What is the difference between Additive Manufacturing (AM) and welding for metals?

In what sense are Additive Manufacturing (AM) and welding of metals similar and in what sense are they different?  If you could only see the pool of liquid metal and arc, laser or electron beam that is melting the metal and the filler metal being added to the weld pool, there is no difference.  The difference is in what the weld pool is intended to do.  In welding, one starts with the parts and then tries to add as little filler metal as possible to make the final structure. As an example, one makes the V-groove in welds as narrow as possible to minimize the use of filler metal and lowers the overall labour coast, which is the key contributor to overall cost of welding. The more filler metal used may contribute to greater distortion and increases the risk of defects.  In additive manufacturing, one starts with nothing and makes the entire structure from filler metal.  Clearly this will be expensive.

In order to find customers who are prepared to pay the high prices for products made by AM, one must provide benefits to justify the high cost. Components of aerospace structures that gain value by minimizing weight and maximizing stiffness are candidates. The military is interested in AM if it can be used to repair ships at sea or equipment in a battlefield by building parts on site to minimize downtime and maximize availability.

Like welding, AM competes with forgings and castings.

Artists have made sculptures using welding because it allows the freedom to create the artwork they choose. Although it is not called AM, it really is. Because industrial AM must be automated to be competitive and to achieve the required quality it is necessarily a complex technology.  AM must not only make the weld metal, it must make the parts.  One approach uses the idea of a thin layer of a bed of metal powder.  Then a laser or electron beam scans the bed melting a thin layer in the desired pattern. When the pattern is completed, a new layer of metal powder is added and the process is repeated.  When the part is completed, the unmelted powder is removed.  The design had to allow the unmelted powder to flow out of the melted and solidified part.  The unmelted powder can be recycled only about 3 times because there are strict limits on the distribution of sizes of the metal powder grains. Structural steel powder for AM costs about $200 per pound and often more than 50% most be thrown away.  Therefore, structural steel parts made by this approach to AM must cost more than $400 per pound.

The other approach to AM is similar to traditional overlay welding. One starts with plate and adds overlay weld metal with wire electrodes or powder metal with an arc or laser heat source.  If the plate is mounted on gimbals, the plate can be rotated in space to keep the weld pool horizontal.  This is more flexible than the bed of powder approach. The powder is cheaper because it does not have to be sized and can be recycled. This has been used for decades to overlay a layer of stainless steel to long heavy-walled steel pipes that are used in nuclear reactors and petro-chemical plants.

In the past 30 years, numerical algorithms called shape optimization, have been developed to optimize the design of 3D structures such as airplanes and automobiles and other parts made by AM.

The difference between AM and welding is more economic and social than technical.  AM is much more capital intensive, usually produces smaller parts and usually is highly automated.  Knowledge gained will continue to flow between welding and AM to the benefit of both technologies.  It is likely that sensor technology developed to control AM will lead sensor technology for welding. Through innovation and competition, the applications best suited to welding and to AM will emerge to make a better world for everyone.

John Goldak President - Goldak Technologies Inc.