Comparing Metals by Weldability: From Easiest to Most Challenging

Welding is arguably the strongest and most critical metal joining technique. However, each metal reacts differently when the heat of an arc, filler, or shielding gas is applied.

Weldability explains how challenging it is to fuse two metals into a solid, uniform, and defect-free bond. Can you join two pieces effortlessly, or do you need additional preparation, heat control, preheating, or post-heating?

In this article, we will compare metals by weldability. Read more to learn which metals are easy to weld and which require special precautions to form strong, defect-free joints.

What Is Weldability?

Weldability is the capacity of a material to be welded into a defect-free, structurally sound joint. Simply put, it explains how much effort you need to put into welding to create high-quality, sound welds.

Source: https://www.youtube.com/watch?v=wHGKmx8xvKQ

• Excellent weldability means you can easily join two metals without special preparation, heat-control methods, preheating, or post-weld heat treatment. A typical example is mild steel, which welds readily without special tools or practices.

• Good weldability still means you don't need any specific tools or preparation, but you need to control the heat or be careful during welding. One example is stainless steel, which usually welds readily, but it can warp or oxidize when exposed to excessive heat.

• Poor weldability is typical of metals prone to cracking or other defects. Metals like cast iron form a hard structure that cracks rapidly when it is heated or cooled. To address it, you need heat control, preheating, post-weld stress relief, or additional techniques to reduce the risk of defects.

Keep in mind that some metals, such as tool steels, can be practically impossible to weld. They are unpredictable, and even if you do everything right, they can still crack.

Therefore, understanding the metal's properties and weldability is essential before tackling any project.

What Affects Weldability?

Several factors affect the weldability of the metal, but the most important ones include:

• Metal composition: Some elements, such as carbon, can make metal notoriously difficult to weld. The more carbon it has, the harder the metal is. However, it forms a brittle structure that is prone to cracking. Alloying elements such as chromium, nickel, or magnesium can also affect weldability and properties of the base metal.

• Welding metallurgy: The study shows how heat from an arc changes the physical, chemical, and mechanical properties of metals. Some metals have a wide heat-affected zone; some dissipate heat; others cause embrittlement or hot cracking.

• Welding method: There are over 30 different welding methods, each better suited for specific applications and metals. Even the four most popular methods can exhibit different characteristics on various metals.

• Surface condition and contamination: Some metals, such as aluminum and titanium, require immaculate cleanliness. Contamination such as rust, dirt, oxidation, mill scale, oil, or moisture can affect weldability, make welds corrosion-prone, or cause porosity and other defects.

• Thermal properties: Some metals, such as copper or aluminum, have high thermal conductivity. They rapidly pull heat away from the joint, requiring higher initial power. In addition, some metals have a high coefficient of thermal expansion. They expand and contract rapidly, causing high residual stresses and distortion.

Comparing Metals By Weldability

Depending on its composition and properties, either mechanical, thermal, or metallurgical, each metal has different weldability. Understanding the weldability helps you prepare for welding and adjust every detail for best results.

Here is a quick chart of metals arranged by their weldability:

Let's further explain the weldability of each metal. We'll also include tips to make each more forgiving to weld.

Mild Steel: Easiest Metal to Weld

Low-carbon steels, also known as mild steels, are generally the most forgiving and weldable metals. Due to their low carbon content (<0.30%), mild steels are ductile and resist cracking when heated. They don't form a hard martensite structure like medium- and high-carbon steels, making them very forgiving.

In addition, mild steels have favorable thermal properties. They don't overheat easily or expand too much once heated. As a result, you don't need complex preheating, post-weld heat treatment, or stress relief to produce sound, crack-free welds on mild steel.

Welding Mild Steel Sheet Metal
Source: https://www.youtube.com/watch?v=zrPIbYnvDdo

Keep in mind that mild steel is not corrosion-resistant. But that means it doesn't form an oxide layer, so you don't need special care. Unlike some metals, you can burn through light surface contamination.

Depending on the conditions and the metal thickness, you can use any of the popular methods for welding mild steel. MIG, TIG, Stick, or Flux-cored can all work, depending on the application.

Stainless Steel: Weldable, But It Requires Heat Control

Stainless steel is a corrosion-resistant alloy of iron that has at least 10.5% chromium. Added chromium forms a thin surface layer when exposed to air, making this metal resistant to corrosion.

Protective Chromium Oxide Layer
Source: https://www.youtube.com/watch?v=zu5NVibNjmg

In general, stainless steel is weldable, but there is a catch. There are hundreds of stainless steel grades, each with a different composition that can affect weldability. To make it easier for welders, grades are grouped as follows:

• Austenitic stainless steels: The most popular group of stainless steel you are likely to encounter as a fabricator or everyday welder. The most common grade, 304, has high chromium and nickel content, which makes it highly corrosion-resistant but generally weldable.

• Ferritic stainless steels: This group has higher chromium but very low nickel content, which makes them magnetic. They are predominantly single-phase and non-hardenable, so you can readily fusion-weld them. However, thicker segments can create a grain-coarsened zone prone to cracking.

• Martensitic stainless steels: These contain moderate amounts of chromium, low nickel, but higher carbon content. As a result, they are generally less weldable than other grades, as they form a brittle structure prone to cracking.

• Duplex stainless steels: This group has an equal proportion of austenite and ferrite structures. Modern duplex steels are readily weldable, but they have a strict heat input range required to achieve the desired structure.

Although weldable, stainless steel can be challenging to weld due to its thermal properties and cleanliness. Below are the issues and solutions explained.

TIG Welding Stainless Steel Pipe
Source: https://www.youtube.com/shorts/JJdAZO92S3g

Challenges and Solutions for Stainless Steel Welding

Nickel: Good Weldability, But Prone to Porosity and Cracking

Nickel and nickel alloys exhibit exceptional corrosion resistance, even at high temperatures. Their favorable properties make them valuable in critical applications, but also expensive.

But, fortunately, nickel and its alloys are generally weldable. Inconel and Hastelloy are common names, but there are two types of alloys based on weldability:

• Solid solution alloys: Pure nickel, or Fe-Ni-Cr alloys that are readily fusion-welded. HAZ does not harden, so they don't need special heat treatment.

• Precipitation hardened alloys: Ni-Cu-Al-Ti, Ni-Cr-Al-Ti and Ni-Cr-Fe-Nb-Al-Ti alloys that are susceptible to post-weld heat treatment cracking.

In general, you can readily MIG- or TIG-weld nickel and its alloys. But they are prone to porosity, surface oxides, hot cracking, and sluggish welds.

TIG Welding Nickel Alloy
Photo by @welder_china (TikTok)

Issues and Solutions for Nickel Weldability

Aluminum: Moderate Weldability, But Very Challenging To Weld

Aluminum is the most abundant metal in the Earth's crust, with low density, excellent electrical and thermal conductivity, and high ductility. Most importantly, it is corrosion-resistant and infinitely recyclable, making it valuable for various industries worldwide.

Aluminum in Modern Industry
Source: https://www.researchgate.net/figure/Spectrum-of-aluminium-alloys-used-in-space-applications-Practically-every-grade-of_fig3_358484374

Aluminum has fair-to-moderate weldability, which mostly depends on the specific alloy series. Here are some examples:

• 1xxx, 3xxx, and 5xxx series: Highly weldable, non-heat-treatable alloys with good corrosion resistance.

• 6xxx series: Very common structural alloys that weld well, though they experience some softening in the heat-affected zone.

• 2xxx and 7xxx Series: High-strength aerospace alloys that are generally crack-prone and difficult or un-weldable via standard fusion methods.

Although most aluminum is weldable, welders consider it one of the most difficult metals to weld, especially for less experienced welders. Aluminum has high thermal conductivity and a low melting point; it forms an oxide layer and is prone to contamination, posing challenges.

TIG Welding Aluminum
Source: https://www.youtube.com/shorts/sewFLh_iL6c

Challenges and Solutions for Aluminum Welding

Copper: Fair to Challenging Weldability

Copper is a highly versatile metal, widely used across industries for its high conductivity, malleability, sanitary properties, and corrosion resistance. Like other metals, copper can be used pure or with alloying elements.

Alloying elements alter the properties of copper, which can also affect its weldability. Here are the most commonly used types of copper (copper alloys) and their weldability:

• Pure copper - needs preheating: Due to high thermal conductivity, pure copper is challenging to weld and needs preheating.

• Free machining copper - unweldable: Severe susceptibility to hot-cracking and porosity.

• Precipitation-hardenable copper alloys - fair to poor fusion weldability: Heat-affected zone (HAZ) softening, localized cracking risks, and high thermal conductivity.

• Brass (copper-zinc) alloys - fair-to-moderate weldability: Fusion welding burns zinc, which releases toxic fumes, and increases risk of porosity and hot cracking.

• Phosphor-Bronze (Copper-Tin) - fair weldability: Susceptible to hot cracking in the stressed condition.

• Aluminum-Bronze (Copper-Aluminum) - Highly weldable: Recommended fusion welding methods (MIG/TIG/Stick).

• Silicon-Bronze (Copper-Silicon) - Excellent weldability: One of the easiest copper alloys to join

• Cupronickel Alloys (Copper-Nickel) - Good weldability: Requires matching 70/30 filler metals, strict surface cleanliness, and no preheating.

TIG Welding Copper-Nickel Pipe
Source: https://www.youtube.com/watch?v=UdLmwDr80so

How to Deal With Copper Welding Challenges

Although most copper alloys are weldable, welding copper can be tricky due to its properties. Here are the challenges and solutions.

• High thermal conductivity: Preheat alloys with high conductivity and pure copper. Control and adjust the heat.

• High thermal expansion: Clamp the pieces to reduce distortion and warping. Preheating also helps reduce the stress that causes distortion.

• Reactive when heated: Thoroughly clean the joints and copper pieces, and use the proper shielding gas to protect the welds.

• Releasing toxic fumes when welded: Weld in a well-ventilated area and control the fumes.

Cast Iron: Poor Weldability and Challenging to Weld

Cast iron is a group of iron-carbon alloys known for high compressive strength, excellent wear resistance, and great thermal stability. It has 2-4.5% carbon, which increases strength but significantly reduces the success of welding cast iron.

Due to its increased carbon content, cast iron forms a hard, brittle martensitic microstructure. During rapid heating and cooling, welds are prone to cracking, so serious precautions are taken.

Tig Welding Cast Aluminum With Helium Mix
Source: https://www.youtube.com/shorts/Iue6BKU4TuM

Like other metals, cast iron comes in different forms depending on the amounts of carbon and silicon it contains. There are different types with different weldability:

• Gray cast iron: Most common form, but it has poor weldability. Successful repair requires strict preheating, specialized filler metals, and controlled slow cooling.

• Ductile/nodular & malleable iron: Greater ductility and better fracture toughness make these types easier to weld than gray iron. However, they are still crack-prone and require special rods and preheating.

• White cast iron: Carbon occurs entirely as iron carbide rather than graphite, making it extremely hard and wear-resistant. However, enhanced hardness makes it brittle and almost unweldable.

Types of Cast Irons
Source: https://www.researchgate.net/figure/Types-of-cast-irons-26_fig1_397955129

Main Issues and Solutions in Cast Iron Welding

Tool Steel (High-carbon Steel): Very Poor Weldability (Almost Unweldable)

High-carbon steels, such as tool steel, as their name suggests, contain a high amount of carbon (up to 1.5% or 2.5% in tool steel). As a result, these are extremely strong, hard, and built to withstand everyday harsh use, wear, and tear.

But, like with cast iron, this much carbon makes them very challenging, almost impossible to weld with fusion welding methods. A hard, brittle microstructure requires adequate heat treatment, including specialized preheating, postheating, and stress relief.

However, even if you do everything right, it doesn't mean you'll succeed. Welds can eventually crack, even with proper technique and heat treatment, so it is reserved for seasoned welders only.

Welding High-Carbon Steel Alloy Train Tracks
Source: https://www.youtube.com/watch?v=5ocKjyVWhak

Issues and Solutions for Poor Weldability of High-Carbon Steel

Final Thoughts

Many metals seem similar at first glance, but they are fundamentally different in terms of composition, mechanical, and thermal properties. Besides applications and everyday use, these also affect weldability.

Weldability refers to the effort required to join two metals. It ranges from the very good weldability of mild steel, which welds readily without special precautions, to high-carbon steels, which require preheating, postheating, specific filler metals, and additional precautions.

Understanding the weldability of each metal helps you prepare. By adapting to the challenges, you are ready to tackle any project.

🧐 Comparing Metals by Weldability: From Easiest to Most Challenging FAQ

1. What makes a metal easy to weld?

A metal is easier to weld when it handles heat well and is less likely to crack. Metals with good weldability also need less special preparation, preheating, or heat treatment.

2. Why do some metals crack after welding?

Welding causes the metal to expand and contract as it heats and cools. If the metal becomes too hard or brittle, these stresses can cause cracks in the weld or heat-affected zone.

3. Do all metals need preheating?

No. Mild steel usually welds well without preheating, while cast iron, high-carbon steel, and some copper alloys may need it. Preheating helps reduce thermal stress and cracking.

4. Can you weld dirty metal?

You can sometimes weld through light contamination, but clean metal usually produces better results. Oil, rust, moisture, and oxides can cause porosity, poor fusion, and other weld defects.

5. Does welding thicker metal require more heat?

Usually, yes. Thick metal absorbs more heat and often needs higher heat input to achieve proper fusion. Some highly conductive metals may also require preheating.

6. Can the wrong welding settings cause cracking?

Yes. Too much or too little heat can create poor fusion, excessive hardness, distortion, or cracking. The correct settings depend on the metal, thickness, and welding process.