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PWL #035 - FOW, Nanofoil Bonding, Welding Stainless, UT TOFD, Robotics, Variable Polarity
July 01, 2006
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Forge Welding, Nanofoil Bonding, Welding Stainless, Ultrasonic Testing Time of Flight Diffraction, Robotic Arc Welding, Variable Polarity Welding Aluminum Pipe and more...

This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers.

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Date: July 2006 - Practical Welding Letter - Issue No. 35


1 - Introduction

2 - Article: Forge Welding

3 - How to do it well: How to weld stainless steel.

4 - Nanofoil Bonding

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article: UT TOFD

8 - Site Updating: Robotic Arc Welding

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contribution: Variable Polarity Welding Aluminum Pipe

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board

1 - Introduction

Hello everyone, thank you for reading on. A reader wondered why he missed the last issues. Well, we assured him that it was not our fault as his address appears on our list. Many things may happen, especially now that all are chasing unsolicited e-mail. So make sure we are not blacklisted.

We thought that the oldest known welding process merited a brief mention, and here it is. Even if you are not going to use it shortly, it is a nice thing to know and remember, especially for young people. Then you will find an example of how NOT to ask a question. If it is not defined the answer cannot be significant.

It is good to be able to present from time to time something new. In fact it may be around already for a few years, but new applications are just now being developed. It is called Nanofoil Bonding and its use will certainly expand.

A specialized ultrasonic technique is briefly introduced. It has some amazing capabilities but still much development has to be accomplished. Its name is UT TOFD, if you need it you will remember and explore it.

We dedicated our new Page of the Month to Robotic Arc Welding. It has much potential, but to reap the profits one must work hard to realize the promise through accumulation of expertise.

Variable Polarity is expanding applications from aerospace to more down to earth uses. It may be helpful to see if it can be used profitably for your next job. Other sections are still at their place.

Enjoy your reading and keep in touch, do not send back to us the whole letter just for using the replay address! Use the Contact Us button instead, that you will find in the NavBar (the navigation bar) at top left of each of our Welding Advisers website pages.

2 - Article: Forge Welding

Forge Welding was a wonderful sight, cherished by myself till now as a vivid child memory dating back to WWII. I remember having seen it as a youngster, from the entrance of their village forge shop, performed by a pair of cheerful strong blacksmiths, the elderly father and the middle aged son.

I wish I could reproduce the scene for a short video, because, among flying sparks and deafening hammer blows, it gave an impression of force, beauty, accomplishment and creativeness. Many would become excited as I was at the view.

It may have been connected to material shortage due to the war, in any case the work I saw consisted in connecting end to end two short pieces of a bar to make a more sizeable one that was needed for some other job.

It looked simple if compared to high tech, but as a mature art it contained ancient wisdom handed down through generations of artisans, and could be performed only with much skill and experience.

The ends of two short bars were introduced into the coal heated hearth fire rekindled by the air of a mechanical blower, and left there until they reached sparking white color temperature, much higher than what needed for regular forging.

The bars were then swiftly removed from the hearth, one by each man, to the anvil. The heated ends were superposed while mighty hammer blows were directed in rapid succession to the lump with perfect timing, by each blacksmith in turn.

One of the two, I suppose it was the father, would slowly turn the bar from all sides while hammering, the other one only supporting it without interrupting his part of blow delivering.

In a few short minutes, the two pieces were now welded in a single bar and the lump was gone.

The bright light that shone white only a few instants ago quickly dimmed to a dark red. The weld finally became a kind of gray but still too hot to be touched.

The two men, sweating at their labor, would now smile satisfied with the outcome and put the newly welded bar aside to let it cool down.

Although the process is less used now than more modern and productive welding methods, Forge Welding (FOW) is still listed in textbooks as the first known welding process. It dates back to about 4000 B.C. (according to the Welding Handbook) and it remained the only known process until late in the nineteenth century.

It is described as a solid state process, meaning that melting temperature is not reached. It is autogenous welding, as additional filler metal from an external source is not needed.

Hammering or exerting pressure by other means is necessary to expel scale and surface contamination and to upset the material to promote intimate contact between the separate parts to realize the welding.

3 - How to do it well: How to weld stainless steel.

Q: What shall I use to weld stainless steel?

A: This question, which I actually received from a reader, is unfortunately not sufficiently defined: therefore it is not possible to give a meaningful answer. One must first describe a few parameters to qualify the solution requested.

Material: as explained in another page on Stainless Steel Welding (opens a new page), there are many different types of steels loosely responding to this category, but they can be grouped in four or five families having important characteristics in common.

Each family/type has to be addressed separately as they behave differently during welding and need specific instructions.

Therefore, before tackling a job, one has to know positively or to have analyzed qualitatively the stainless steel type involved, for identifying at least its family.

Joint: type and dimensions of the needed joint must be stated. One should pay attention to the deformations that may develop as a consequence of welding and take proper precautions.

Process: If we think of a small shop and of an occasional job coming up once in a while, we will try to adapt whatever process is available that will give an acceptable solution. If we have a larger shop with plenty of equipment to choose from, and with experienced workforce with the needed skills, we will be able to select in complete freedom. If we are planning for mass production we will be able to purchase the equipment capable of the most cost effective welding.

Consumables need to be suitable both for base material and for process selected.

Of the common processes that can be used to weld stainless steels we will consider only three:

  • the Shielded Metal Arc Welding (SMAW) or Manual Metal Arc (MMA) with covered electrodes, see Shielded Metal Arc Welding Tips (Opens a new page). This manual process is the first to think of, if the material is not extremely thin. By using multiple passes one can weld substantial thickness.

  • the Gas Tungsten Arc Welding (GTAW or Tig) with nonconsumable tungsten electrode, see Tig Welding Tips (Opens a new page). This manual or mechanized process can produce very clean welds, as needed for food or pharmaceutical industries. It is not used, generally, for thick materials except for the first pass.

  • the Gas Metal Arc Welding (GMAW or Mig) with consumable electrode, see Mig Welding Tips (Opens a new page). This process provides higher deposition rate than the two above, and is best used for industrial applications on substantial thickness or over a root pass made by GTAW or GMAW. Can be used for Robotic Arc Welding (Opens a new page).

One should note that the old time Oxyacetylene Gas welding process, requiring the use of fluxes for removing the oxidized layer, should not be considered for quality welds.

Filler metals for stainless steels are briefly introduced in an article in Practical Welding Letter No.02 for October 2003, visible by clicking on PWL#002.

Standards for stainless filler metals are listed in the page on Stainless Steel Welding indicated above.

4 - Nanofoil Bonding

A new joining technology, emerged from long academic research and industrial development, is now being introduced for high temperature brazing or soldering of critical heat management components in electronic applications. Other uses are currently developed for military and space usage.

The heart of the method is a foil built of hundreds of alternate layers of aluminum and nickel, each layer a few nanometer thick, obtained by vacuum deposition using physical vapor techniques such as magnetron sputtering and electron beam evaporation.

When the foil is ignited by one of several means, it develops an exothermal self propagating reaction at controlled speed and temperature depending on its exact makeup. This precisely localized heat source can be utilized for joining applications, in a way similar to other long known thermite reactions.

If the foil is sandwiched tightly pressed between preplaced solder or brazing alloy layers deposited on the surfaces of metallic or ceramic elements to be joined together, the localized heat will produce complete melting of the filler metals and will finish the joining process without heating the delicate structures involved.

A page summarizing the history of this development can be seen at

For more information on the Company and the People behind the technology see:
A list of downloadable Technical Papers is available at

An article describing the development of an application of this technique is reported in the first item of the following section.

5 - Online Press: recent Welding related Articles

Metallic bonding of Ceramic Armor using Reactive Multilayer Foils

Hybrid Laser Mag Welding of thick Steel

From AWS Inspection Trends - The Answer is...

Hardfacing by Oxyacetylene Welding

Rapid Prototyping

6 - Terms and Definitions Reminder

Brazing Procedure Specification (BPS) is a document specifying all the parameters to be established and controlled during a brazing operation for a specific application.

Constricted arc is a plasma arc squeezed by the passage through the constricting orifice of a nozzle.

Dilution is the modification of the chemical composition of a welding filler metal by the intermixing with the portion of base metal melted during the fusion welding process.

Discontinuity, also called flaw, is any occasional or inadvertent lack of physical continuity in the structure. That imperfection is considered a defect only if it may prevent the designed performance of the weldment , consisting in sustaining and transmitting the service stresses without risk of failure.

Fixture is a device that holds elements together in their correct relative position before and during welding.

Protective Atmosphere is a means of insulating parts from the deleterious influence of the normal air atmosphere during high temperature operations. It may be a vacuum or a controlled gaseous envelope in which the treated parts are immersed.

Stop-off is a material locally painted or sprayed to cover selected surfaces for preventing the influence of process materials or elements in those areas.

Weldability refers to the ease of a material to be welded under a specified procedure into a defined structure, capable of sustaining the designed service stresses without failure.

7 - Article: UT TOFD

The nondestructive Ultrasonic Testing technique called Time of Flight Diffraction, is a recent development suitable for examination of welds on thick pipes and pressure vessels.

Time of Flight Diffraction (TOFD) inspection employs two longitudinal (compression) wave angle beam transducers or probes placed symmetrically in tandem, one facing the other on both sides of the weld.

During inspection, the transducers (one transmitter, the other receiver of acoustic energy), connected as a pair by some mechanical means, are moved along the weld while maintaining the relative positions.

The signals arriving at the receiver after a pulse of ultrasonic energy is emitted by the first probe and displayed as Radio Frequency (RF) signals on a screen or on a strip recorder are as follows:

The first signal recorded represents the surface (lateral) wave (generated through mode conversion) trace that runs along the upper surface.

The last one represents the back echo from the far side. Actually the back echo appears as two signals, one caused by direct reflection, the other created by mode conversion on the far side surface.

In between would appear signals, diffracted in all directions by both extremities of a discontinuity if present. First the signal representing the beginning of the flaw, near to the top surface, then the far side signal, emitted by the other end. Their presence along the time (of flight) coordinate permits establishing the exact flaw location.

This technique shows promising capabilities, especially relative to the determination of defects size and location, but is not without problems.

The presence of surface signals obscures those of eventual flaws near both (near and far) surfaces by generating dead zones, so that the volume inspected must exclude a certain thickness at both sides.

Detection depends on flaw geometric shape. Best found are edge delimited flaws, like cracks. But rounded weld defects, like pores or holes that do not generate diffracted signals, will not be detected by TOFD.

While scanning signals are detected in real time, their interpretation requires complex post processing and interpretation that tends to take time. For exact location, ultrasound velocity should be independent of propagation direction, but this may not be the case for anisotropic materials. Correcting this condition for finding the real location may need further post processing.

Small transducer size and narrow beams improve spatial resolution by reducing pulse width, but lengthen the time for inspection. Furthermore, smaller transducers display lower sensitivity or lower signal to noise ratio.

It is stated that, before the method can be included in inspection standards, more experience must be collected on calibration procedures and on detection and location capability.

In conclusion it seems safe to assume that the full impact of UT TOFD will be realized in the future once the difficulties of interpretation will be solved by the use of proper post processing algorithms.

An integrated solution should probably combine the results of this method with those of classical ultrasonic testing to cover also those flaw shapes and locations now beyond detection with the present technique.

8 - Site Updating: Robotic Arc Welding

The new Page added this Month to, our Welding Advisers Website, is dedicated to the application of robotic means to perform arc welding.

It should be understood that, while the introduction of automatic welding may dispose of the need of skilled manual welders, it requires the presence of much more expert operators to take care of the complex systems interactions.

The single outstanding characteristic of a robotic welding facility is increased productivity relative to manual operations. In order to achieve the full scope of possible improvements there is a need to fully understand the process used, normally GMAW, to exploit the capabilities to their maximum.

Higher currents, higher speeds, minimum oversize welding, use of larger size filler metal, reduction of unproductive time, these are among the most important parameters that contribute to productivity. It may not be easy to push them so that the process be successful, robust and reliable.

Robotic welding will provide sizeable benefits if implemented correctly with full understanding of the conditions necessary to secure the best results.

Moreover preventive maintenance must be devised and enforced to assure that no cases of unscheduled downtime compromise production.

For exploring this opportunity, now within reach for many industries, providing the potential of a profitable leap forward, one can read the new page by clicking on Robotic Arc Welding (Opens a new page).

For seeing a structured index to all pages, click on Site Map (Opens a new page).

For obtaining regularly this publication, click on Subscription (Opens a new page).

9 - Short Items

9.1 - Dry Hyperbaric Welding is conducted in a submersible welding pressure chamber. A complete system developed in Norway for exploitation of oil fields located deep at sea, includes also equipment for pipe manipulation, a gas unit for the control of chamber atmosphere, a Gas Tungsten Arc Welding Head capable of welding in all positions, assisted by a cold wire feeder, power source, video cameras for observation and computers for conducting the process.

In this particular installation, called pipeline repair system (PRS) and described at page 52 of the June 2006 issue of the Welding Journal, welding can be assisted by a diver, but is conducted remotely under computer control from the surface support vessel.

9.2 - Failure Analysis starts with the systematic observation of the outcome of a mechanical failure, in the careful collection of the data relative to the original production of the failed structure including possible repairs, and of the environmental and operating conditions at the time of failure.

Special inspections and tests may be required to find out the specific modes of failure and the stress levels prevailing at the time. The main purpose is to assess the primary cause of failure so that all possible preventive measures can be taken in time to avoid future new occurrence of failures in similar structures.

Although the preliminary investigation is generally conducted by a professional metallurgist with experience in this type of work, depending on the complexity of the structure and on the presence of additional causes, the analysis may be difficult and such that the services of experts in several branches of engineering and physical science may be required.

It is important that the preoccupation with eventual responsibilities and associated costs do not bias observations and conclusions of the investigators.

9.3 - Hybrid Laser-GMAW Process combines unique advantages of two unrelated technologies to produce speed and economy of production in as diverse industries as shipbuilding and automobile manufacturing.

High power lasers are now available at reduced costs. Due to objective characteristic of lasers, however, straight application of laser welding is restricted to applications of high precision edge preparation and setup. They cannot accommodate gaps and edge mismatch normally found in industrial production.

Gas Metal Arc Welding presents interesting characteristics like the capability to bridge gaps and to modify the composition of the weld metal by filler metal addition. It has however limited penetration so that joining of thick sections requires multipass welds.

The skillful combination of the two unrelated processes can provide advantages unthinkable of, when using each one by itself. Thick section hybrid welding solutions have demonstrated the advantages of the synergic operations of the technologies also with the reduction of deformation. Specific parameters must be developed for optimizing results for all new applications.

The second item in section 5 above is an article from TWI on this subject.

A short note on this process was presented in section 7 of Issue No 16 of Practical Welding Letter for November 2004, available by clicking on PWL#016.

9.4 - Mold Cavity is the space of a mold where the liquid metal is poured through channels called sprues, gates, runners and risers, to form a casting upon solidification.

9.5 - Pry Checking Test as used for ongoing production control, consists in an attempt to break faulty (stick) resistance spot welds by the forceful introduction of a chisel or wedge between welded sheets to exert a separating force.

A recent investigation reported that this type of test may introduce an irreversible damage in perfectly acceptable spot welds. The method should be perfected and instrumented in order to provide meaningful results without damaging sound welds.

9.6 - Rapid Prototyping is the construction of physical models laid down in a progressive fabrication as a stack of cross sections created in physical space, one after the next until the object is finished.

The machine reads data from a drawing and derives the geometric and dimensional features of each cross section. It then lays down successive layers of liquid or powdered material following the exact shape and dimensions, and promotes their progressive solidification.

The solid prototypes can then be used as models for preparing molds where metals can be cast to obtain corresponding metallic objects.

A commercial company offers Case Studies of specific applications. See the last item in section 5 above.

10 - Explorations: beyond the Welder

News of the Best of Times and the Worst of Times

Audio Interviews with leading Scientists and Journalists

Popular Science: Download a digital sample

Wisdom Quotes

Beaches of the Caribbean

11 - Contribution: Variable Polarity Welding Aluminum Pipe

Variable Polarity Gas Tungsten Arc Welding and Plasma Arc Welding were introduced in the aerospace industry about three decades ago. They were slow to spread to other industries because the more traditional Alternating Current (AC) process, although not really satisfactory, was considered sufficient for most applications.

One of the fields where the more complex Variable Polarity welding technology was recently successfully applied is that of piping and vessels to store and distribute clean gases to strict cleanliness standards.

Variable polarity inverter based power supplies with integrated computer managed controller, allow connection of the tungsten electrode to precisely programmable waveforms.

This is accomplished continuously and reliably by precise high speed switching between the alternating positive and negative portions of the square wave cycle, where the single parameters (frequency, duty cycles, levels of current and duration) are programmed individually.

When power supplies with these capabilities are combined with orbital welding heads, the resulting joints meet the most demanding X-Ray requirements for high quality manufacturing of aluminum pipelines.

A note on this subject was published in the April 2006 issue of the Welding Journal at page 42. Interested readers are urged to seek the original article.

12 - Testimonials

From: Tim Prusak (e-mail omitted)
To: Welding Advisers
Date: 02 Jun 2006, 02:25:46 AM
Subject: response to an earlier question: Welding H-13


In regards to a question as far as welding H-13 to mild steel handles that you sent me procedures for, I'd like to let you know that all worked great!

No problems at all! [...]

I enjoy your articles Elia and look forward to writing you again!

Tim Prusak

From: Sukhendu Samanta (e-mail omitted)
To: Welding Advisers
Date: 17 Jun 2006, 08:19:24 AM
Subject: Re: PWL

Dear Elia Levi,
thanks a lot for providing me such a great &
fantastic material about modern welding...

13 - Correspondence: a few Comments

My Correspondence desk is busy almost daily with a few queries. While I try to be of help by providing a few references or resources from website pages that cover quite a bit of welding technology and metallurgy, I often find that essential pieces of information are missing.

Being left in the dark is quite frustrating, and the purpose itself of the query remains unclear. Therefore it is a refreshing gift to receive a message like the first one of the two above confirming that the advice actually helped.

I wish I could always help, and I will always be grateful to the kind few that let me know of it. It is a welcome energy infusion that helps me continue.

I have no other way to communicate to my readers than by the e-address they provide. I suspect that some of you do not receive my letters, for one reason or other, possibly for unsuited blacklisting. So please check with your ISP.

If you do not wish to receive these letters any longer please unsubscribe. When our message bounces back from your address we do not know why (your inbox could be full momentarily) and how to deal with it.

14 - Bulletin Board

14.1 - Sept. 13-15: 2nd Intl. Conf. on Engrg. Failure Analysis
Toronto Ontario Canada.

14.2 - Oct. 31-Nov.2: FABTECH Intl. and AWS Welding Show
Georgia World Congress Center

14.3 - In case you missed the Mid June Bulletin, dedicated to
Careers, Certifications, Jobs - an Online Resource List,
you can still download it by clicking on PWL#034B. (Opens a new page).

14.4 - Ready for an amazing journey? Take yourself the time for a Quicktour. It is refreshing and promising.


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Click on this Logo NOW!

Copyright (c) by Elia E. Levi and
All rights reserved.

See you next time.

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