Exploring the realm of metallurgy has a wide range of challenges in welding, with certain metals posing formidable obstacles even to skilled welders. In this exploration, we delve into the question: What is the hardest metal to weld? From ordinary metals to exotic alloys and refractory metals, we navigate the difficulties welders face in their pursuit of joining the unyielding substances shaping our modern world.

Factors Affecting Weldability:

1. Chemical Composition:

Alloying elements significantly impact weldability. Metals like high-carbon steel can be hard to weld due to their composition. For instance, carbon content above 0.30% can lead to increased hardness and susceptibility to cracking.

2. Melting Point:

High melting points require more heat input, risking distortion, while low melting points may lead to overheating. For example, metals like tungsten and molybdenum have extremely high melting points, making them challenging to weld without specialised equipment.

3. Thermal Conductivity:

Influences heat distribution during welding, affecting bead size and joint shape. Metals with high thermal conductivity, like copper, dissipate heat quickly, posing challenges in controlling the welding process and preventing distortion.

4. Metallurgical Structure:

Certain structures, like martensite in high-carbon steels, can make metals harder to weld. Martensite formation during rapid cooling after welding can result in brittleness and cracking in the weld area.

5. Impurities:

Contaminants increase the likelihood of defects. Metals such as aluminium are highly susceptible to contamination from atmospheric gases and surface impurities, leading to porosity and reduced weld quality.

6. Hydrogen Embrittlement:

Some metals, like high-strength steels, are susceptible to hydrogen absorption during welding. Hydrogen can diffuse into the metal, compromising its mechanical properties and leading to embrittlement and cracking.

Issues Caused by Low Weldability:

Lower weldability of the specific metal can result in weak welds prone to cracking or other defects that affect their structural integrity. As a result, you can see holes that eventually progress into cracks and leaks. Thermal conductivity and melting points can cause metals to warp and distort, or the heat-affected zone can dissipate heat without fusing the pieces correctly.

Metals like cast iron or high-carbon steel are exceptionally hard and suitable for industrial applications. However, as the metal’s hardness increases, the weld joint’s crack resistance is lower. Trying to match the tensile strength and yield strength can lead to severe cracking into the weld joint, so achieving high-quality welds can be a nightmare.

Hardest Metals to Weld:

Welding is a metal joining process that can be used to fuse various metal types and alloys. However, some of them, such as mild steel, are easier to weld, while some, such as aluminium, cast iron, or titanium, can be a nightmare.

Aluminium:

Aluminium is a non-corrosive, lightweight, and aesthetically pleasing metal commonly used in fabrication. However, aluminium is soft, highly sensitive, and is insulated by a tough oxidised layer. Higher thermal conductivity and low melting point, combined with other properties, make it versatile but also more challenging to weld.

• Oxide layer: Aluminium resists corrosion due to the oxide layer that forms once the bare aluminium is exposed to air. The aluminium oxide layer repels corrosion, which makes it highly versatile in various applications. However, the oxide shows drawbacks when welding. Pure aluminium melts at 650°C (1,200°F) , while the oxide melts at 2,037°C (3,700°F), which makes aluminium welding without removing a protective layer impossible.
• High thermal conductivity and low melting point: These two properties can make aluminium welding an absolute nightmare. You can quickly burn through the pieces if you fail to control the heat during the welding job. A smaller window of workability can make tracking and indicating weld progress and quality complex.
• Vulnerable to contamination: Aluminium is a sensitive metal highly vulnerable to impurities and contamination. Any dirt or contact with atmospheric gases such as oxygen or hydrogen can contaminate the weld and make it prone to corrosion, porosity, or cracking. So, to weld aluminium, you must thoroughly clean and carefully store aluminium pieces.
• Porosity: Aluminium absorbs hydrogen quicker the more it heats up. Once the metal starts to cool down, the hydrogen escapes and creates small holes inside the weld, known as porosity. Porosity is not only an aesthetic issue. It can compromise the structural integrity of the weld, eventually leading to cracks and failure.

Aluminium Welding

Knowing where the challenges in aluminium welding come from will significantly increase your chances of welding it successfully. Firstly, knowing that aluminium is highly vulnerable to contaminants puts an extra focus on cleanliness and weld preparation and cleanliness. Before welding aluminium, thoroughly clean the pieces and remove the oxide layer. To do so, use a stainless steel brush that you’ll use only for aluminium. Also, ensure you have high-purity shielding gas such as 100% Argon.

To ensure proper heat control, you should use TIG welding or MIG welding. TIG welding aluminium is done with AC TIG welders such as Primeweld Tig225X, and it can be more challenging compared to MIG. However, the TIG welding allows you to fine-tune the heat, providing features such as AC balance and pulsing, producing the best aluminium results and precise control. 

Gas Metal Arc Welding (MIG) is a semi-automated welding process with fewer options but is easier to use. However, pulse and double-pulse MIG welders will produce TIG-like welds. An aluminium filler material such as ER4043 or ER5356 will do the job in most applications.

So, to avoid welding defects when welding aluminium, clean the pieces, provide good shielding gas coverage, and control the heat to provide proper fusion and avoid burning through. Even though it sounds complicated, you can get the job done efficiently with some practice.

High-carbon steel:

Carbon steel is an alloy composed primarily of iron and carbon. It’s the most common type of steel and typically contains low amounts of other alloying elements, improving its mechanical properties. Welding steel is generally straightforward, but things can get complicated once the carbon content surpasses 0.30%. As the carbon content increases, the steel becomes harder, corrosion-resistant, and resistant to wear and tear.

Structural changes in a heat-affected zone make high-carbon steels and other metals with high carbon content exceptionally hard to weld. Rapid cooling during the welding process can result in martensite formation. This microstructure is very hard but brittle, which makes the weld prone to cracking. Additionally, diffusible hydrogen can lower the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement and, eventually, breaking.

Additionally, high carbon content can also cause warping during the cooling. As the metal cools, it can significantly deform. Weld discoloration can also emerge, and it is not only an aesthetic issue in carbon steel welding. It can also indicate other problems, such as a lack of fusion, porosity, etc.

Welding High-carbon Steels

Welding carbon steel in a room-temperature environment will cause rapid cooling, which leads to hardening and, eventually, cracking. Therefore, you will need to preheat the pieces prior to welding and postheat them after welding. Preheating the carbon steel before welding will promote slow cooling, remove moisture and hydrogen, and reduce the thermal stress into pieces. The preheat temperature will vary on the thickness and grade of the steel, and it is typically 150°C to 260°C.

Post-heating the pieces after the welding will prevent martensite formation, a very hard but brittle microstructure in the weld area. Post-heating carbon steel can be done by full, soft, or stress-relief annealing. The key is to heat the pieces to create an austenite microstructure, which is malleable and ductile, and then let the parts cool slowly to avoid residual stresses and cracking.

MIG welding is preferred for high-carbon steel, but you can also use Stick/Arc for thicker or TIG for thinner pieces. When selecting a filler material, ensure it has low amounts of diffusible hydrogen and undermatch its tensile strength to avoid creating a hard weld pool prone to cracking.

Cast Iron

Cast iron typically has a carbon content of 2% – 4%, roughly 10 times as much as most steels. The high carbon content causes the carbon to form flakes of graphite. Like tool steel, cast iron is one of the most difficult metals to weld due to its high carbon content, and the issues occur due to rapid cooling. Cast irons are hard, strong, and machinable but have poor ductility, making them incredibly difficult to weld.

Cast iron typically has a carbon content of 2% – 4%, roughly 10 times as much as most steels. The high carbon content causes the carbon to form flakes of graphite. Like tool steel, cast iron is one of the most difficult metals to weld due to its high carbon content, and the issues occur due to rapid cooling. Cast irons are hard, strong, and machinable but have poor ductility, making them incredibly difficult to weld.

Welding Cast Iron

Most welding on cast iron involves repairs to castings, not joining two metals. Broken cast iron parts are not unusual, given the brittle nature of most cast iron, and the most common type is grey cast iron. Cast iron repairs are usually done by stick welding, and an experienced welder will use nickel rods (either 99% or 55%). Steel rods can be used for minor repairs, but welds can be less machinable.

There are two ways to weld cast iron, with or without preheating. When preheating, heat the pieces 260-650 degrees C, but never over 760 C. During the welding, limit the heat to minimise admixture and residual stresses, and use short, 1″ welds. Without preheating, you can heat the base metal up to 100 F and weld shorter welds with limited current. If you use nickel rods, you can apply compressive stress by penning.

A critical temperature in most cast iron is about 790 degrees C. Over this temperature, most issues and cracks are developed. The heat generated by the arc will surpass the critical temperature, but keeping pieces heated above it can cause defects.

To avoid post-weld defects, allow the part to cool slowly. You can cover it with an insulating blanket or bury it in dry sand to slow cooling rates and reduce the risks of cracking. Keep in mind that cracks can appear even if you do everything right.

Titanium:

Titanium is a unique material that is lightweight but can bear significant load. High strength-to-weight ratio and corrosion resistance made it a new wonder metal in the past. However, titanium shows specific properties that make it extremely hard to weld.

Firstly, Titanium is a highly reactive metal. Oxygen and nitrogen diffuse into titanium at temperatures above 400°C, causing severe embrittlement. Additionally, oxygen and nitrogen from the air can contaminate the weld and shielding gas, and hydrogen derived from moisture or surface contaminants can be a real problem.

Titanium also has an extremely high melting point and low thermal conductivity, which makes it hard to weld without damaging the pieces and using special equipment. High strength and low weight can cause warping or burning through without controlling the heat, which once again requires operator experience.

Welding Titanium

To successfully weld titanium, you must ensure maximum cleanliness and adequate heat transfer during welding. Due to reactivity, cleaning and degreasing both base metal and filler wire is crucial. Consumables should be stored in clean, dry conditions and not unprotected in the fabrication atmosphere.

Because of its affinity for embrittling elements, titanium is usually welded by TIG welding and MIG welding with inert gas shielding. Argon and argon/helium gas shielding must adequately cover the weld, but there is a catch. Above the critical temperature of 515 C, molten titanium will absorb oxygen and nitrogen, so you’ll need a trailing shield to protect it from contamination before it cools down.

Even though the weldability of titanium is decent, avoiding contamination is crucial. You can use dedicated titanium rods and techniques such as GTAW or GMAW.

Copper, Brass, And Bronze

Copper, brass, and bronze are different materials, but fundamentally, they are made of copper and alloying metals such as zinc or tin. It is difficult to weld copper due to its high thermal conductivity, which can cause it to cool quickly during welding, forming a brittle weld joint or dissipating the heat, causing a lack of fusion. Additionally, copper’s high thermal conductivity is also conducive to heat deformity and damage, making it very easy to warp or distort parts.

Once you add alloying elements such as zinc or tin, you get brass and bronze, which can have various grades, different types, and compositions. Even though you can successfully weld or braze all copper-based metals, due to high heat requirements, you can easily warp, distort, or burn pieces as a beginner.

Magnesium

Magnesium is not one of the metals you’ll encounter in your everyday hobby of welding, but it is worth mentioning why magnesium is a risky and difficult metal. It has a low melting point and good thermal conductivity, which can make controlling the welding process challenging.

Like aluminium, magnesium is highly reactive to contamination and can easily ignite when exposed to oxygen or moisture. It burns at a temperature of 2205 C, and the fire can only be extinguished by starving it of oxygen or using a Class D fire extinguisher, so magnesium welding is best left to professionals.

Final Thoughts

Understanding each metal’s properties is crucial for successful welding. By addressing potential issues through proper cleaning, shielding, and heat control, welders can tackle even the most challenging metals with confidence.