Product Design Engineering: Weldments

Weldments:
Industries commonly use weldments as an alternative to cast or machined parts. A welded steel structure is generally stronger and more rigid than a cast iron structure because the tensile strength of steel is approximately twice that of cast iron. In addition, castings generally include extra thickness to allow for defective material or shifting cores. The higher strength to weight ratio of a weldment is clearly a benefit where weight or strength is a concern. Also, the added cost and time to create a mold and pattern for a casting can be less cost effective than the added processing time to manufacture a weldment in small number production runs. Creating a weldment involves more than just slapping pieces of steel together. The manufacturing personnel must cut materials to size, prepare them for welding through cleaning or grinding, assemble, weld, and finally relieve the weldment of stress deformation caused by heating and cooling. Therefore, you should carefully consider the advantages and disadvantages of using a weldment in your design.

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Differences between Welding and Brazing:

A brazed joint is made in a completely different manner from a welded joint. The first big difference is in temperature – brazing does not melt the base metals. This means that brazing temperatures are invariably lower than the melting points of the base metals. Brazing temperatures are also significantly lower than welding temperatures for the same base metals, using less energy.
If brazing doesn’t fuse the base metals, how does it join them? It works by creating a metallurgical bond between the filler metal and the surfaces of the two metals being joined. The principle by which the filler metal is drawn through the joint to create this bond is capillary action. In a brazing operation, you apply heat broadly to the base metals. The filler metal is then brought into contact with the heated parts. It is melted instantly by the heat in the base metals and drawn by capillary action completely through the joint. This is how a brazed joint is made.
Brazing applications include electronics/electrical, aerospace, automotive, HVAC/R, construction and more. Examples range from air conditioning systems for automobiles to highly sensitive jet turbine blades to satellite components to fine jewelry. Brazing offers a significant advantage in applications that require joining of dissimilar base metals, including copper and steel as well as non-metals such as tungsten carbide, alumina, graphite and diamond.
Comparative Advantages. First, a brazed joint is a strong joint. A properly made brazed joint (like a welded joint) will in many cases be as strong or stronger than the metals being joined. Second, the joint is made at relatively low temperatures, ranging from about 1150°F to 1600°F (620°C to 870°C).
Most significant, the base metals are never melted. Since the base metals are not melted, they can typically retain most of their physical properties. This base metal integrity is characteristic of all brazed joints, including both thin- and thick-section joints. Also, the lower heat minimizes danger of metal distortion or warping. Consider too, that lower temperatures require less heat – a significant cost-saving factor.
Another important advantage of brazing is the ease of joining dissimilar metals using flux or flux-cored/coated alloys. If you don’t have to melt the base metals to join them, it doesn’t matter if they have widely different melting points. You can braze steel to copper as easily as steel to steel. Welding is a different story because you must melt the base metals to fuse them. This means that if you try to weld copper (melting point 1981°F/1083°C) to steel (melting point 2500°F/1370°C), you must employ rather sophisticated and expensive welding techniques. The total ease of joining dissimilar metals through conventional brazing procedures means you can select whatever metals are best suited to the function of the assembly, knowing you’ll have no problem joining them no matter how widely they vary in melting temperatures.
Also, a brazed joint has a smooth, favorable appearance. There is a night-and-day comparison between the tiny, neat fillet of a brazed joint and the thick, irregular bead of a welded joint. This characteristic is especially important for joints on consumer products, where appearance is critical. A brazed joint can almost always be used “as is,” without any finishing operations needed – another cost savings.
Brazing offers another significant advantage over welding in that operators can usually acquire brazing skills faster than welding skills. The reason lies in the inherent difference between the two processes. A linear welded joint must be traced with precise synchronization of heat application and deposition of filler metal. A brazed joint, on the other hand, tends to “make itself” through capillary action. In fact, a considerable portion of the skill involved in brazing is rooted in the design and engineering of the joint. The comparative speed of highly skilled operator training is an important cost factor.
Finally, brazing is relatively easy to automate. The characteristics of the brazing process – broad heat applications and ease of filler metal positioning – help eliminate the potential for problems. There are many ways to heat the joint automatically, many forms of brazing filler metal and many ways to deposit them so that a brazing operation can easily be automated for almost any level of production.

Welding of EN24T:

We have identified an issue with all tools manufactured using welded EN24T material in that this particular material is impossible to weld in the correct conditions.

So we’ve begun a plan to re-design the few existing tools we have in this condition to remove the need to weld.

I am a big fan of brazing where stress considerations allow. For 2 main reasons:

As you say it is much easier to repair.
It creates much lower thermal stress so avoids stress relief in most cases. In the 1970’s and 1980’s when I used to work on racing motorcycles, most of the modifications and repairs we did were brazed, just welded the high strength tubes.

You can get brazing alloys with strengths of 500 MPa plus, a well-designed braze can outperform a weld in many ways.

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Stress-relieving after welding:
Stress-relieving is the process generally specified after welding of most materials. Removing or reducing the residual stresses generated by welding is required for improving the dimensional stability of weldments. In certain applications, internal residual stresses can sum up with those generated by externally applied loads. Then, if the yield strength of the material is exceeded, unacceptable plastic deformations will occur.

Non symmetrical machining may need to be performed on welded assemblies. Then, following material removal, the redistribution of internal stresses may cause unwanted distortions.
Furthermore, without Stress-relieving, the material may suffer from service problems such as stress corrosion cracking. Stress-relieving occurs by diffusion of atoms within solid materials. Naturally, the material slowly approaches its equilibrium state. Heat, if supplied, will increase the rate of diffusion by providing additional energy.
The movement of atoms has the effect of redistributing and eliminating linear imperfections, called dislocations, in crystalline arrays. This alteration allows metals to deform more easily, so that their ductility is improved. The relief of internal stresses is a thermo-dynamically spontaneous process. However, at room temperatures, it is a very slow process.
Stress-relieving should always be considered:
Residual Internal Stresses can be reduced or removed either thermally or mechanically.
At higher temperatures the process will occur with an accelerated pace. The treatment is not intended to produce significant changes in material structures or mechanical properties. Therefore it is normally restricted to relatively low temperatures.
Most commonly thermal Stress-relieving is applied by heating the welded part in a furnace.
At elevated temperature the yield strength of materials is reduced, relative to that at room temperature. Since the residual stress cannot exceed the yield stress, plastic flow will reduce the excess to the level of yield strength at that temperature.
In other words, a significant reduction of residual stresses is possible by heating weldments. As materials are weaker at higher temperatures, (remember "Strike while the iron is hot"), plastic yield will occur. For steels such a temperature is around 620 °C (1150 °F). Most Stress-relieving operations are carried out in air furnaces.
In air, alloys are subject to discoloration or scaling depending on the alloy and temperature used. Post-treatment cleaning or scale removal treatments are therefore often required. Time dependent stress relaxation will further reduce most of remaining residual stresses.
A rule of thumb requires that the time of the metal at temperature be at least one hour per 2.5 cm (1 inch) of thickness. At higher temperatures the time required to reach a given lower stress level is shorter.
Stress-relieving may be skipped if other specialized thermal treatments are planned for other reasons. For very large parts it may be unpractical and most expensive to find a suitable large enough furnace and the Stress-relieving treatment risks to become very costly. In those cases heating may be performed locally near the welds with a gas flame torch, or using resistance heated blankets or by induction systems heating. (See Videos further down this page).
The local strains that produce high residual stresses can also be eliminated by plastic deformation at room temperature. Sheet metal, plates and extrusions may be stretched beyond the yield stress to relieve by yielding the differential strains. In other cases a working operation generating a different residual stress distribution may be superposed.
It will modify and correct the pattern of the original residual stresses to a more favorable equilibrium. Such may be rolling or shot peening that are known to introduce compressive stresses.
In certain applications, like in the welding of tool steels, it is recommended to peen the weld while it is still hot to reduce the level of dangerous tensile stresses. Manually applied mechanical systems are available, intended to peen welds in large constructions to reduce the residual stresses.
See Ultrasonic Impact Treatment at:
http://en.wikipedia.org/wiki/Ultrasonic_impact_treatment
Another form of mechanical Stress-relieving is called VSR or Vibratory Stress Relieving.
The induced vibration level, to be effective, should be in the lower, or sub-harmonic, portion of the harmonic curve, just before the amplitude quickly rises and reaches the part's natural resonance. It is at the right frequency that the vibration has the greatest dampening effect, at which point it removes the stresses caused by welding.
A list of downloadable Articles on Vibratory Stress Relief is available at:
http://www.vsrtechnology.net/resources/library-articles/
See an article showing typical applications of induction heating for Stress-relieving:
Welders Turn To Induction Heating For Preheating, Stress-Relieving
http://www.millerwelds.com/resources/articles/index?page=articles24.html
On this subject, an article introducing ASM Handbook Volume 4C on Induction Heating and Heat Treatment was published (2) in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

Weld Types:

There are an overwhelming number of welding processes used in industry today, but the most common are gas, electrical arc (including wire-feed), and resistance. Of those processes, the two most common weld types are the Bevel weld and the Fillet weld. It is important for you to know what type of weld is to be used so that preparatory operations, such as chamfers or grooves, can be modeled on parts as needed.

Selection of Stock:

There are many commercially available steel forms that should be used whenever possible to avoid weldments. A standard piece of I-beam stock is cheaper and in most cases stronger than an equivalent weldment.

Machining Weldments:

In situations where you need to machine a welded member, placing the weld outside of the machined area increases tool life, reduces the machined surface deformation, and produces a more appealing surface finish.

Modeling Weldments:

Weldments occasionally create a problem for new designers because of its very nature. How do you handle a project that is neither a true assembly, nor a component that you machine or cast as one piece? Using CATIA also poses a unique challenge; you cannot store geometry in the assembly file. To address this issue, there are a couple of different methods for creating weldments; the one you use ultimately depends on the standards set by your company.

Modeling a Weldment as a Complete Part:

Some situations might allow you to model a weldment as a single complete part. An example of this situation might be for a lever that connects two shafts. The part shown in this figure would be an excellent candidate for a casting. Although, if demand calls for a small production run, a weldment is the wise choice. The main disadvantage to modeling the weldment as a single part is that it is difficult to detail the individual pieces of the weldment on a drawing, and tracking the drawing number of initial parts that make up the weldment is not possible.

Creating Multiple Bodies in One Part File:

Many companies allow designers to model the multiple bodies of a weldment in a single part file. This technique can be used in order to simplify a situation, such as a support bracket. Depending on your approach, the bodies can be associative to the original parts. Be advised that using this method to create a weldment, consisting of a large number of complex parts, can cause large file sizes, which, in turn, decrease computer performance. Also, depending on where components are modeled, (absolute vehicle position or at the origin of the part), you may need to relocate the bodies to their assembled locations.

Creating a Weldment from an Assembly:

Another method of creating a weldment is to create all of the pieces in individual CATPart files, and then constrain them together in an assembly.

However, in the case of a weldment assembly, you cannot perform secondary operations to an assembly, so a final step is to create a CATPart (the weldment file) from this assembly, using the command: Generate CATPart from Product. This method allows you to create detail drawings of each component and is well suited for instances where the weldment must display the physical welds. However, to create the weldment file, you lose associativity to the original parts.

Creating a Weldment Assembly:

The most popular method of creating a weldment is to create all of the pieces in individual component files and, using the Master Model concept, mate them together in an assembly. This method allows you to create detail drawings of each component and is well suited for instances where the components must have preparatory machining performed.

Performing Secondary Operations on Weldments:

In many cases, you must drill holes, machine surfaces to an acceptable tolerance, or perform other secondary operations to a finished weldment. This is not a problem if you create the weldment as an individual part. However, in the case of a weldment assembly, you can not perform secondary operations to a component. Actually, NX has created two provisions for just that situation: Promote and WAVE Geometry Linker.

Promote Command:

Promote a component to a solid body within an assembly in order to perform operations that are secondary to a weldment.

Q: If a hole is created through a weldment after promoting the components, does that hole become part of the original component file?

A: No. Any operations performed on a promoted body do not affect the original component file however changes to the original component file do appear on the promoted body.

NOTE: Use WAVE Geometry Linker as a better alternative to Promote.

Weldments Using WAVE Geometry Linker:

Link the solid bodies of weldment components into the assembly file in order to unite the bodies and perform secondary operations.

Using WAVE Geometry Linker in conjunction with a mated assembly is the most effective and parametric method of creating a weldment. This method allows you to create detailed drawings of weldment components, conform to the Master Model concept, and perform subsequent operations to a weldment within an assembly. This is the recommended method to use in all cases.

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Weldment Symbols: