Welding-overview
for basic understanding
of distinguishing features
Welding-overview should be probably the first page of any publication on welding.
That is if one aims at explaining the fundamentals of the processes used to bring about joining operations.
These may be loosely grouped under the name of Welding.
Most of times the material involved are metals.
Some types of plastic materials can occasionally be welded by special processes.
In fact even before starting to speak on Welding-overview, one should try to understand what is meant by Welding.
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The best explanation is the official one, formulated by the American Welding Society (AWS) in its document AWS A3.0 - Standard Welding Terms and Definitions, as follows:
Welding. A
joining process that produces coalescence of materials by heating them
to the welding temperature, with or without the application of pressure,
or by application of pressure alone, and with or without the
application of filler metal.
The next step would be the clarification of the term Coalescence, from the same document:
Coalescence. The growing together or growing into one body of the materials being joined.
While this explanation is correct, it does not address the needs of people who have no idea of metallurgy, who cannot imagine crystals and grains, and never thought that materials can "grow together" like siblings in a family.
An example follows further down this page.
New readers may remain baffled and dubious, but the best way to learn what Welding-overview is all about is to keep reading on, so that slowly the picture will clear itself by adding new facts and information.
The word Science is more and more associated to Welding technology. To grasp the fundamentals, though, it is not urgent to deal with all aspects at one time.
It is important to note that not all the processes described in Welding-overview can be applied directly to all materials.
Special attentions may be needed to weld some of them.
Certainly one should have a working idea of a process before considering how to apply it practically to any given material.
Assuming that every reader be ready to accept, at least provisionally, that welding involves connecting somehow separate elements into a unified structure, capable of maintaining its shape, and even to bear and transmit loads, we will try in this Welding-overview to convey general concepts, that, to be understood without confusion, need further detail and refinement.
Having already implied that welding technology is based on several physical principles and that one can easily assemble a library of many books to cover only its most important aspects, we will try to simplify this Welding-overview by organizing the matter within specific subjects to help memorize essential facts.
In the following, readers are advised to understand the different categories, and then to look for specific pages using the given links.
The proposed pages are meant to contribute additional knowledge readily absorbable, but readers are advised to proceed slowly, planning to take their time, to cover as much information as they feel useful to their learning purposes.
A general Welding-overview can start by grouping together processes that have common characteristics.
Fusion welding.
It is known that metals, like many other materials, can undergo a change of state, from solid to liquid, upon being heated to a sufficiently high temperature.
Upon reaching that temperature, a solid metal melts.
Melting can be local, if it takes place only in a limited space.
Welding processes based on melting or fusion can be grouped together, meaning that their capability to be joined depends on their change of state, at least locally, where the joint is performed.
Now the concept of coalescence can be better understood, to mean that molten materials can intermix, and then solidify in a common joint as the temperature is reduced below the melting range.
As most metals react at high temperature with the gases of normal atmosphere, suitable means must be applied to protect the molten metal, from atmospheric reactions likely to impair joint properties.
That applies also to metal which is still quite hot even if already solid.
Each process in turn is performed by using various types of protective atmosphere as applicable, at least just around the melting zone.
Fusion welding processes can be further qualified based on their heat source.
The electric arc is a practical and economic source of local heat that can be used for welding.
As explained in other pages, to produce a welding arc one needs a suitable power source, a proper electric circuit, and a fit electrode capable of generating and maintaining the electric arc of the right properties for welding.
It is further characterized by the electrode type and by the means to protect molten metal from reaction with the atmosphere.
See Arc_Welding_Processes.
The oxy-fuel flame is another source of local heating for melting, with a long history of successful applications.
The hot flame is generated in a burner that provides the correct proportions of oxygen and of fuel gas, and it is manipulated to provide the exact temperature required in the place where it is needed.
Exothermic chemical reactions are fundamental to one more fusion process, called Thermite_Welding that has several specific applications.
Other processes resorting to melting are based upon high energy devices, like laser beam or electron beam, requiring special means to be generated and applied.
See High_Energy_Welding_Processes.
Resistance welding.
A large class of welding processes is identified by the heating mechanism, which occurs because of the passage of electric current in a suitable electric circuit.
It may be somewhat confusing the fact that these processes are not called fusion processes although there is indeed fusion or melting, but localized within the workpieces.
See Resistance_Welding_Processes, and
High_Frequency_Resistance Processes.
Solid state welding
Solid state welding are processes where the workpieces to be welded are not melted.
Different versions are available. An ancient and historic process is now called forge welding; it consists in heating the parts to forging temperature and then bringing them in intimate contact by means of powerful hammering together of the elements to be joined.
See Forge_Welding.
Modern methods depend of heat generated by friction between parts, which are then brought in contact together by powerful hydraulic presses.
See Friction_Welding_Processes.
In principle, when atoms of separate bodies come in close contact, interatomic attraction develops and the elements become united.
In practice many interposed extraneous substances limit this ability, unless all disturbing factors are removed.
Solid state processes based on the above principle are performed cold, without any heating. Among them are:
Other joining processes not included in Welding.
It may be important to appreciate that certain processes used to join materials, lack essential characteristics and are therefore excluded from what is defined as welding.
Brazing (from AWS A3.0) is a group of joining processes that produce coalescence of materials by heating them to the brazing temperature in presence of a filler metal having a liquidus above 450°C (840°F) and below the solidus of the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action.
See Brazing_Processes
and also the pages dedicated to brazing specific metals.
Soldering (from AWS A3.0) is a group of joining processes that produce coalescence of materials by heating them to the soldering temperature and by using a filler metal having a liquidus not exceeding 450°C (840°F) and below the solidus of the base metal. The filler metal is distributed between closely fitted faying surfaces of the joint by capillary action or by wetting the surfaces of the workpieces.
See Soldering.
Adhesive Bonding (from AWS A3.0) is a joining process in which an adhesive, placed between faying surfaces, solidifies to produce and adhesive bond.
See Adhesive_Bonding.
Mechanical Fastening is any method requiring the use of special mechanical elements called bolts, nuts, rivets, screws and many other items, that perform the function of joining elements by being applied one by one on properly prepared joints.
Many important constructions like ships, bridges, towers, were built in the past and are still used nowadays without recurring to welding at all.
On Mechanical Fastening see
Conclusion
This closes our Welding-overview.
The list of subjects that must be mastered to grow a practical understanding of the main theme has only been hinted at.
Go beyond the Welding-overview!
Every reader is encouraged to pursue his/her learning needs by browsing through the titles listed in our Site_Map and by trying to assimilate as much as possible from each page.
It is possible that certain subjects generate confusion, by not being sufficiently elucidated in this Welding-overview.
In that case readers are urged to ask their questions by explaining which assertions baffled them.
Use the Contact Us Form. We will then try to clarify the issues by answering the queries and by updating our pages.
Welding is a fascinating discipline that touches to unexpected aspects of human knowledge.
It can easily fill the intellectual needs of anybody for a whole lifetime.
For those who love the subject it is also highly rewarding.
We hope that our efforts with this Welding-overview contribute to help interested people to improve their knowledge and familiarity with Welding.
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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter,
click on Welding Topics.
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Friction Welding Processes
Friction Stir Welding
Friction Surfacing
Flash Welding Process
Stud Welding
Upset Welding
Resistance Welding Processes
High Frequency Resistance Welding
Projection Welding
Gas Welding Processes
Arc Welding Processes
High Energy Welding Processes
Laser Drilling
Micro Welding Processes
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