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Study of Weld Imperfections on Thin Aluminum Tubes

According To ISO 10042

Summer Student internship – 30/06/2014 till 22/08/2014

Presented by:

Rabel Rizkallah

NOTRE DAME UNIVESITY-LOUAIZE, LEBANON

Faculty of Engineering

ME Department

EN/MME/FW

&

EN/MME/MM

Supervisors:

Said Atieh

&

Jean-Michel Dalin

1

Table of Contents

Acknowledgements: ............................................................................................................... 2

Abstract .................................................................................................................................. 3

Introduction ............................................................................................................................ 4

Part I

Aluminum’s Importance ................................................................................................ 5

Used Material ................................................................................................................. 8

The GTAW Process or TIG Welding Process ............................................................... 9

Aluminum Welding: The Difficulties and Precautions ................................................ 11

The Objective and Actual Work .................................................................................. 14

Radiographic and Visual Testing ................................................................................. 16

Metallographic Testing ................................................................................................ 19

Sectioning: .................................................................................................... 19

Mounting: ...................................................................................................... 21

Polishing: ...................................................................................................... 22

Etching: ......................................................................................................... 23

Macroscopic Images and Results ................................................................................. 25

Preparation of the Catalogue’s Forms .......................................................................... 28

Form One ...................................................................................................... 31

Form Two ...................................................................................................... 32

Form Three .................................................................................................... 33

Form Four ..................................................................................................... 34

Finishing of the Catalogue ........................................................................................... 35

Part II .................................................................................................................................... 36

Leak of the VJ Vacuum Chamber of ATLAS .............................................................. 36

Micro-Tomographic Testing ........................................................................................ 37

Advantages and Perspective ......................................................................................... 40

Conclusion ............................................................................................................................ 42

2

Acknowledgements:

I would like to thank both of my supervisors, Said Atieh and Jean-Michel Dalin, for

their constant support and encouragements, and for their availability and help during my

work.

I also wish to thank Aline Piguiet and Alexandre Porret, my office colleagues, who

always found time to help and support me even with their many obligations.

Many thanks also to Frederic Excoffier and Pascal Mesenge who provided me the

samples used for the project and found time to do so even though they had a lot of more

urgent jobs to attend to.

My thanks also go to Alexandre Gerardin for his help and advices regarding the

metallographic testing notably with the polishing and etching processes as well as the use of

the Leica microscope for the macroscopic views.

I also wish to thank the entire MM section with the likes of Stefano Sgobba, leader of

the MM section, as well as Gonzalo Arnau Izquierdo, Ana Teresa Perez Fontenla, Ignacio

Aviles Santillana, Stefanie Langeslag, Barbora Bartova and Floriane Léaux.

3

Abstract

The following report presents the work done as part of my summer student internship

at CERN in the EN/MME group, and is divided into two parts. Part I of the report is a

continuation of the work started previously by the student Quentin Drouhet, which aims at

studying and classifying welding imperfections on various materials of small thicknesses.

Drouhet worked on the welding imperfections found on thin Stainless Steel plates of

thicknesses of 2 and 6 millimeters, welded using the GTAW (or TIG welding) process. My

job was to conduct a similar study on thin Aluminum tubes welded using this same process.

The samples used for the study were made of EN AW 5083 and came with two different

thicknesses of the tubes, of 1.5 and 2.2 millimeters. Studying the welding imperfections was

based on visual testing, radiographic testing and metallographic testing. The classification of

these imperfections in a reference catalogue is important as it helps engineers and technicians

recognize and identify the various imperfections that could be encountered during welding

and compare them to the norm (the ISO 10042 was used) regulating the quality levels. This is

even more important for people working in welding or with the welds, and not familiar with

the norms and standards. Part II of the report will introduce a new method for defect analysis

that is still not available at CERN: the micro-tomographic testing. This method, which

analyses the cross section of a weld for every 15 microns increment in a direction

perpendicular to it, will be tested on a defect found on one of the VJ vacuum chambers of the

ATLAS experiment that was spotted during a leak test. The micro-tomographic testing was

chosen because it allows spotting very small defects that the conventional methods used in

Part I cannot spot. Both Part I and Part II of the report were successful in the sense that the

imperfections and defects were clearly observed and identified. Seven imperfections on four

macroscopic views were obtained in Part I of my work, which went beyond expectations

taking into consideration the lack of available time to work on the project. However, it didn’t

allow the completion of the reference catalogue as more imperfections need to be obtained.

The completion of the catalogue will be done during a future student internship. As for the

micro-tomographic testing, the method allowed us to identify a second leak that wasn’t

spotted at CERN, and presented us with excellent quality radiographic images of the weld

which were used to create a simulation of the defect as one moves through the cross-section,

along the weld. The operation of both leaks was clearly understood, and the method also gave

us the benefit of an important time saving as compared to what would be required if

conventional methods were adopted instead.

4

Introduction

Welding is a very common and important process that enters in virtually any

engineering design at CERN. Also, Aluminum, which is one of the most used materials at

CERN, happens to be one of the most difficult metals to weld because of some of its

properties and characteristics that affect the welding process. Moreover, most of the

Aluminum that is welded at CERN comes in the form of tubes, which is more critical than

plate welding, as imperfections are more susceptible to appear on tube welding. This is the

motivation behind the study of welding imperfections on Aluminum tubes and their

classification in a reference catalogue. The samples used for creating the imperfections to be

studied were thin Aluminum tubes made of EN AW 5083 alloy, and came with two

thicknesses of 1.5 millimeters and 2.2 millimeters. The long term objective of the project

would be to recreate as many imperfections as possible and classify them in a reference

catalogue, which would help engineers and technicians identifying weld imperfections and

compare them to the quality requirements as regulated by the ISO 10042 norm. The work

done in order to realize this reference catalogue will be the subject of Part I of the report.

The procedure used in Part I of the report to study the welding imperfections is based

on visual testing (VT), radiographic testing (RT) and metallographic testing. However, these

methods take time, and don’t allow the observation of very small defects. In the latter case, a

different approach has to be applied in order to be able to see some defects that even if small

can prove critical and lead to important problems. This is addressed in Part II of the report

where a small leak was identified during a leak test on an Aluminum weld in one of the VJ

vacuum chambers of the ATLAS experiment. The conventional methods of Part I proved

insufficient to analyze the defect responsible of the leak, and micro-tomographic testing was

used instead. Details on the obtained results and the advantages this new method provides

will be discussed later on in Part II.

5

Part I

Aluminum’s Importance

Aluminum is replacing Stainless Steel as a material in the newest vacuum chambers

as can be seen in the ATLAS vacuum chambers and studies are ongoing to even replace

Beryllium with some new transparent Aluminum alloys. The reason why we are interested in

Aluminum as a material is because of its excellent mechanical and physical properties. While

Aluminum based alloys and Stainless Steels both present excellent cryogenic properties,

Aluminum has some important advantages over Stainless Steels in the applications related to

vacuum and ultra-high.

Figure 1: Aluminum is the Future

One of the biggest advantages of Aluminum over Stainless Steels lies in its high

strength to weight ratio. Typical density of an Aluminum alloy is of around ,

which is roughly three times less than the density of a typical Stainless Steel material that is

around . On the other hand, Aluminum alloys are typically only two times less

hard than steels, with some alloys even matching steels’ hardness. The combination of this

very low density and quite high strength makes Aluminum an excellent material for

engineering applications, as we still get the required strength, with much less weight. And

less weight means less energy and less money spent. Aluminum is also very ductile and easy

to machine; much easier than steel for instance.

Aluminum vacuum chambers have excellent vacuum and ultra-high vacuum

properties. Aluminum is for instance ten millions of times less permeable to Hydrogen than

Stainless Steel is. Aluminum alloys have also less carbon in their composition than Stainless

6

Steels (all steels have carbon in their composition). This means that Aluminum vacuum

chambers have much less H2, H2O, hydrocarbon vapor and carbon based gas contamination at

ultra-high vacuum level. Aluminum also presents a thin oxide layer that protects the material

from oxidation since this oxide layer is saturated and cannot withstand further oxygen atoms.

Moreover, this saturated oxide layer serves as a barrier that prevents diffusion and desorption

of Hydrogen, Oxygen and Carbon and therefore protects from contamination at vacuum and

ultra-high vacuum levels.

Outgassing rates on Aluminum vacuum chambers have also reached excellent levels

with rates of less than now possible. Aluminum also presents very high

thermal conductivity, in the range of , while all steels have a thermal

conductivity falling between and . This means that baking and then

cooling an Aluminum vacuum chamber will be done much faster than a similar chamber

made of Stainless Steel. Also, because of the high thermal conductivity of Aluminum, a

complete bake out is possible with no risk of condensation of gases on local cool spots as the

entire system will have a uniform temperature change.

Figure 2: Aluminum Tube Chamber

Aluminum is also an amagnetic material which is an advantage when working on

beams made of charged particles, as the Aluminum won’t affect nor the trajectory of the

beam neither the magnetic field of the concerned control magnets. Stainless Steels on the

other hand do exhibit some residual magnetism, as it is an Iron alloy. Moreover, Aluminum

has a much faster induced radioactivity decay compared to Stainless Steels. If bombarded

with the same flux, residual radioactivity one can find in the Aluminum material will be

typically between one and two orders of magnitude lower than what will be found on a

Stainless Steel material.

Finally, let’s point out that ongoing studies are even trying to replace the Beryllium of

the vacuum chambers with some new Aluminum alloys. The reason for this is that Beryllium

7

is toxic, very expensive and difficult to obtain. The main advantage of Beryllium is that it is

extremely transparent, which is a very important property as the detectors are placed outside

the vacuum chamber. New Aluminum alloys do exhibit similar characteristics and research is

ongoing to improve their weldability and mechanical properties.

8

Used Material

The Aluminum alloy used for this part of the report was the EN AW 5083 (or Al

5083) which is one of the most common Aluminum alloys with excellent mechanical and

cryogenic properties. According to the ASM standards, its melting zone goes from to

with a thermal conductivity of and a density of . Its

composition is given in the following table:

Unspecified

other

elements

Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Ga

V

Specified

other

elements

Ti

Each

Total

Al,

minimum

Al

5083

0.40

- 0.7

0.40 0.10 0.4-

0.10

4.0 -

4.9

0.05 -

0.25

… 0.25 … … … 0.15 0.05 0.15 rem

Table 1: EN AW 5083 Composition

In Part II of the report, the sample taken from the VJ vacuum chamber of the ATLAS

experiment was made of two Aluminum alloys welded together: the EN AW 5083 and the

EN AW 2219. The 2219 is also a very used Aluminum alloy which has according to the ASM

standards a melting zone going from 543 to 643 , a thermal conductivity between

and (depending on the thermal treatment) and a density of

. Its composition is given in the following table:

Unspecified

other

elements

Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Ga

V

Specified

other

elements

Ti

Each

Total

Al,

minimum

Al

2219

0.20 0.30 5.8-

6.8

0.20-

0.40

0.02 … … 0.10 … 0.05-

0.15

0.10-0.25

Zr

0.02-

0.10

0.05 0.15 rem

Table 2: EN AW 2219 Composition

9

The GTAW Process or TIG Welding Process

The used process for welding the Aluminum tubes is known as the Gas Tungsten Arc

Welding (GTAW) or more commonly known as the Tungsten Inert Gas (TIG) welding. The

process uses a non-consumable and refractory tungsten electrode and the main idea is to

create an electric arc between it and the base metal. The base metal which is Aluminum in

this case will melt around while the tungsten electrode won’t as its melting

temperature is much higher than that of Aluminum at about .

Figure 3: Schematics of the GTAW Process

During the process, a filler metal is added, to compensate the lost material during

welding. For instance, Magnesium is very easy to evaporate and does just that when the base

metal melts. The filler metal will provide extra Magnesium to cancel out the material loss.

The filler metal will also create the welding bead.

The process uses also inert gas as a protection to the weld pool against oxidation. The

gas used is usually Argon or Helium, or a combination of both, and doesn’t react with the

base metal in any way. This inert gas serves another function as well. That is, it helps

10

maintain a stable arc between the tungsten electrode and the base metal, as it ionizes during

welding.

The GTAW process is used on Aluminum quite extensively as it allows welding of a

large interval of thicknesses going from 0.25 millimeters to 150 millimeters, and it is

relatively slow with a slow solidification in comparison with electron beam welding for

instance. This facilitates the welding of tubes and pipes, and allow for higher degassing and

limits risks of lack of fusion.

The welding quality obtained is affected by many parameters. One of these

parameters is the current intensity or input energy. As a rule of thumb, the current must

satisfy the expression where is the current intensity and the thickness of the

base metal. This will give us a compromise between intensity and penetration depth. Another

important parameter is the choice of the filler metal. The filler metal has to be as close as

possible to the base metal in terms of mechanical properties and composition, in order to

obtain a certain mechanical and structural homogeneity in the weld. Also, the flux of inert gas

has to be proportional to the diameter of the collet with per millimeter of the collet

diameter. The position of the torch is also important, as well as the advance speed and the

type of alimentation, i.e., AC or DC current, and direct or reversed polarity.

11

Aluminum Welding: The Difficulties and Precautions

There are many types of weld imperfections or defects that can occur during welding.

Even though many of these imperfections are common to all welding processes for all

different materials, they are affected by the material that is being welded and by the welding

technique. For instance, tungsten inclusion won’t be found on samples welded with electron

beam welding, and root porosity which can occur on Stainless Steel welding is almost never

found on Aluminum welding.

Aluminum welding presents more difficulties as compared to other materials like

Stainless Steel because of some of its characteristics.

The first difficulty we face when welding Aluminum is the formation of a layer of

Aluminum Oxide at the contact between the Aluminum metal and the atmosphere. This

Aluminum Oxide comes in two layers. There’s the first layer, or inner layer, of Al2O3 very

strongly bounded to the metal, which plays the role of barrier, is saturated and prevent further

oxidation. The second layer, or outer layer, of Al(OH)3 and Al2O3.H2O, respectively bayerite

and bohemite, much thicker than the first layer. Removing the oxide layer before proceeding

to welding is a necessity because its melting temperature of is much higher than the

melting temperature zone of the Aluminum base metal which is around the mark as

was already seen. If the oxide layer isn’t removed, what can happen is that the base metal will

melt without the oxide layer which will lead to a lack of fusion. Moreover, oxide inclusions

can occur. Also, since this oxide double layer formation is extremely fast, it needs to be

removed just before welding. This can be done by mechanical or chemical means. However

removing the oxide layer mechanically is the most adopted solution, and is safer than the

chemical approach as there’s no risks whatsoever of letting the Aluminum base metal

reacting with the etching solutions. During welding, it is also strongly recommended to weld

using alternating current (AC current) to allow for cathodic bombardment. That is, during the

positive half-cycle, the base metal will be bombarded by ions instead of electrons, which will

break and disperse the forming oxide. This doesn’t mean that using DC current isn’t possible

of course, and DC current is indeed used to weld Aluminum with Helium as the inert gas

because it is much hotter than argon.

Another difficulty of Aluminum welding is the big Heat Affected Zone, or HAZ as

compared to other metals like Stainless Steel. This is due to many properties of Aluminum

including the big thermal expansion and the spread of the melting zone, as well as the

welding technique, as the TIG welding leads to a higher HAZ than electron beam welding for

example. Because of this, and because Aluminum doesn’t show any color change before

melting, special attention needs to be given to the input energy and the advance speed. If the

energy is too high, or the advance speed to low, the HAZ will be bigger, allowing segregation

to happen, which will weaken the base metal, and may lead to cracks. Preheating can improve

the penetration depth and the advance speed, which will facilitate welding.

12

Finally, the biggest source of imperfections in Aluminum welding is the huge

solubility of Hydrogen in melted Aluminum. When Aluminum is in its solid state, Hydrogen

solubility is negligible and almost inexistent. But as soon as Aluminum melts, this solubility

increases drastically. The following plot shows the solubility jump of Hydrogen in pure

Aluminum which melts at .

Figure 4: Hydrogen Solubility in Aluminum

Hydrogen solubility in Aluminum alloys follows the same logic. And because of the

big thermal conductivity of Aluminum, the Hydrogen has little time to escape, and gets

trapped inside the Aluminum as it solidifies. This is the main source of porosities and is often

seen in Aluminum welding. This is a much more critical issue with electron beam welding

because as was mentioned already, TIG welding is slower with relatively slow solidification

as compared to electron beam welding which has a very fast solidification rate. This is why

one usually goes to TIG welding to avoid trapping Hydrogen in Aluminum. But porosities

due to trapped Hydrogen in the base metal are observed in TIG welding nevertheless, and

special attention should be taken in order to avoid it. Humidity, oils, paints… are all sources

of Hydrogen and must be removed before proceeding to welding. Preheating the base metal is

also recommended as it helps remove humidity from it.

Of course, as for all other metals, Aluminum welding is affected by the welding

parameters used. The energy input, the advance speed, the inert gas protection and choice of

the gas, the choice of filler metal, the position of the torch, etc. … are all parameters that

13

affect the welding process. If these parameters aren’t well chosen, welding imperfections and

defects can occur in the weld.

The following table lists both causes and precautions of weld imperfections

commonly obtained with TIG welding off Aluminum.

Weld imperfection Cause Precaution

Cracks Weld energy too high, rapid cooling,

incompatible filler metal…

Preheat before welding, reduce the

weld energy and advance speed,

choose a compatible filler metal…

Porosities Humidity, oils, paints… Insufficient

gas protection

Preheat the base metal, remove all oils

traces…

End crater pipe Brutal end of the weld Reduce the energy intensity

progressively as the weld end is

reached and make sure to fill the crater

Lack of fusion Energy too low, bad positioning of

the electrode, bad removing of the

oxide layer

Increase the energy, reduce the

advance speed, readjust the electrode

position, make sure to remove the

oxide layer and weld immediately

after doing so, allow cathodic

bombardment

Lack of root penetration Energy too low, advance speed too

high

Increase the energy and reduce the

advance speed

Continuous/intermittent

undercut, shrinkage

groove

Energy too high Increase the advance speed, reduce the

energy

Excess penetration Energy too high, separation of the

two pieces to be welded too big,

advance speed too low

Reduce the energy, reduce the distance

between the base metal pieces,

increase the advance speed

Sagging/Incompletely

filled surface layer

Energy too high Reduce the energy

Root concavity Protection gas flux too important Adjust the gas flux

Uniformly distributed

porosity

Tungsten inclusions Contact between the Tungsten

electrode and the base metal

Avoid the contact between the

electrode and base metal during

welding

Oxide inclusion Bad removing of the oxide layer Make sure to fully remove the oxide

layer just before welding, weld using

cathodic bombardment

Lack of penetration Energy too low, advance speed too

high

Increase the energy, reduce the

advance speed

Table 3: Welding Imperfections’ Causes and Precautions

14

The Objective and Actual Work

The objective of Part I is to recreate as many defects as possible on welded

Aluminum tubes, welded using the GTAW process. For this, two EN AW 5083 alloy tubes

were used. One tube with a 1.5 mm thickness and an 83 mm diameter, and a second tube with

a 2.2 mm thickness and a 124.6 mm diameter. Each tube came with three sample welds on

them: on the 1.5 mm thickness tube, the three welds were named respectively A, S1 and S2,

and on the 2.2 mm thickness tube, the three welds were named A2, S3 and S4.

Figure 5: View of Weld A Figure 6: View of Weld A2

Figure 7: View of Weld S1 Figure 8: View of Weld S2

Figure 9: View of Weld S3 Figure 10: View of Weld S4

15

The procedure to analyze these welds consisted in running both visual and

radiographic testing on the welds, to identify the imperfections and choose the typical ones to

be used in the reference catalogue we’re aiming at. Once the imperfections are chosen, the

welds will be sectioned to isolate the imperfections on small samples that will be prepared for

the metallographic testing that comes before the macroscopic observation. Each imperfection

will be listed in a form of the reference catalogue, along with its corresponding radiographic

and macroscopic views, as well as an actual picture of the weld as it looks from the outside

and inside of the tube at the position of the imperfection, and a comparison to the ISO 10042

to assess its quality level.

The following sections will detail these procedures.

16

Radiographic and Visual Testing

The radiographic testing happened at the radiographic lab of building 376. The choice

of the technique and parameters are regulated by the norm ISO 17636-2. For all of the six

welds, the perpendicular technique was used with a current intensity of 4 mA and a distance

source to film of 50 cm. For welds A and A2, the voltage was of 45 kV, while for welds S1,

S2, S3 and S4, the voltage was of 50 kV. The exposure time for each of the three welds found

on the 1.5 mm thickness tube, that is welds A, S1 and S2, was of 60 seconds. The exposure

time of the other three welds on the 2.2 mm thickness tube, that is A2, S3 and S4 was of 120

seconds.

Figure 11: Setup to Take the Radiographic Images

17

Also, in accordance to the ISO 17636-2, an image quality index (IQI) was used to

assess the quality of the radiographs and determine whether or not interpretation of the

radiographs is possible. The IQI used was the Al 13.

This way, 33 digital radiographs were obtained as follows:

7 radiographs for weld A

6 radiographs for weld A2

7 radiographs for weld S1




Figure 12: Radiographic Image of Weld A2

Figure 13: Radiographic Image of Weld S1

18

Figure 14: Radiographic Image of Weld S3

Even though it was initially planned to do argentic radiography too and compare both

the digital and argentic radiographs, argentic radiography was judged unnecessary and wasn’t

carried out. This is because the welding defect were very clear on the digital radiographs and

no complementary information was needed to judge what type of imperfection or defect we

were dealing with.

Simple visual testing (VT) was used to confirm what was observed on the digital

radiographs, and helped identify some imperfections that weren’t apparent on them. For

instance a superposition of an excess penetration and a sagging made it invisible on the

radiographic images and was identified by VT instead. On the other hand, one imperfection

that will be analyzed and shown later on in this part of the report seemed to be a continuous

undercut on the radiographic image, while visual observation led us to believe it was a

sagging instead. After the macroscopic view was taken, it appeared to be both of these

defects.

The VT is regulated by the ISO 17637 norm. According to this norm, the minimum

luminosity requirement should have a value of 350 Lux, with a preference of a luminosity of

the order of 500 Lux. The norm also recommends to observe the samples with a 30° angle

between the eye and the sample’s surface. To facilitate the VT and improve its quality, a

magnifying glass and an additional light source were used.

It is by combining both the radiographic images and the visual observations that the

choice of typical imperfections to be analyzed was done.

19

Metallographic Testing

After identifying the weld imperfections using visual and radiographic testing, typical

imperfections from each type were selected to be analyzed and included in the reference

catalogue. This section of the report will detail the successive steps that led to the

metallographic testing and macroscopic observation of the imperfections. The procedure

consists of four steps before the macroscopic observation: sectioning, mounting, polishing

and etching.

Sectioning: The first step consists in sectioning the Aluminum tube to isolate the desired

imperfection. Since there were three welds on each tube, the tubes were first cut in three rings

each, each of the rings containing one weld. After that, each weld was sectioned at the

position of the typical imperfection previously chosen. Before obtaining the welded tube

samples, I tried cutting some Aluminum tubes on the cutting machine of the mechanics lab of

building 376.

Figure 15: The Cutting Machine in Building 376

However, this turned out to be risky as the machine isn’t optimized for cutting tubes

and is more appropriate for plates’ cutting. Moreover, because of the low thicknesses of the

tubes, cutting with a straight section wasn’t easy if not undoable as the tubes bent the further

20

the blade penetrated the material. So it was decided to section the tubes by hand, with a saw,

to avoid damaging the imperfection during this process.

Figures 16 and 17: Cutting the Samples with a Saw

And because the samples are to be polished later, and since polishing does remove

some material, sectioning was carried out at about 2 millimeters away from the actual

imperfection. This way, 15 samples were created and named according to the corresponding

weld and their position on this weld. For example, the sample S2 08-10 is from the weld S2

of the 1.5 mm thickness tube, at a position between 8 and 10 centimeters from the chosen

origin on this particular weld.

Figure 18: Sectioning of the 1.5 mm Thickness Tube

21

Mounting: The sectioned samples were mounted in a methyl methacrylate resin to allow

manipulation for polishing. This resin is transparent and created by mixing two doses of

powder with one dose of liquid of “Technovit 4006”. Each sample is placed in a small mold

and the resin is poured on them to fill the mold completely. Note that the cross-section to be

analyzed has to be placed in contact with the bottom of the mold so that it remains a free

surface after the resin solidifies. The molds are then placed in a recipient creating a high

pressure of 2 atmospheres for 30 minutes, which allows the resin to solidify without humidity

contamination from the atmosphere. All 15 of the sectioned samples were mounted this way.

Figures 19, 20 and 21: Mounting in the Resin under High Pressure

Figure 22: The 15 Mounted Samples

22

Polishing: Polishing the Aluminum samples happened on successive rotating disks, each

mounted with an abrasive paper of silicon carbide with different grit size for each disk. The

six disks that were used are in order the P80, P180, P320, P400, P600 and P1200 grit. The

grit size went down with each of the disks. Finally, two finishing steps of polishing were

carried out, using successively a two micrometer diamond paste and a one micrometer

diamond spray.

Figure 23: Polishing with the 2M Grit

The following table shows the evolution of the grit size on the successive polishing

steps:

Polishing step Grit size Average particle

diameter (µm) Lubricant

1 P80 201

Water

2 P180 82

3 P320 46,2

4 P400 35

5 P600 25,8

6 P1200 15,3

7 2M 2 Diamond

8 1M 1

Table 4: Polishing Parameters

23

The objective of polishing is to obtain a perfectly mirror surface with no rays

whatsoever. For this, polishing on a disk isn’t stopped until all the rays are in the same

direction. And when we move to another disk, the sample is rotated by 90 degrees and

polishing is carried out until all the previous rays have disappeared. After polishing with the

2M grit, no rays should be observed on the sample.

Due to lack of time, only four samples out of the fifteen mounted samples were

polished, and therefore only these four were etched as will be described below. The four

samples in question are:

S1 06

S2 22-25

S3 38-40

S4 02-06

Figures 24, 25 and 26: Respectively Polished Samples S2 22-25,

S1 06, S3 38-40 and S4 02-06

Etching: Etching is an electrolytic attack done on the samples in order to reveal the

microstructure and details that weren’t visible, or at least weren’t clear, previously. For our

purpose, etching will help accentuate the shape of an imperfection to make it easier to see.

Figure 27: The Etching Setup

24

Etching is regulated by the ASM-E407 standard that lists the possible attacks with

their functions for various alloys and type of materials. The attack used on the EN AW 5083

samples was etching with HBF4, the tetrafluoroboric acid, etchant number 5 according to

table 2 of the E407 standard, as shown in Table 4 below.

Metal Etchants Uses

Aluminum

Base:

Pure Al 1a, 2, 3

4, 5

1b

general structure

grain structure under polarized light

grain boundaries and slip lines

1000 series 1a, 3, 2

4, 5

6, 7

general structure


phase identifications

2000 series 3, 2, 1a

8a, 6, 7

general structure


3000 series 3, 1a

4, 5

8a, 6, 7

general structure



4000 series 3, 1a general structure

5000 series 3, 1a, 2, 6, 8a

4, 5

general structure


6000 series 3, 1a, 2, 6, 8a,

222

4, 5

1a, 2, 7, 6, 8a

general structure



7000 series 3, 1a, 2

4, 5

3b, 6

general structure



Table 4: Possible Etchants with Their Functions According to Table 2 of the E407 Standard

The following table resumes the parameters chosen for etching.

Used Acid HBF4

Potential Difference 22V

Current Intensity 2.2A

Etching Time 30 s

Table 5: Etching Parameters

All four of the polished samples were etched this way.

25

Macroscopic Images and Results

Following the etching of the four samples, macroscopic images were taken using the

stereomicroscope Leica MZ16 found in building 599.

Figure 28: Taking of the Macroscopic Views

From the observation of these macroscopic views, seven imperfections were identified,

some of which weren’t predicted by simply observing the radiographic images. The obtained

imperfections were:

Lack of penetration

Alignment defect

Linear porosities

Localized gas pore

Excess weld material

Root undercut

Sagging

The following images show these seven imperfections on the four macroscopic views

taken.

26

Figure 29: Macro of S1 06

Figure 30: Macro of S2 22-25

Lack of Penetration

Alignment Defect

Linear Porosities

27



Gas Pore

Excess Weld Metal

Root Undercut

Sagging

28

Preparation of the Catalogue’s Forms

It is by combining the corresponding radiographic and macroscopic images of a particular

imperfection that a form is created in the reference catalogue. In addition, real pictures of the

weld from both the upper part and root part are included. Finally, since the imperfection is

already identified, what remains is measuring its size and comparing it to the quality level

requirements of the ISO 10042 norm, i.e., quality levels B, C and D, where the level B is the

most strict and the level D the least one.

The design of the catalogue’s forms took the forms of the Stainless Steel reference

catalogue based on the ISO 5817 as a model, and slight changes were made. The changes

mainly concerned the removing of the argentic radiographs that weren’t taken, and turning

the form to portrait view.

In order to compare the imperfections to the quality level requirements B, C and D of

the ISO 10042 norm, the size of the imperfections has to be measured. This was done directly

on the stereomicroscope Leica MZ16. To illustrate this, here are the pictures showing the

obtained measures for the four polished samples that have been analyzed.

Figure 33: Imperfections’ Dimensions on S1 06

29

Figure 34: Imperfections’ Dimensions on S2 22-25

Figures 35 and 36: Imperfections’ Dimensions on S3 38-40

30

Figure 37: Imperfections’ Dimensions on S4 02-06

With these measures of the imperfections’ dimensions, comparing with the quality

level requirements of the norm ISO 10042, i.e., quality level requirements B, C and D, was

done, and the results were placed in a table in a similar fashion as what can be seen in the

Stainless Steel reference catalogue based on the ISO 5817. Reading the results from the table

can be done by following the convention shown in Table 6 below.

B, C + D = Acceptance levels. a = Reference No. of ISO 10042. b = Reference No. of

ISO 6520-1. RT = Radiography testing. VT = Visual examination. S = Macroscopic

testing. X = not permitted.

Table 6: Convention for the Forms’ Table Reading

Next are the four forms that were created from the work described up till now.

31

FORM ONE

Designation Quality level

a b D C B

Su

rfa

ce I

mp

erfe

cti

on

s

Cracks 1.1 100

Crater cracks 1.2 104

Surface pore 1.6 2017

End crater pipe 1.7 2025

Lack of fusion 1.8 401

Lack of root penetration 1.9 4021 X X X

Continuous undercut 1.1 5011

Intermittent undercut 1.1 5012

Excess weld metal 1.11 502

Excess penetration 1.1 504

Sagging / Incompletely filled surface layer 1.2 509 / 511

Root concavity 1.18 515

Shrinkage groove 1.18 5013

Inte

rn

al

Imp

erfe

cti

on

s Cracks 2.1 100

Gas pore 2.3 2011

Uniformly distributed porosity 2.4 2012

Clustered (localized)porosity 2.5 2013

Linear porosity 2.6 2014

Elongated cavities / wormholes 2.7 2015 / 2016

Tungsten inclusions 2.9 3041

Lack of penetration 2.11 402

Imp

erfe

cti

on

s

in J

oin

t

Geo

metr

y

Linear misalignment 3.1 507 VT/S VT/S X

B, C + D = Acceptance levels. a = Reference No. of ISO 10042. b = Reference No. of ISO 6520-1. RT = Radiography testing. VT =

Visual examination. S = Macroscopic testing. X = not permitted.

Radiography Parameters

Technique Perpendicular with inner film

Distance from Source 50 cm

Current 4 mA

Voltage 50 kV

Exposure Time 60 s

Tube Characteristics

Base Metal EN AW 5083

Filler Metal EN AW 2319

Thickness 1.5 mm

Diameter 83 mm

Etching

Acid HBF4

Voltage 22V

Current 2.2A

Time 30 s

32

FORM TWO


a b D C B

Su

rfa

ce I

mp

erfe

cti

on

s

Cracks 1.1 100





Lack of root penetration 1.9 4021








Inte

rn

al

Imp

erfe

cti

on

s Cracks 2.1 100

Gas pore 2.3 2011



Linear porosity 2.6 2014 X X X




Imp

erfe

cti

on

s

in J

oin

t

Geo

metr

y

Linear misalignment 3.1 507






Current 4 mA

Voltage 50 kV

Exposure Time 60 s




Thickness 1.5 mm

Diameter 83 mm

Etching

Acid HBF4

Voltage 22V

Current 2.2A

Time 30 s

33

FORM THREE


a b D C B

Su

rfa

ce I

mp

erfe

cti

on

s

Cracks 1.1 100








Excess weld metal 1.11 502 X X X





Inte

rn

al

Imp

erfe

cti

on

s Cracks 2.1 100

Gas pore 2.3 2011 S S X







Imp

erfe

cti

on

s

in J

oin

t

Geo

metr

y







Current 4 mA

Voltage 50 kV

Exposure Time 120 s




Thickness 2.2 mm

Diameter 124.6 mm

Etching

Acid HBF4

Voltage 22V

Current 2.2A

Time 30 s

34

FORM FOUR


a b D C B

Su

rfa

ce I

mp

erfe

cti

on

s

Cracks 1.1 100










Sagging / Incompletely filled surface layer 1.2 509 / 511 X X X


Shrinkage groove 1.18 5013 X X X

Inte

rn

al

Imp

erfe

cti

on

s Cracks 2.1 100

Gas pore 2.3 2011







Imp

erfe

cti

on

s

in J

oin

t

Geo

metr

y







Current 4 mA

Voltage 50 kV

Exposure Time 120 s




Thickness 2.2 mm

Diameter 124.6 mm

Etching

Acid HBF4

Voltage 22V

Current 2.2A

Time 30 s

35

Finishing of the Catalogue

The four forms that were done aren’t enough to complete the reference catalogue.

More forms showing more weld imperfections and comparing them to the ISO 10042 norm

need to be created, in a similar fashion as what was done here in Part I of the report. These

forms can use the mounted samples that weren’t polished yet to study the other desired

imperfections, as the samples that were sectioned were selected in order to show as many

types of weld imperfections as were available on the initial tubes. That mean that the

remaining mounted samples should present us other types of weld imperfections that would

complete the catalogue. The samples need to be polished and etched, before taking

macroscopic views.

Note however that other welds might be necessary if all possible imperfections that

could be obtained on Aluminum welding want to be referred to in the catalogue. Indeed,

some imperfections like the lack of fusion weren’t spotted on the radiographic images.

This will be the subject of another student internship.

36

Part II

Leak of the VJ Vacuum Chamber of ATLAS

One of the most important roles the tests of the MM section have is spotting leaks.

Vacuum chambers need to be perfectly isolated from the outside to create as much as

possible vacuum or ultra-high vacuum in the inside. Vacuum tests consist in running

Hydrogen in the vacuum chamber and then check if this Hydrogen is detected by the

detectors placed outside of the chamber. If it is, then the vacuum chamber is leaking.

This is how a leak was spotted on one of the VJ vacuum chambers of the ATLAS

experiment. The defected piece consisted of two types of Aluminum alloys, the EN AW 5083

and the EN AW 2219, welded by electron beam welding.

The weld was isolated on a ring as shown on Figure 38, and sent to be analyzed by

radiographic testing and penetrant testing. However the order of magnitude of the leak was of

10-7

mbar.l/s which was too small to catch by the testing methods available to the NDT team

at CERN.

Figure 38: The Defective Weld of the VJ Vacuum Chamber

37

Micro-Tomographic Testing

The weld was therefore sent to RX-solutions to follow a micro-tomographic testing.

The micro-tomographic testing consists in taking high quality radiographic images of the

cross section of the weld by moving 15 micrometers for every other new image. This way, a

complete mapping of the weld is done, and the radiographic images are used to recreate the

weld from various viewing angles, as well as creating a simulation of the cross-section of the

weld as we move along the welding bead by looking at the cross-section.

For this, the weld was sectioned to isolate the defected piece where the leak was

known to take place. A small piece of Copper, as can be seen on the purple mark in Figure

39, was attached to the sectioned sample to serve as a reference point during the testing. That

is because Copper appears much brighter than Aluminum on the radiographic views, due to

its higher density.

Figure 39: The Sectioned Piece Containing the Defect

The quality of the obtained radiographic images from the micro-tomographic testing

was as high as it could get, and the leak as well as its operation appeared clearly on them.

Moreover, it turned out that there were two main leaks on the weld.

The first leak, which was already spotted at CERN, was due to a huge crack that went

from one surface of the defective weld through the entire thickness all the way to the other

side. That led to the outer surface of the tube being connected to the inner surface through

38

this crack, and therefore created the leak. The micro-tomographic testing also showed us that

this crack emerged to the inner surface at several locations too.

The following three figures will show the leak due to this crack going from outer

surface to inner surface with Figure 40 showing various viewing angles of the defect, and

figures 41 and 42 pointing it out with a red arrow.

Figure 40: Leak Zone One


39


The problem faced at CERN with analyzing the crack of this defected piece is

that it ran along the extremity of the welding bead and was therefore difficult to see during

the radiographic testing. Penetrant testing didn’t work quite as expected as well because a

leak of the order of 10-7

mbar.l/s is just too small for such techniques. That’s why it was

decided to rely on the micro-tomography.

40

The second leak, that wasn’t detected at CERN and was only spotted thank to the

micro-tomographic testing, was due to the same crack having one of its many ramifications

running into a gas pore that was itself connected to a surface pore. This again connected the

outer surface to the inner surface, creating a leak.

The next figure will show the second leak zone, which is the one due to the crack

running into two porosities with the second of these porosities emerging at the surface. Note

that the size of the porosities is quite large. Porosities are more difficult to avoid in electron

beam welding as compared to TIG welding because of the much higher energy involved and

the faster cooling rate. This means that the outgassing is less effective in electron beam

welding and therefore, gas is more easily trapped. This is why some research is being

conducted nowadays to try to modify electron beam welding by reducing the input energy

and find a compromise that will increase the outgassing rate and therefore reduce the risk of

obtaining porosities.

Figure 43: Leak Zone Two

Advantages and Perspective

The micro-tomographic testing is fast. Analyzing this defect took one hour and a half

to completion. On the other hand, if we wanted to study it by applying the procedure I

followed in Part I, that is, cutting, mounting and then polish every 15 micrometers and

etching, before polishing again 15 micrometers and etching again till we finish the entire

piece, it would have easily taken a month to finish. This is considering that the work is so

precise that we won’t miss the leak by polishing too much. By “too much” I mean more than

15 micrometers.

41

Moreover, the quality and precision of the obtained images were incredibly high,

much higher than what can be attained at CERN.

Micro-tomographic testing however is expensive. Testing this small weld section of

about three centimeters long has cost 1100 euros. However, taking into consideration the time

saving and the results’ quality, this is probably worth the money.

42

Conclusion

Part I of the report ended up being the start of the reference catalogue’s development.

Because of lack of time since the weld samples didn’t arrive before the last two weeks of my

internship, the complete catalogue couldn’t be finished. However, what was done was very

satisfying with seven imperfections identified on four polished samples. Another eleven

samples containing other types of imperfections are already mounted, and only need to be

polished and etched before taking the macroscopic views. Also, four catalogue forms were

created and will serve as a model to follow for the rest of the catalogue. This will be the

subject of another student internship.

The development of this catalogue is just another step of the work started by Quentin

Drouhet and myself, as other reference catalogues of welding imperfections are planned to

follow, concerning both welding of thin plates and thin tubes, and on various materials like

Stainless Steels, Aluminum and Copper, and extending to other welding procedures than TIG

welding like electron beam welding for instance.

Part I of the report relied on the conventional methods for welding imperfections’

analysis, notably RT, VT and metallographic testing. These procedures, along with penetrant

testing and ultrasonic testing, are effective for relatively large imperfections and provide us

satisfying results in that case. Using micro-tomographic testing to analyze the samples used

in Part I would be unnecessary and pointless as the method is too precise for this, and that

would be a huge waste of money. However, these methods can’t quite deal with very small

defects like the leak of the VJ chamber of ATLAS showed us. The leak was identified during

a leak test, but studying the defect turned out to be very difficult because of the placement of

the crack and the order of magnitude of the leak which at 10-7

mbar.l/s was too small. This is

where micro-tomographic testing becomes interesting as it provides a degree of precision in

defect analysis that cannot be reached with the conventional methods, along with an excellent

quality of imaging and a huge time saving.

Now it is true that the engineering department of CERN isn’t a very big one that can

test everything or move to mass production for example. The EN department’s main function

is testing, and if it proves too difficult, the samples are sent to an outside company to be

analyzed there since not all the techniques are available at CERN which isn’t an engineering

facility anyway. However, it is very important to be able to analyze the smallest of defects as

they can be the cause of huge problems with catastrophic consequences. We all remember

what happened in 2008 at the LHC because of a defective solder joint. So the addition of a

technique like the micro-tomography, allowing CERN to spot and analyze such small defects

as the leak observed in Part II, might prove a good investment for the future.

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