark band...The white line is caused

by deviation of the transmitted beam by the diffraction process, thus decreasing the

total transmitted radiation in a particular line or band — The diffraction process is

not limited to the weld structure illustrated, but will occur in any exposure when

crystalline planes are properly oriented with respect to the X-ray beam. Likewise,

this process is not limited to aluminum welds."

Figure 2 illustrates the general features of the Rummell model. The interdendritic particles shield the film producing the light line and the diffracted beam produces the dark band. Large grains of this type are encountered occassionally.

Since the solidification mode is generally dendritic and the interdendritic constituent

is of an eutectic type, the second phase particles are likely to be coherent, or at

least highly aligned, with the dendritic branches. Finally, since the film is generally

placed very close to the sample, the physical separation of the direct and diffracted

beams on the film is also close.

large grain with

interdendritic

particles ,

normal

backgrou

diffraction

enhanced

Rummel, et al.

Figure 2. X-ray diffraction as a cause of the dark line

weld radiograph enigma.
Hirosawa, et al. [5] reviewed the literature, including the work of Rummell,

et al. and Tucker, et al., who reported similar findings and conclusions to those of

Rummell [6]. Rabkin, et al. also reported dark band enigma formation in Al-Mg alloy

welds [7], Issiki reported similar effects in aluminum alloy castings [8], and Irie,

et al. reported similar features in stainless steel welds [9]. Hirosawa, et al. reported

an analytical procedure related to magnesium content in aluminum alloys [5].

It is evident that the enigma, ghost defect, or linear anomaly has been encountered for a number of years and in a number of alloy systems and welding processes. It appears to consist of dark or light lines, separately or together, primarily straight, in alignment with the welding direction, but also may have a curvature. The curvature is due to changing conditions along the weld path. All reports

place these features in the fusion zone. The dominant questions to be answered in

finding the solution to the weld radiograph enigma problem are:

1) What is the enigma?

2) What produces the effect?

3) How can it be recognized?

4) How are weld properties affected?

The question about the effect on properties is fundamental. However, it must be

realized that even if there is a visible feature in the sample there are established

size tolerances. Perhaps the greatest problem is the inability to distinguish the

enigma from a weld defect. A method was evaluated for this purpose and is described

in a later section. There are many manifestations of the enigma. It will be shown

to be a fine structure in the weld radiograph that is related to a real condition in the

weld. The occurrence of this fine structure will.be related to such factors as concentration gradients, shape effects and possibly features as severe as porosity. Even

these latter, at levels to produce a visible feature, are probably within the established

size tolerances. The correct interpretation of these features required an understanding of the principles of welding physical metallurgy and image formation in x-ray

radiography.

EXPERIMENTAL

The Space Shuttle external tank is fabricated, primarily, of aluminum alloy

2219-T87. The initial joining process was TIG welding which is being replaced by

VPPA welding. The filler material is 2319 aluminum alloy wire. Weld radiograph

enigmas were reported for TIG welds as well as the VPPA process. TIG weld radiographs show a higher level of density variation than VPPA radiographs, therefore,

fine structure is not as easily detected.

In most thicknesses, two weld passes are used, both from the same side. The

root pass is in the keyhole mode and the cover pass in partial penetration, frequently referred to as "in the TIG mode." The plasma has a remarkable cleaning

action. The heavy ions break up the tenacious oxide layer and the fragments are

swept away by the plasma jet. If the torch is properly aligned and the parameters

properly set, a flawless weld results. Even the cover pass maintains the high level
of structural integrity due to the action of the keyhole pass which leaves a clean

system. The shape of the liquid puddle during the vertical up keyhole pass is shown

in Figure 3.

liquid .

Figure 3. General shape of the keyhole mode weld po'ol

(the shaded portion is the liquid).

The final weld structure is characterized by crown and root reinforcements making a smooth transition to the base metal plate on both surfaces. Figure 4 describes

the general appearance of the weld after the root and cover passes. The root reinforcement is produced in the keyhole pass and the top surface of the weld is left

approximately flush. Thinner plates are welded in only a single pass leaving reinforcements on both surfaces. The cover pass remelts a portion of the fusion zone of

the previous keyhole pass and part of the base metal. The symmetry shown in the

sketches of Figure 4 can be expected as the result of symmetric torch alignment.

The heat affected zone exhibits only a minor change of grain size. The hardness

profile across the weld from one base metal plate to the other shows a generally

symmetrical decrease in hardness towards the weld centerline with a moderate increase

close to the fusion boundary in the heat affected zone. This small increase is due to

an age hardening effect.

X-ray radiography uses a tungsten target tube operated at a tube voltage of

80 kV and a tube current of 15 mA. The typical standoff distance is 40 in. In most

cases the film is placed in direct contact at the back of the sample. However, in one

of the fixtures this space is restricted so that the film is approximately 1.25 in. away.

The exposed film is processed automatically and is read and interpreted visually.

Other NDE procedures are employed to clarify and decide specific conditions. Present

practice requires 100 percent radiographic inspection.

Three densitometers were used to obtain the line traces across the weld pattern.

These are the MacBeth TD 102 densitometer with an aperture of approximately 3 mm

diameter, an instrument which is designed for analyzing spectroscopic plates, and the

Perkin Elmer PDS Microdensitometer developed for analyzing stellar plates. The Per kin
after root pass after cover pass

Figure 4. Shape of the weld after the root pass (left) and the

cover pass (right) in VPPA welding.

Elmer instrument was operated with a square aperture of 125 ym side dimension at

contiguous fields. An offset density of 1.5 was uniformly used for all measurements.

The data was printed out and manually transferred to a personal computer for processing. The second instrument could not be used due to its inability to work with

high density radiographs.

The scanning electron microscope with an EDAX chemical analysis attachment

was used to map the concentration of copper and iron over transverse sections of the

weld.

Radiography, based on the fluorescence of alloying elements, was used to map

concentration distribution. In another investigation of fluorescence effects, the

standard x-ray facility was operated at a standoff distance of 40 in. over a range of

tube voltages. The film (DuPont Cronex 45) was placed over blocks of pure copper

and lead, placed side by side. The exposure was controlled by setting tube current,

exposure time and using various thicknesses of 5052 aluminum alloy plate.

CONTRIBUTING FACTORS

Welding

Welding exerts its influence on the condition of the final solid through all of its

operating parameters. Conditions in the liquid pool control weld geometry, grain

shapes, and final distortion and residual stresses. The hot, just solidified alloy

behind the pool, contracts as it cools, whereas the base metal away from the weld

may not have any tendency to contract. If the effective center of this initially hot

metal is out of line with the center of mass of the plate, the contraction produces a

net bend and leaves a serious pattern of residual stresses as shown in Figure 5,

which illustrates the directions of these distortions. This is explained in detail by

Masubuchi [10]. These stresses, in concert with applied stresses, can lead to premature service failure. If a stress riser, or other mechanically sensitive feature is

in the field of influence of the stress pattern, cracking can occur.

Fixturing can be used to control or reduce distortion. It also exerts an effect

on the final microstructure by alteration of the heat sink configuration. This modifies

the pattern of grain growth and affects properties. The effect on "grain growth

direction is illustrated in Figure 6. In a previous study [11], the application of
Figure 5. Shrinkage distortion in a butt weld (includes

peaking and longitudinal curvature).

CHILL

Figure 6. The effect of heat sink configuration on the

weld metal grain structure.

cooling blocks along a short region at the side of the weld produced a finer microstructure and increased the mechanical properties in that section.

Welding power and speed affect the final weld structure through their action

on final cooling rate. Power must, of course, be set to control penetration and width.

The higher the speed the more power is required to penetrate to the proper depth.

At higher welding speeds, the base metal is not heated to the same extent and has

a greater cooling effect after the torch passes. This changes weld shape and affects

the direction of heat flow. The desired ellipsoidal shape of the weld pool becomes

pointed at the back as illustrated in Figure 7. The consequence is that the direction

of columnar grain growth in the fusion zone is affected. If the condition is sufficiently pronounced, a plane of weakness is formed along the weld centerline. Cooling

stresses may" then result in centerline cracking.

At the other extreme of welding speed, the final fusion zone structure will be

too coarse due to the slow cooling conditions supported by the well-heated base plate.

Coarse microstructure is associated with lower mechanical properties. In a previous

research it was shown that an increase in interdendritic particle size from 10 to 150

ym correlates with a 10 ksi decrease in weld tensile strength [11]. The effect of

welding speed in electron beam welding is dramatically illustrated in a brief review

note by Komizo, et al. [12]. These same effects are also characteristic of any moving

source fusion welding process, such as VPPA welding.

Weld metal morphology, which is primarily dendritic, is controlled by the solidification process. Grains have columnar shapes directed along the line of heat transfer.
Figure 7. The effect of welding speed on shape of the fusion zone

and weld metal microstructure. The relative lengths

of the arrows represent welding speed.

For a given materials system, welding speed has the greatest influence on the grain

shapes and directional pattern in the fusion zone as well as the size of the structural features. The secondary dendrite branch dimension is the critical size parameter in the fusion zone microstructure. The product of welding speed and the square

of the size parameter is a constant for a given materials system and solidification

process.

Stresses are developed by differential cooling from a condition where pronounced

temperature differences exist within the system. Under these conditions any possible

stresses are released due to the enhanced plasticity at the elevated tempeature. As

the hot metal cools, its strength increases and its contraction, relative to the condition of the initially cool metal, causes locked-in stresses to develop. The magnitude

and direction of these stresses is influenced to a lesser degree by variations in

solidification conditions.

Solidification

Solidification is the most significant process in weld metal structure and properties. The basic weld characteristics that are directly affected include:

1) Chemical segregation

2) Second phase production

3) Porosity.

It is not possible to remove solidification factors from welding process factors

since they are totally interdependent. They must be considered together in any

given application. However, selected topics in the physical metallurgy of weld metal

solidification will now be considered.
Chemical segregation is a dominant factor and can be readily explained in relation to the aluminum-copper phase diagram (Fig. 8), which applies to the 2219 alloy.

The nominal copper composition is 6.3 percent by weight. Reference to Figure 8

shows that the composition line is to the right of the maximum solid solubility point

of 5.62 percent copper (548°C). Therefore it can be expected that there will be free

particles of CuAl2 intermetallic compound in the microstructure, regardless of the

treatment after solidification. Under steady state welding, the weld pool composition

corresponds to this same 6.3 percent Cu value.

660

63

X Cu

Figure 8. Aluminum rich end of the Al-Cu phase diagram.

The composition of 2219 aluminum alloy and the solid that

forms initially are shown. The eutectic constituent (E),

is a coherent mixture of aluminum solid solution and

CuAl~ intermetallic compound.

The composition of the first solid to form on cooling is S.. (marked on the diagram) and is formed at a temperature of 640°C. As solidification continues, the

copper that is not incorporated in the first solid enriches the liquid so that it freezes

out at a lower temperature, corresponding to a further position along the solidus line.

The last solid to form is an eutectic mixture, with a composition of 33.2 percent

copper. It freezes at 548°C. It is this liquid that contains the highest copper content and is most likely to flow against the coolest surfaces of the cooling weld. It

has the greatest capacity to fill any cracks or other openings in the weld metal.

The solid-liquid boundary surface of the weld pool is not isothermal but varies

from a temperature corresponding to the freezing temperature of S1 at the positions

where solidification commences and drops to the eutectic solidification temperature at

positions directly behind the moving source.

Second phase production is an important aspect of solidification. In this alloy

system the second phase can be formed in two constituents. The intermetallic compound phase (53 percent Cu) is formed in either fusion zone or heat-affected zone

and is in the parent metal microstructure. The second constituent, the eutectic

8
mixture is only in the fusion zone — a product of solidification. It is this constituent

that is important in some forms of enigma and this is a major controlling factor on the

weld tensile strength. It does not influence the yield strength very strongly since

this property is dominated by the solid solution that comprises most of the microstructure.

Bayless [13] produced weld structures to demonstrate the light line enigma.

His method involved the intentional misalignment of the torch for the root pass, thus

producing a deep undercut to one side of the weld and an exaggerated buildup of

metal to the other side. The normal and misaligned final profiles are shown in

Figure 9.

/

J

(A) (B)

Figure 9. The desired (left) and misaligned (right) shapes

of the root pass weld metal.

When the cover pass was applied, the final structure showed the enigma due to

the concentration of eutectic mixture in the undercut. The cover pass only partly

repaired the initial improper shape. The deepest part of the undercut, which was not

remelted was filled with eutectic liquid during welding. This left a residual copperrich band of the form described above in the enigma simulations. The first three

microdensitometer traces were made on the radiographs of the Bayless simulation

samples. Lancaster discusses other methods to cause these effects [14].

Porosity is a type of second phase that results from solidification. There are

several causes including:

1) The release of gases dissolved in the liquid.

2) The entrapment and release of hydrogen that is produced by the decomposition of water or hydrocarbons left on the surface of the metal or filler wire prior to

welding.

3) The failure to fill the interdendritic regions due to insufficient eutectic

liquid.

4) The freezing over of interdendritic regions, thus preventing access by the

available eutectic liquid.

The first two sources produce a more uniform distribution of porosity throughout the new solid but the latter two develop concentrated openings in the vicinity of

the center line. None of these are typical of VPPA welding of 2219-T87 aluminum

alloy. Easterling [15] gives more information about porosity in welding.
Radiography

The initial x-ray beam is directed normal to the principal plane of the sample

and is uniform in intensity. This uniformity is achieved by employing a long distance

between source and film.

Passage through the sample reduces the intensity of the entire spectrum in

proportion to the sample thickness and its absorption characteristics, which are a

function of wavelength. The beam is further attenuated as it passes through the

film. The energy lost in the sample produces the pattern seen in the film after it is

developed. The radiation absorbed in the film causes the darkening. The thinner

the sample, the more radiation penetrates to reach the film, and the more particles of

silver that are developed in the emulsion to darken the film.

These features are represented in Figure 10. The thickness of the lines representing the x-ray beam, that is moving downwards, indicate the relative intensities.

The points in the film cross section represent particles of silver, after development.

The effect of the reinforcement is to increase the absorption and reduce the final

silver content in the corresponding parts of the film.

I I

x-radiation, movln

I I I I

towards sample and film

sample

film

Figure 10. Representation of the general configuration for the

preparation of an x-ray weld radiograph. The distance

between the sample and film is exaggerated in this figure.

The physical processes involved are outlined in Figures 11 and 12. Cullity [16]

gives a comprehensive presentation of these processes. The spectrum of the initial

beam is shown in the first block at the left of Figure 11. The spectrum of the final

beam is shown at the right. Under the typical 80 Kv tube operating conditions,

spectral content of the initial radiation is primarily a continuous distribution of x-rays,

generally identified as continuous, white or brehmsstrahlung. A characteristic of

tungsten and other high atomic number target material tubes is the suppression of

characteristic radiation peaks. There is probably a small component of characteristic K or K0 radiation. Such a peak is shown schematically. There is only a small

a p

energy content in these characteristic radiation components.

10
\

A

I

Incident Spectrum, I0 LJ Transmitted Spectrum,

Sample '

I - I ~^

Figure 11. The spectrums of the initial and transmitted

x-ray beams in radiography.

X

Figure 12. Typical absorption curve.

After passage through the sample, the intensity of radiation is generally diminished, as shown in the block diagram to the right. This attenuation is most pronounced in the vicinity of the absorption edge of the important chemical components

in the sample. The relationship between thickness, materials characteristics, and

initial and reduced intensities is written below the sample.

Each element has characteristic absorption characteristics in the form shown in

Figure 12. Energy is absorbed when specific electronic processes are induced within

the elements of the sample. The energy invested in these processes can be recovered

in different ways. Copper, for example, has an absorption edge at 1.38059 A and

uses this energy to produce fluorescent radiation at 1.54 A wavelength. In this case,

the characteristic radiation is added to the spectrum. These processes are most

active when there is energy at the lower wavelength side of the absorption edge.

In this case, the sample acts as a source of secondary radiation. This usually has

the undesireable effect of increasing the general background density of the film and

tending to obscure the features of the image.

Diffraction is a possible contributing factor because diffraction can cause extra

radiant energy to be concentrated along specific directions. Rummell referred to

one specific diffraction effect. Another, that has been mentioned, and should be

11
considered, is the development of Kossell lines, which are a phenomenon in the class

of wide angle scattering [17-19]. If the linear anomaly in the radiograph is actually

due to a diffraction effect, the features will be characteristic of that effect and can

be recognized by appearance.

In one application, the Kossell pattern is developed by placing a thin layer of

fluorescent material, such as copper, against the side of the single crystal sample that

is towards the source. When the narrow beam of x-rays strikes the sample, this

layer then provides an x-ray source at that close location to produce divergent

diffracted beams. These rays are contained within a conical surface, intersecting

the film in arcs. This type of enigma structure has not been reported.

Anomalous transmission, known as the Borrmann effect [20-23], can also make

a contribution to the intensity of x-rays delivered to alloying elements concentrated

close to the film within the weld. The necessary conditions are a large, favorably

oriented grain of a single crystal phase. Such large grains have been observed in

weld metal. They are formed due to the maintenance of conditions favoring stable

growth, as opposed to nucleation. These conditions include a favorable alignment of

a high symmetry crystal direction with the vertical, along which the x-ray beam propagates. In aluminum, as is typical of most alloy systems, the favored growth direction is <100> which has the highest symmetry. There is a greater likelihood that for

a grain that grows in a <100> direction parallel to the weld surface that it will also

have another <100> type direction aligned vertically. Such an alignment would be

naturally favored and would tend to prevent the growth of grains of other alignments.

The formation of large, highly aligned grains should be no surprise in a solidification

process as controlled as that in a VPPA fusion zone.

The natural conclusion is that a number of different factors contribute to the

variation of radiographic film density across the weld. A number of these are summarized in Table 1 with respect to the direction of the darkening effect of each.

TABLE 1. ABSORPTION

Factor

Shape and Thickness

Macro Openings

Chemical Accumulation

Porosity

Fluorescence

Diffraction

Anomalous Transmission

Darkening

4- (Cu in 2219 Al)

t

t

-t (possibly with I )

t

It was found that persons can unambiguously distinguish adjacent light and

dark areas when the density difference is only 1 percent (e.g., 2.98 and 3.01).

The significance is that many features are observed that are only of secondary

significance.

12
RESULTS AND DISCUSSION

The above factors can produce a complicated visual pattern when combined.

The following are microdensitometer traces made across welded panels of 221&-T87

aluminum plate. Figure 13 is a trace across the VPPA root pass made under conditions representing one set of torch alignment conditions marking the limit of acceptable

practice. These traces are plots of data taken at a spacing of 125 ym along a line

across the weld radiograph. There are 204 points per inch. The sample surface

conforms to the desired shape, but the effect of the misalignment is visually obvious

as is the lack of symmetry in the trace about the weld centerline. There is a minor

degree of undercut along both edges of the weld (see 1 and 2). The undercut at 2

is more pronounced and wider. The peak at 3 corresponds to a surface marking that

runs along the side of the root reinforcement.

4 --

CO

QJ 1

LL

o \

Position (inches)

Figure 13. Microdensitometer trace across radiograph of nearly symmetric

root pass in a VPPA weld.

Figure 14 is a similar trace across another unfinished weld (root pass only)

made in the same plate. However, the torch misalignment was intentionally set to

produce an extreme undercut along one edge. This is represented by the pronounced

peak at the right. The deep valley at the left represents the region where the weld

metal accumulated. This trace provides a good representation of the weld profile.

Figure 15 is across another weld made in the same plate, under the same conditions of misalignment, but with the cover pass applied. Even here the misalignment

is detectable in the microdensitometer profile, although it is not visually detectable

on the finished weld. Features 4 and 5 correspond with a weld root marking (see 1

in the first trace) and a surface scratch on the underside of the plate.

Figure 16 is a trace across the production radiograph showing two light line

enigmas (marked 6 and 7). The appearance of the radiograph matches the appearance

of the radiograph of Figure 15. The sample is not available. There is considerable

detail, or fine structure, in the trace. These features can be related to the factors

listed in Table 1.

13
4 ••

0 1

Position (inches)

Figure 14. Microdensitometer trace across radiograph of root pass

made under conditions of severe torch misalignment.

CO

c

2..

r*S —'

~r

n

Position (inches)

Figure 15. Microdensitometer trace of weld radiograph with

cover pass over a misaligned root pass.

14
4 --

ro

c

O) 7

0 ^

c

iZ

ORIGINAL PAGE IS

OF POOR QUALITY

0 1 2

Position (inches)

Figure 16. Microdensitometer trace across production weld radiograph.

Figure 17 displays a trace made across the radiograph of a TIG welded 221&-T87

plate with a definite centerline end crack. The appearance of the crack feature in

the radiograph is very similar to the dark line enigma with the difference that the

darkening associated with the real crack is much stronger in intensity.

4 -•

zr>

0.' -r

Q -'

A-1 '-.",_-•-

I j

Position finches)

Figure 17. Microdensitometer trace across center line crack.

(TIG welded 2219 alloy)

Table 2 summarizes the density differences of the features noted in the microdensitometer traces.

15
TABLE 2. THE EFFECT OF ENIGMAS AND CRACKS

ON RADIOGRAPHIC FILM DENSITY

Sample and Feature

Fig. 13, feature 1

Fig. 13, feature 2

Fig. 13, feature 3

Fig. 14, total

Fig. 15, feature 4

Fig. 15, feature 5

Fig. 16, feature 6

Fig. 16, feature 7

Fig. 17, crack

Type

light

light

dark

shape

dark

dark

light

dark

defect

Density Increment

0.09

0.05

0.05

1.81

0.18

0.10

0.13

0.18

0.50

A series of measurements were made to evaluate the possible contribution of

fluorescence. The film was placed over a composite background, with copper to one

side and lead to the other. The density of the resulting film was measured at five

points within each field and averaged. The results are plotted in Figure 18 as

darkening ratio versus tube voltage. At low voltages the copper produces a greater

darkening, but this changes pronouncedly at a voltage between 80 and 120 KV. The

L absorption edge for lead is 0.95073 A, which indicates a critical wavelength just

longer than 130 KV. The nearest edge for copper is activated most effectively at

tube voltages of 8900 V and slightly higher.

LJ

0.'

cOQ..

Q) ' ' ^

a

•/I

a>

40 80 120 160 200

Tube Voltage (Kv)

240 280

Figure 18. Ratio of backscatter over Cu to backscatter over Pb

as a function of tube voltage.

Along with these measurements, a radiograph was made of a large grained pure

aluminum sample. This showed a general pattern of detail corresponding to the grain

structure, although none of the grains is separately identifiable. This is evidence

of the possible contribution of diffraction or anomalous transmission.

16
ENIGMA SIMULATION

It is possible to simulate enigma formation. This is done by calculating the

transmission ratio I/I , through the plate at each position across the weld. The basis

of the calculation is Lambert's Law, I = IQe~yX. In these equations, I is the intensity of the transmitted radiation and I its initial intensity on the front side of the

sample, e is the base of the Napierian logarithms, p the absorption factor, and x

the distance of transmission through the plate. Since human sensitivity relates to

Inl, this response is directly represented by the magnitude of the exponential quantity, px. Absorption is a characteristic of each chemical species and a function of

wavelength. The values are generally available, usually listed as the mass absorption

coefficient, p/p , where p is the density. Thus, response is measured by the product, (p/p)px . In any mixture, the net mass absorption coefficient is the linear

average of the values for the elements, in proportion to their weight fractions in the

mixture.

Figures 19 through 21 are density traces generated in this way through the

sample. In each figure, the sample is shown above and the film density representation directly below. It can be seen how the pattern of complexity in the radiographic

film develops with the inclusion of complexity in sample shape and chemical segregation. This illustrates the origin of the enigma as well as the feasibility of simulating

and interpreting these effects.

sample

backed weld

film

film densitu

Figure 19. Computed film density for the sample shape shown.

Note that in Figure 19 the density of the radiograph does not vary across the

ends and has a curvature that matches the sample curvature. The sample shape

corresponds closely to that of an actual weld, but the outline was drawn freehand on

graph paper. The dimensions were read directly from this drawing and entered into

the computer for the calculation. The result of the calculation was plotted and processed in a similar manner to that used for Figures 13 through 17. The variation in

film density represented by the thickness of the cross hatched section is exaggerated

several times.

17
The addition of the root reinforcement adds a bit of detail to the film density

profile. The width of each reinforcement is clearly displayed in Figure 20.

sample

plain weld

film

normal density

variation

Figure 20. Film density variation across weld with both

crown and root reinforcements.

sample

weld with

eutectic band

film

film showing

grey line

enigma

Figure 21. Computer simulated radiograph density across a weld

with both reinforcements and a longitudinal band

of eutectic mixture.

Figure 21 was generated from the same weld configuration with an arbitrary

addition of eutectic mixture. The angle of the deposit cross section, shown in the

weld cross section, its thickness (0.5 mm) and the effective mass absorption coefficient, are reasonably close to experimental values, but are used primarily for illustration. The result is exaggerated.

18
The gray line enigma of Figure 21 corresponds to that demonstrated by Bayless.

CONCLUSIONS

The enigma is fine structure in the x-ray radiograph representing a real condition in the weld metal. There are many types of weld radiograph enigmas. The

microdensitometer traces show that some features of radiograph fine structure can be

related to visible features on the weld. The correspondence between radiograph fine

structure and weld metal condition awaits direct verification, although the present

indications are promising. The microdensitometer trace method can be used to reconcile these features of weld radiographs. Initial measurements indicate that the

enigma represents a density difference of the order of 0.1 and that the crack defect

represents a difference of the order of 0.5. It is suggested that this method be

developed for weld evaluation and possible in-process control.

Weld radiograph fine structure is generally aligned in the welding direction,

hence when it is sufficiently pronounced it is classed as a linear anomaly, or enigma.

Changing conditions along the weld can produce a general curvature, which can

explain curved enigma features. The full effect on properties is not known. The

enigma cannot be eliminated and is not a feature of VPPA welding alone — it is a

general phenomenon.

19
REFERENCES

1. Duren, P. C. and Risch, E. R.: Radiographic Interpretation Guide for Aluminum Alloy Welds. NASA Technical Memorandum TM X-53939, May 10, 1970, p. 8.

2. Johnston, L.: VPPA "Black Line" Anomaly Indication on X-Ray Films. Memo

for the Record, August 21, 1984.

3. Kinchen and Brown: Evaluation of VPPA Radiographic Anomalies in 2219 Aluminum Weldments. Laboratory Test Report, Metallurgical Analysis, 85A205,

June 3, 1985.

4. Rummell, W. D. and Gregory, B. E.: "Ghost Lack of Fusion" in Aluminum

Alloy Butt Fusion Welds. Materials Evaluation, XXIII, 1965, p. 586.

5. Hirosawa, E., Naoe, M., and Fukui, T.: "Ghost Defects" in Radiographs of

Aluminum Alloy Welds. Materials Evaluation, XXIX, 1971, p. 99.

6. Tucker, M. S. and Larssen, P. A.: Markings in Radiographs of 2014 Aluminum

Alloy Gas Tungsten-Arc Welds. Welding Journal, Vol. 47, 1968, p. 223.

7. Rabkin, D. M., Bukalo, L. A., Kirzhova, V. Ta., and Deniyanchuck, A. S.:

Weld Heterogeneity in Aluminum-Magnesium Alloys. Avt. Svarka, No. 5, 1966,

p. 74.

8. Issiki, S., Ko, J. , Kataoka, K. , and Yamazawa, T.: The Abnormal Pattern in

Radiographs of Aluminum Alloy Castings. Hihakai-Kensa. Vol. 15, 1966, p. 257.

9. Irie, M., Fujii, M. , and Yamashita, S.: The Influence of Structure on the

X-Ray Radiograph of Stainless Steel Welds. Hihakai-Kensa, Vol. 12, 1963, p. 263.

10. Masubuchi, K.: Residual Stresses and Distortion, Chapter 6. Welding Handbook. Volume I, 7th Edition, American Welding Society, 1981, p. 223.

11. Jemian, W. A.: The Strength and Characteristics of VPPA Welded 2219-T87

Aluminum Alloy. Final Report, NASA-ASEE Summer Research, 1984.

12. Komizo, Y., Punshon, C. S., Gooch, T. G., and Blakeley, P. J.: Effect of

Process Parameters on Centre Line Solidification Structures of Electron Beam

Welds in Steel. Welding Institute Members Report No. 252, 1984.

13. E. O. Bayless, personal communication.

14. Lancaster, J. F.: The Physics of Welding. Pergamon, 1984.

15. Easterling, K.: Introduction to the Physical Metallurgy of Welding. Butterworths, 1983.

16. Cullity, B. D.: Elements of X-Ray Diffraction. Addison-Wesley, 1978.

17. Taylor, A.: X-Ray Metallography. Wiley, 1961, p. 770.

18. Birks, L. S.: Electron Probe Microanalysis. 2nd Edition, 1971, p. 140.

20
19. Yakowitz, H.: The Divergent Beam X-Ray Technique. Electron Probe Microanalysis. Academic Press, 1969, p. 361.

20. Borrmann, G.: Rontgenwellenfelder. Trends in Atomic Physics, Risch, Paneth,

Roosbaud, Editors, Interscience, 1959, p. 262.

21. Hunter, L. P.: X-Ray Measurement of Microstrains in Germanium Single

Crystals. J. Appl. Phys. , Vol. 30, No. 6, 1959, p. 874.

22. Batterman, B. W.: Imaginary Part of X-Ray Scattering Factor for Germanium

Comparison of Theory and Experiment. J. Appl. Phys., Vol. 30, No. 5, 1959,

p. 998.

23. Guinier, A.: Small Angle X-Ray Conference Proceedings. J. Appl. Phys.,

Vol. 30, No. 5, 1959, p. 601.

21
CONCURRENCE

ON THE DETERMINATION OF THE ORIGIN OF LINEAR ANOMALY IN THE

MACROSTRUCTURE OF VPPA WELDED 2219-T87 ALUMINUM —

PRELIMINARY STUDY

By W. A. Jemian

Auburn University

E. O. BAYLESS, JR/

Metals Processes Branch

.

A. C. NONES, JR. /

Metal Processes Branch

P. H. SCHUERER

Process Engineering Division

22
APPROVAL

ON THE DETERMINATION OF THE ORIGIN OF LINEAR ANOMALY IN THE

MACROSTRUCTURE OF VPPA WELDED 2219-T87 ALUMINUM —

PRELIMINARY STUDY

By W. A. Jemian

The information in this report has been reviewed for technical content. Review

of any information concerning Department of Defense or nuclear energy activities or

programs has been made by the MSFC Security Classification Officer. This report,

in its entirety, has been determined to be unclassified.

N- fyl^ X-^t^w-y^^-w^— -m^t

R. J. SCHWINGHAMER

 j

Director, Materials and Processes Laboratory

. S. GOVERNMENT PRINTING OFFICE 1986-631-058/20108 23