The Weldability of Copper and Its Alloys

Copper Alloys

Copper-based alloys are categorized into families based on their chemical makeup. UNS assigns a number designation based on this. These numbers range from 10000 to 99999. Also, these alloys often have a C in their name, for example, C11000. Wrought metals fall between 10000 and 7999. Cast metals are between 80000 and 99999.

These numbers are often stylized by dropping the last two zeroes off. This makes them easier to read and takes up less space in writing. Online Metals follows this convention. As an example, we have copper C10100 listed as copper 101. For consistency’s sake, the rest of this article will also follow this format.

Wrought: 101 to 130 | Cast: 801 to 812

Fusion welding is not recommended for free-machining copper because they are prone to cracking. Nor is it suggested for precipitation-hardenable copper alloys. High temperatures will weaken the heat affected zone. Commonly weldable coppers are typically paired with Cu 1897 and Cu 1898 filler metal.

Brass Alloys

Wrought: 205 to 28580 | Cast: 833 to 858

All brasses are weldable except the alloys containing lead. However, the lower the zinc content, the more easily welded it is. Low-zinc brasses with less than 20% zinc have good weldability. By comparison, high-zinc brasses with over 20%, have only fair weldability. Lastly, cast brasses are only marginally weldable. The recommended fillers for low-zinc brasses are Cu 6328 and Cu 6560.

Wrought: 404 to 486 | Cast: 833 to 848 “Tin Brass”

Unleaded tin brass alloys have fair weldability. However, they are prone to hot cracking and forming oxide films on the weld pool. Therefore, high welding heat inputs, high preheat, and slow cooling rates should be avoided. Leaded tin brass alloys are generally considered unweldable.

Bronze Alloys

Wrought: 501 to 524 | Cast: 902 to 917 “Phosphor Bronze”

Unleaded Phosphor Bronze alloys have fair weldability. But, under stressed conditions they are subject to hot cracking. So, like Tin Brass, high heat inputs, high preheat, and slow cooling rates should be avoided. You can carefully weld leaded Phosphor Bronze using SMAW. Keep in mind that weldability of copper alloys decreases as lead content increases.

The most frequently used phosphor bronze alloy is best suited to Cu 5180 filler metal.

Wrought: 608 to 64210 | Cast: 952 to 959 “Aluminum Bronze”

These metals have low electrical and thermal conductivity, which improves weldability. However, it is crucial to remove all aluminum oxide from the surface of the material before welding.

For Aluminum Bronze alloys with less than 7.8% aluminum, Cu 6240 and Cu 6100 are ideal filler metals. While alloys with aluminum content greater than 7.8% are better suited with Cu 6180 and Cu 6100. 642 Aluminum Silicon Bronze is best paired with Cu 6100.

Wrought: 647 to 661 | Cast: 870 to 87999 “Silicon Bronze”

These are arguably the easiest of all the bronzes to weld. Unlike many other copper alloys, their thermal conductivity is relatively low, and you can use high welding speeds. Silicon Bronze alloys should be stress relieved or annealed prior to welding. Next, they should be slowly heated to the desired temperature. Then rapidly cooled through the critical temperature range. Silicon bronzes are readily weldable with Cu 6560 filler metal.

Wrought: 701 to 72950 | Cast: 962 to 969 “Copper Nickel”

These alloys are commonly used in welded fabrication projects. Phosphorus and sulfur levels must be less than 0.02% to ensure good welds. Most Copper Nickel alloys do not contain a deoxidizer. Therefore, fusion welding requires the addition of a deoxidized filler metal. This lowers the risk of porosity in the weld. For copper nickel with a 10% nickel composition, Cu 7071 or Cu 7158 fillers are recommended. For copper nickel with a 30% nickel composition, a Cu 7158 filler is recommended.

Nickel Silver Alloys

Nickel Silver is neither nickel nor silver. Actually, it is a brass alloy. But because people often list Nickel Silver as its own category, we are doing the same here.

Wrought: 735 to 799 | Cast: 973 to 978 “Nickel Silver”

These alloys possess a weldability similar to other brass. Also, the weld quality decreases if lead is present. Unleaded Nickel Silver alloys are considered suitable to weld. But leaded alloys are not. Like other brasses, alloys with lower zinc content have better weldability.

These low-zinc alloys are readily weldable with Cu 6328 and Cu 6560 filler metals.

Common Copper Alloy Welding Defects

Because the weldability of copper alloys changes so much, it can be intimidating. Let’s explore some of the more common defects and how to avoid them.

Porosity

Copper and its alloys are most susceptible to this defect. Pure copper, brass, phosphor bronze, copper nickel, and aluminum bronze are particularly so. To combat porosity in coppers, phosphor bronzes, and copper nickels, do not use autogenous welding with a matching filler. Instead, use filler wire containing deoxidants, such as aluminum, silicon, manganese, titanium, or phosphorus.

Phosphor bronzes are best paired to fillers with a high level of deoxidants.

Copper nickels are best paired with fillers containing 0.2-0.5% titanium.

Reduce the risk of porosity in brasses by using a zinc-free filler. Either silicon bronze or aluminum bronze filler is best suited with brasses. Additionally, higher welding speeds reduces pore size in the weld.

Aluminum bronze alloys have a strong aluminum oxide film that forms on their surface. This film gives them strong corrosion resistance. But it also causes oxide film entrapment which increases its risk of porosity. Remove the film to prevent this. Rigorously clean the material surface with wire brushes or scrapers before welding these alloys.

Lack of Fusion

Brasses with less than 20% zinc are vulnerable to fusion defects because of their high thermal conductivity. Therefore, preheat low-zinc alloys before welding. Aluminum bronzes are also susceptible to lack of fusion due to the aluminum oxide film. Again, it’s crucial to thoroughly clean the surface to remove this film before welding.

Hot Cracking

Coppers containing chromium or beryllium alloys, brasses, and aluminum bronzes with low aluminum content, less than 8.5%, are susceptible to hot cracking. Carefully preheat copper with chromium/beryllium and aluminum bronze to reduce the risk. However, brass containing prominent levels of zinc doesn’t require preheating. Unlike their low-zinc counterparts, slow cooling rates can reduce their cracking risk.

How to Weld Copper to Brass

Get the best results when welding copper to brass and other items that require a full penetration weld.

TIG Brazing Copper to Brass

This video covers how to heli-braze brass to copper. To join these two materials, use silicon bronze filler material. It is important to remember that you are not trying to melt the base material, which you do in traditional welding. Instead, you are creating a puddle from the filler material.

SAFETY NOTE: Working with brass generates a lot of fumes. Therefore, it is important that your work area is well-ventilated. Ideally, have ventilation and some sort of fume extraction system. It is especially important to not overheat when working with brass. Doing so creates even more fumes than usual.