Laser Cladding vs. Hardface Weld Overlays
A Comparison of Laser Cladding versus PTA, TIG, and MIG Weld Overlays
While you won’t find "hardfacing" in the dictionary, it’s an industry standard word that anyone looking to repair and remanufacture their machine parts would benefit to know. Hardfacing is any metalworking process where harder or tougher material is applied to a base metal i.e. a machine part surface. This describes anything from thermal spray to laser cladding to weld overlaying. All of these use various heat sources to apply metallized coatings.
In a previous post we compared laser cladding to thermal spray and briefly described the difference between a mechanical and metallurgically bonded coating. In most industrial scenarios, a mechanical bond will suffice but what about when it doesn’t? Laser cladding isn’t your only metallurgically bonded option, outside of spray and fuse thermal spray. So, let’s take a look at how laser cladding compares to the different hardface weld overlay process on the market today.
Laser Cladding and Weld Overlay Process Comparison
Each process is performed differently, with laser cladding being the most different in application. Laser cladding is the use of electrically produced laser light to heat wire or powder material and apply it to a substrate surface. Each of the weld overlay types are relatively similar with small but significant differences between them. MIG (Metal Inert Gas) welding, or GMAW (Gas Metal Arc Welding), uses electricity, a shielding gas and a consumable electrode. This consumable electrode is typically the material used to overlay the substrate. TIG (Tungsten Inert Gas) welding, also referred to as GTAW (Gas Tungsten Arc Welding), uses the same raw materials except that, instead of a consumable electrode, it uses a nonconsumable tungsten electrode and an additional rod of the overlay material.
A more recent type of welding process, PTA (Plasma Transfer Arc) Welding, is the similar to TIG welding in raw materials but instead of a rod filler material, it uses a powder material and a specialized nozzle to ionize the shielding gas and create a plasma arc to apply the material to substrate surface. When it comes to overlaying capabilities, PTA is the most comparable to laser cladding; but that does not discount the other two.
Each of these processes produces an overlay that is metallurgically bonded to the substrate. This means that the overlay material and the substrate material fuse together at a molecular level. This has many advantages including the decreased surface porosity that results in high performance, corrosion resistant coatings. Laser cladding and weld overlays also result in extremely wear resistant coatings.
Differences Between Cladding and Weld Overlays
Weld Overlay vs. Cladding Thickness
Each of the weld overlays range in thickness from approximately 0.05 inches to about 0.1 inches. Laser cladding has a slightly wider range at 0.008 inches to 0.15 inches thick.
The most black and white difference between the four process is the materials it can overlay. MIG welding is the most limited, only applying steels and stainless steels, nickel, flux cored wire and hardface wires. TIG welding, along with PTA welding and laser cladding, can apply the popular overlay metal, Stellite® 6 and other cobalt based alloys, as well as the nickel and tungsten carbide-based alloys of the Colmonoy® alloys, including Colmonoy® 88. TIG welding can also apply 316 stainless steel, carbon steel and other wire metals. PTA welding and Laser cladding can also apply other stainless steels such as 300 and 400 series stainless steels.
Heat Input and Heat Affected Zone
A huge advantage of Laser Cladding is something called Heat Affected Zone, or HAZ. HAZ is the zone of the substrate metal that has not melted but, has been subjected to the elevated temperatures of the process for a brief period of time. This alters the molecular structure of the metal in this area and it no longer possesses the desirable properties of the substrate material, but since it has not melted, it hasn’t assumed the properties of the solidified, overlay. This HAZ is a weakened layer that affects the hardness and porosity of the underlying microstructure.
Typically weld processes with a higher heat input have a larger heat affected zone. Laser cladding, since it requires the lowest heat input of each of these processes, has the smallest HAZ. Despite having the highest heat input, PTA has the next best HAZ. This is due to the faster cooling and quenching of the overlay material. It does, however, still have the highest heat input of the weld processes, so heat distortion can still be a concern since it is performed at approximately 50,000°F.
Each process has specific benefits and limitations in respect to deposition rate, positioning and the control of process variables. Laser cladding is typically a robotically controlled process which provides unparalleled consistency but also limits it in deposition rate and positioning. PTA welding is the fastest overall with a skilled operator. TIG weld overlay, while slower than PTA weld overlay, is faster than MIG weld overlay and can be done in most positions, although overhead welding can be difficult and requires highly skilled operators. MIG weld overlay cannot be done on vertical or overhead surfaces. These variables can of course be eliminated by careful planning and tooling when applying harface weld overlays.
TIG welding provides good temperature control while PTA welding and laser cladding provide excellent control over their process parameters. MIG welding can be difficult to maintain consistent temperature and arc, requiring skilled craftsmen to create a quality overlay.
The Main Differences Between Weld Overlays and Laser Cladding
So, while each process has its pros and cons, they all produce high quality, corrosion and wear resistant coatings. All of the weld overlays; MIG, TIG and PTA, can produce coatings similar to laser cladding with PTA being the closest by far. Laser cladding and weld overlays vary greatly in heat input and the resulting coatings vary slightly in thickness capabilities and materials. Laser cladding excels when heat distortion is an issue for smaller parts or parts made of heat sensitive metals. In situations where temperature isn’t an issue and laser cladding might be overkill, weld overlays provide an economical, hardfacing solution.
For more information about PTA Welding, check out our other blog post: Plasma Transfer Arc Welding for Abrasion Resistant Coating or check out the welding services that HTS Coatings offers.