April 1st, 2009

PWL#068 - Integrated Laser Welding & Cutting, Oxyfuel Gas Bevel Cutting, Filler Metals for Thermite Welding, Cracks in Welds, Shockwave Induced Spraying, Thermite Welding, Brazing Magnesium and more...

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1 - Introduction

2 - Article - Integrated Laser Welding and Cutting

3 - How to do it well: Oxyfuel Gas Bevel Cutting

4 - Filler Metals for Thermite Welding

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article - Cracks in Welds

8 - Site Updating: Thermite Welding, Brazing Magnesium

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contributions: Shockwave Induced Spraying

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board

1 - Introduction

This 68th issue of Practical Welding Letter opens with a short report on a new successful application of laser beam where the same equipment can now be used for cutting and for welding, with only minor adaptations in the programming.

It appears that the capability described is very useful and economic for automotive body building and possibly also for other fabrications where the two processes can be combined.

A question on beveling with oxyacetylene flame is answered by pointing to the all important parameters which have to be correct for acceptable results. As often is the case, there is nothing wrong with the process, provided the conditions are right.

Then, with a reference to a new page of our website, some indications are given about possible sources for supplies of filler metals for thermite welding. Although this is quite a special process it can be used for certain difficult jobs.

We have then an article on cracking, initiated from a blog post that made reference to a news item. It seemed that some clarification might be needed.

The Pages of the Month refer this time to two Processes.

One, Thermite Welding is well established since a long time for a few known niches.

The Other, Brazing Magnesium, while current for traditional applications, is in a frenzy of development of new and improved low temperature filler metals.

These are needed for permitting successful joining of a new class of magnesium based materials, namely Magnesium Matrix Composites (MMC). They promise to offer exceptional mechanical properties for automotive and aerospace applications at very low density.

Shockwave Induced Spraying is a new cold spraying process, claimed to have significant economic advantages over competing alternatives. Reference is given to a full review and presentation.

In the Comments we report the sorrowful complaint of a reader for the theft of their working tools. Unfortunately we can only blame the thieves responsible for the wicked deed and offer to the victims our sympathy and participation.

Other departments can be found at their usual place. We invite your feedback, comments and contributions. Write to us using the form at Contact Us.

2 - Article - Integrated Laser Welding and Cutting

New developments in the construction of high brightness fibre lasers permit to combine the two different functions of Welding and Cutting in a single machine head for swift change between processing modes. The importance of this innovation is especially appreciated in those manufacturing sequences that include both operations in a single setup, in a robotic cell.

The equipment permitting this feat is customarily called 3-D Laser Processing System, where 3-D stands for 3 Dimensions, to indicate that laser operations are not limited to a plane (2 Dimensions).

The important features that support 3-D performance are fiber-coupled beam guidance, high laser beam quality and efficiency, an optimized combination laser head and a suitable modulation control, together with smaller size and lower costs.

These characteristics bring about the advantages of high flexibility, shorter processing time and better accuracy. High brightness permits increased operating freedom in changing head to workpiece distance.

The programmable laser modulation control allows drastic variation of speed, as needed for following varying contours with the required accuracy, at the correct power for any value of the instant speed. On straight paths the speed may be 10 to 20 times that used around small radii.

The combination head allows the instant change of process by automatic switching between gas type and flow rate, focal and nozzle position and laser power and speed. These capabilities are best exploited in the fabrication of automotive details.

The flexibility of utilization realizes economic advantages like short and integrated process sequences, high equipment utilization, cost-efficient variants production and savings in handling, setting up and clamping parts.

One of the best benefits derives from the dual utilization of an identical path for cutting, then welding with a given offset as needed, because the same coordinates are already logged in the system. The gain is improved accuracy and speed.

The integrated equipment for 3-D manufacturing has to meet the requirements of accessibility, speed, acceleration and accuracy, the head must allow good accessibility to the workpiece, the process parameters must assure good quality results over the whole speed range, and cutting parameters must be tolerant of distance variation from the workpiece surface.

A complete presentation of all the advantages of the combined equipment briefly described here can be found in an article published at page 38 in the March 2009 issue of the AWS Welding Journal. Interested readers are urged to seek it.

3 - How to do it well: Oxyfuel Gas Bevel Cutting

Q - In carbon steel, welding preparation of bevel with gas cutting is advisable?

A - All suitable cutting processes must be used with correct parameters for providing bevels of acceptable quality. Depending on the amount of carbon in the steel and on the thickness, oxyfuel gas cutting can be advisable if it permits to achieve a suitable cut quality, sufficiently smooth bevel, without excessive oxidized scale or deep decarburized layer.

It is the careful balancing of all cutting variables, more than 20 according to certain counts, that helps obtain a smooth edge.

In general if bevels are finish machined there is more tolerance for eventual imperfections. Finally it is the result of further welding that helps to decide if the cut quality is acceptable or not.

4 - Filler Metals for Thermite Welding

As reported in Section 8 further down, a new page on Thermite Welding was recently added to the www.welding-advisers.com website.

It is perfectly possible to prepare pyrotechnic mixtures capable of exothermic reactions out of separate materials, essentially for demonstrations.

An interesting website:
http://www.amazingrust.com/Experiments/how_to/Thermite.html
provides information and tips to do just that.
WARNING! Don't play with fire without proper safety precautions.

However, for performing serious work with actual Thermite welding it is recommended to get detailed instructions and to procure the premixed materials from recognized suppliers with long experience and know how like the following:

http://www.elektro-thermit.de/en/1_5_1.php

http://www.ameco.net/pdf/Thermite_Welding.pdf

http://www.thermitwelding.demon.co.uk/profile.html

http://www.orgothermit.com/products.html

Note that different compositions are adjusted for specific applications, to provide the mechanical properties needed for the various rail alloys, according to national standards, or per other requirements.

5 - Online Press: recent Welding related Articles

Take Your Pulse: Time to Examine Your Welding Equipment
http://millerelectric.cmail5.com/t/y/l/dukdkl/iuutvydd/jd

Compressor-Generator-Welder Combination
http://www.millerwelds.com/education/articles/story188.html

Video Library
http://www.millerwelds.com/resources/video_library/

Welding of HSLA Steels
http://www.twi.co.uk/content/jk98.html

Connect - January/February 2009
http://www.twi.co.uk/content/conjan09.pdf

6 - Terms and Definitions Reminder

Arm in resistance welding equipment is an adjustable beam projecting from the machine frame, designed to sustain the lower electrode and the compressing force applied during welding.

Bonding Force is that holding two atoms, resulting from an energy decrease when atoms are brought closer.

Cutting Attachment is the device applied to convert an oxyfuel welding torch into a cutting torch.

Detonation Flame Spraying is a thermal spray process where a controlled explosion of fuel and oxygen entrains a surfacing powder and propels it onto a substrate.

Electrode Lead is the conductor joining the power source and the electrode holder.

Friction Stir Welding is a process that produces a weld between two abutting edges by friction heating and plasticised material displacement produced by a rotating tool traversing along the joint.

Joint Spacer is a metal insert placed in a joint root, to maintain the required root opening and to be used as backing.

Plasma Arc Welding is a powerful welding process that uses a constricted arc from a non consumable electrode, either transferred (to the weld pool) or non transferred (within the torch) under gas shielding.

7 - Article - Cracks in Welds

A recent article posted on a blog and titled "Cracks in Welds",[Note: The link was recently removed by the source
http://www.thefabricator.com ]
suggests me to spend a few words to clarify the issue.

Let us start with the official AWS definition:
"Crack is a fracture type discontinuity characterized by a sharp tip and high ratio of length and width to opening displacement".

Most specification require that cracks be removed from welded structures by grinding and/or by any acceptable repair. There are no doubt or uncertainties in this requirement.

The problems arise when inspectors find, by any one of the non destructive testing methods prescribed for the structure considered, one or more linear indications of discontinuity which may or may not be interpreted as cracks. The final decision depends on the experience and skill of the inspector and on the help in interpretation that the organization can provide.

It is certain that any decision can have significant economic consequences (either cost or savings). That is why inspectors may be subject, to their vexation, to heavy pressure, if their decisions are not lenient as management would obviously prefer.

"In case of doubt ask" - this is probably the best recommendation one could give, if there is at all whom to ask.

It should be understood that the size of the detectable indication is dependent upon the sensitivity of the inspection methods employed, and that with sufficiently sensitive methods one would find almost always indications, even if only at the molecular level.

One can imagine that an indication of the smallest detectable size is less dangerous than a sizeable one. But where is the limit?

There is no thumb rule permitting to decide once and for all which indications can be safely accepted, but there are a few general guidelines that can help.

Acceptability limits should be spelled out either in general standards or in specific instructions. The designer is the person in charge of establishing those limits, hopefully basing the selection upon a long experience of satisfactory service life of structures of similar type.

The only way to prevent catastrophic failure of complex machinery or structures is to perform complete non destructive inspection at frequent intervals. That is done routinely in aerospace systems.

The difficult economic problem is to establish the correct frequency of inspection that will detect dangerous developments of faulty conditions before they reach the threshold beyond which failure becomes imminent.

Too much inspection is costly, if not really needed. Sparing on inspections may be economic but also risky, especially if failures can cause loss of human life.

A whole new science called fracture mechanics studies the conditions that will promote the growth of instabilities like cracks to the point where failure becomes unavoidable.

It is by using the lessons taught by fracture mechanics, based on scientific principles, that one can establish the required frequency of inspection and the dangerous size of rejectable indications, that must be detected for avoiding failure.

The quite recent trend to study the fitness for service of costly aging power plants and other structures, is a tentative answer to these economic pressures.

Fracture mechanics is called in to find out the most economic maintenance plan to be implemented, assuming that scheduled shut-downs designed to detect repairable units before their sudden catastrophic failure is preferable to unplanned outages involving partial destruction of entire units.

An article related to the subject discussed in this note and titled Risk Assessment was published (7) in Issue 29 of Practical Welding Letter for January 2006. Click on PWL#029 to read it.

Readers and inspectors with first hand information on the subject outlined here are invited to contribute from their own experience.

8 - Site Updating: Thermite Welding, Brazing Magnesium

The new Pages of this Month refer both to specialized processes. The first, Thermite Welding, is mostly used to join track rails, but has also a few other applications, mainly to repair or to build massive construction parts in a way similar to casting.

The main difference is that the material is obtained as superheated molten metal from exothermic chemical reactions. Its long history of successful applications is a guarantee that, if applied correctly, it should be expected to perform satisfactorily and economically.

This new page is found by clicking on Thermite Welding.

The second new page, on Brazing Magnesium, shows how this process is applied for castings. However a breakthrough is badly needed for brazing successfully Magnesium Matrix Composites, an advanced class of composites exhibiting exceptional mechanical properties, comparable to those of high strength materials.

What is still lacking but worked on with feverish dedication, is the development and testing of new alloys capable of low temperature brazing with acceptable mechanical properties, to permit the realization of scores of applications in automotive and aerospace fields.

This second new page is found by clicking on Brazing Magnesium.

9 - Short Items

9.1 - Adhesive Wear is due to transfer of material from one surface to another during contact with relative motion of the two, because of solid-phase welding. Particles that are removed from one surface are either permanently or temporarily attached to the other surface.

9.2 - Deburring means removing burrs, sharp edges, or fins from metal parts by filing, grinding, brushing or rolling the work in a barrel containing abrasives suspended in a suitable liquid medium.

9.3 - Dual-Phase Steels are a class of high-strength low-alloy steels (included in a larger group of Advanced High Strength Steels - AHSS), characterized by a tensile strength value of approximately 550 MPa (80 ksi) and by a microstructure consisting of about 20% hard martensite particles dispersed in a soft ductile ferrite matrix. Ferrite and martensite are predominant but small amounts of bainite, pearlite, or retained austenite, may also be present.

9.4 - Elongation is a term used to describe the amount of extension of a test piece when stressed. In tensile testing it is the increase in the gage length, that is the difference between the length measured after fracture of the specimen within the gage length and the original gage length, divided by the original gage length, usually expressed as a percentage. A large value of elongation can be interpreted as substantial ductility.

9.5 - Quench Cracking is the fracture of a metal during quenching from elevated temperature. Most frequently observed in hardened carbon steel, alloy steel, or tool steel parts of high hardness and low toughness. Cracks often start from fillets, holes, corners, or other stress raisers and result from high stresses due to the volume changes accompanying transformation to martensite.

9.6 - Restraint is the prevention of a part or weldment from moving to accommodate changes in dimension due to thermal expansion or contraction, by application of an external mechanical force to clamp it in a fixture.

10 - Explorations: beyond the Welder

NIST and the World Trade Center
http://wtc.nist.gov/

Clean, Safe, Affordable Power
http://www.hyperionpowergeneration.com/

Automatic Particles Analysis
http://www.clemex.com/Multimedia/PS3/PS3.html

Bumble Bees in Decline
http://www.xerces.org/

Pandemics Are Avoidable
http://www.gvfi.org/

11 - Contributions: Shockwave Induced Spraying

A recent article, appearing at page 32 of the March 2009 issue of Advanced Materials & Processes, an ASM International Publication, introduces a new solid state spray process capable of depositing thick coatings of a number of different materials on a variety of substrates.

Although similar to Cold Gas Dynamic Spraying, a process of whom two versions were developed (one low- and one high-pressure) the new Shockwave Induced Spraying (SISP) process depends on the generation of trains of shockwaves in a straight nozzle by the fast opening and closing of a control valve downstream of a high pressure gas container.

The shockwaves compress the gas in front of them creating a pulsed (at 10 to 30 Hz) heated supersonic flow, where each pulse catches and heats a metered batch of spray powder, and accelerates it toward the substrate to be coated.

The process is claimed to provide high deposition efficiency for many materials, mainly due to the heating in flight process that reduces the critical velocity for solid state bonding. Gas and energy reduced consumption compared to similar processes, contributes to the economy of operation.

Different gases, like helium, nitrogen or dry compressed air, provide different attainable gas velocities and spraying conditions for various materials. Specific applications where high productivity and low capital and operating costs can be achieved, should be investigated for their efficiency and economy.

Interested readers are urged to read the original article and to contact the authors for further information.

12 - Testimonials

On Mon Mar 02 06:34:01 2009, the following results were submitted from the "Form 5" on welding-advisers.com:

Name: John Diehl
E-mail Address: removed for security
Country: United States

Questions and Feedback:
Your Newsletter is tops. I truly enjoy every issue.

John

From: Horacio Navarro
To: Welding Advisers
Date: 14 Mar 2009, 10:36:12 AM
Subject: 10Ksi electric hoist beam

Elia,
thanks again for your hints.

Horacio Navarro C.

13 - Correspondence: a few Comments

This is a moving complaint I would like to share with all readers.

"Sunday March 15, 2009
From Tess Falasco
E-mail Address: removed for security
Country: United States
Introduce Your Organization: Self
Subject: Stolen welders
We live in Northern CO and someone is stealing welding machines of all sizes. My husband's truck was stolen 3/11/09 and hasn't been recovered.
I did my own research and have heard that many others from job sites have been stolen in just the last 10 days. One guy had 2 stolen from his yard, got them on video but cops do nothing. A shop in Littleton broken into and had 11 stolen and 2 big ones from a job in Parker, CO.

Maybe you can tell your readers to lock up tighter as thieves are in droves now due to economy or drug habit maybe. I hope I catch them first!

I hate thieves! Sharing the wealth has a whole new meaning and we're not sharing anything! This is our lively hood and many more here in CO that work hard."

It is really wicked to steal working tools from honest workers. Unfortunately we can only express our sympathy and participation. Have our readers any practical suggestions to offer?

14 - Bulletin Board

14.1 - 12th Aluminum Welding Conference
May 5 - 6, 2009 - Toronto, Ontario Canada
www.aws.org

14.2 - 66th Annual World Magnesium Conference
May 31 - June 2, 2009 - San Francisco, Calif. USA
www.intlmag.org

14.3 - Contribute from your Experience to your fellow Readers!
Click on Welding Talk.

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