The most important aspect of a weld is its reliability. Welds in critical applications, such as on aircraft or medical devices implanted in someone’s body, can’t fail. That’s why there are so many specifications and requirements governing all aspects of a weld.
At EB Industries, we specialize in welding services for customers in high-tech and high-stakes industries such as aerospace, defense, medicine, and alternative energy. Our process has been designed to minimize potential problems and situations that may arise during the welding process. However, understanding the common causes of weld failure is important for design engineers and quality assurance personnel. This knowledge is critical in ensuring that your parts have a long and effective lifespan.
The Causes of Weld Failure
High Melting Point Metals and Alloys
Many critical applications require metals that can maintain their strength at high operational temperatures. The high melting points of these materials pose unique challenges during the welding process. Controlling temperature during welding can be challenging due to higher melting temperatures and increased heat input requirements. This can lead to welding defects such as warping and cracking during both the welding and cooling processes. Overheating risks damaging the base metal or the weld, and the problem compounds when different materials are welded together. When precision and stability are crucial, mitigating these challenges using various heat control techniques becomes even more critical.
Disparate Thermal Expansion Characteristics
Critical applications often require making parts that have properties that are an amalgamation of specific characteristics of various component materials. A part might require high tensile strength in one area, but another section of it might require precision machining that can best be performed on a more malleable material, and often the weld is the interface between the two. Often these welds are fusions of materials with different thermal expansion coefficients making welding a complex task. Different expansion rates can lead to warping, cracking, and other defects. Some of the most challenging welds we execute at EB Industries involve complex combinations of materials.
Reactivity is another factor that can make precision welding difficult. When welding, some metals like aluminum and titanium can react with the air and cause surface oxides to form. These oxide layers can act as barriers, preventing complete fusion, contaminating the weld pool, and weakening the joint. In some instances, such as during laser welding, oxide layers can interfere with the laser’s ability to couple with the metal and efficiently transfer energy into the weld area. Reactive metals can also generate gases during welding, leading to porosity—small voids in the weld that further diminish its strength and integrity. Some of the most difficult materials we weld at EB Industries are reactive and have high melting points. Titanium, a commonly used material in many high-tech/high-stakes applications, is a perfect example of a material that requires special handling and care to get high-quality, pure welds.
Tantamount to avoiding temperature-related welding issues is controlling heat levels in the workpiece. To ensure the entire assembly is strong, durable, and reliable, it is crucial to maintain the optimum heat levels during the welding process. Inadequate heat control can give rise to a range of problems that compromise the structural integrity of welds. Poor control of heat and energy results in the following issues:
- Warping or cracking: If the heat input during welding is too high, it can cause excessive thermal expansion and contraction, resulting in the warping or cracking of the metal.
- Overheating: Overheating the metal during welding can cause it to become brittle and reduce its strength and ductility.
- Lack of fusion: If the heat input during welding is too low, the materials to be welded might not fully melt and properly fuse resulting in a lack of fusion and a weak weld joint.
- Excessive hardness: Welding at high temperatures can cause the metal to become hardened, reducing its toughness and making it more prone to cracking.
- Reduced mechanical properties: Poor heat control during welding can also reduce the mechanical properties of the metal, such as its strength, toughness, and ductility, reducing the overall quality of the welded joint.
The keys to controlling heat are accurate placement and control of either the electron beam or laser beam. Because of the nature of this type of welding, there is the possibility of tremendous computer control of virtually every aspect of the weld—from power to placement, to feed rate, and to even more esoteric factors, such as the geometry of the beam itself, which can be modified to affect different areas within the weldment differently—more power here, less power there, etc.
Electron and laser beams not only have control and accuracy, but they also have the potential to deliver tremendous amounts of energy into a workpiece. This means that high-temperature materials can be heated quickly and precisely, which vastly improves weldability, and minimizes the HAZ (heat-affected zone), the area around the weld that is heated and cooled during welding. Minimal HAZs mean less potential damage to the part and to sensitive components that might be within the part.
Another key to controlling heat during welding is the proper development of tooling and fixtures. EB welding generally must occur in a vacuum chamber, so parts must be manipulated via specialized tooling. Our tooling is generally part of the heat control strategy for the part, along with careful placement of the beam in consideration of the melting points of the materials, precise control of power and feed rates, etc.
Porosity is the presence of small voids or holes within a weld. The voids in the weld can act as stress concentrators and reduce the strength and integrity of the joint, which makes the weld more prone to failure under load. In fusion welding, porosity is typically caused by the release of gases during the welding process. The gases typically result from the vaporization of contaminants in the weld area, such as moisture, hydrocarbons, oxides, or other contaminants in the base metal or filler material. Gasses in the weld area, especially oxygen, also contribute to porosity issues during welding. Some materials, such as aluminum, are porosity prone, which makes them especially challenging to weld.
Several factors can contribute to porosity in welding, including:
- Contaminants: Contaminants in the base metal or filler material, such as moisture, oil, or grease, can generate gases during the welding process that can become trapped in the weld and form porosity.
- Poor shielding: Insufficient protection of the weld area from the air can cause oxidation and the release of gases that can become trapped in the weld and form porosity.
- Incomplete penetration: If the materials to be welded do not sufficiently melt and fuse, the result can be a lack of fusion and porosity in the weld.
- Welding too quickly: Welding too quickly can cause the metal to cool too quickly, trapping gases in the weld and resulting in porosity.
The first step to controlling porosity is the proper handling of materials to be welded. Pre-cleaning of parts before welding using appropriate solvents and techniques, and storage and handling of parts in specific environments are necessary to maintaining pristine weld areas. Potential contaminants, especially gases like oxygen, must be avoided when welding reactive materials. When laser welding, particular care must be paid to the use of shielding or cover gases. A cover gas is typically an inert gas, such as helium or argon, which is used to flood the weld area and drive out any air or other potentially contaminating gases. Gas contaminants are much less of a concern when electron beam welding, as the process generally occurs in a vacuum where gas is a non-issue. Electron beam welding is typically selected for materials like titanium, which require high temperatures as well as a contaminant-free environment to ensure a quality weld, but excellent results can also be obtained using laser welding in a glovebox environment.
A Welding Process is Only as Good as the Inspection Process
We cannot stress enough the importance of weld inspection in our overall process. EB Industries performs all weld inspection in-house utilizing the latest inspection technology operated by AWS certified weld inspectors. This is an essential final step in the making of a part.
We also perform most Non-Destructive and Destructive testing in-house. We have a full sectioning lab as well as full range of NDT equipment.
Testing and inspection at EB Industries are not a final step or an after-thought: they have been incorporated into our manufacturing process. Inspection areas are an integral part of all our electron beam and laser welding stations, and generally, a trained operator and a trained inspector work as a team on all welding projects. This allows for continuous monitoring of quality with no impairment of production speed.
Testing is continuous as well. Parts and raw materials are tested as they come in-house, at required points during manufacturing, and then extensively before parts are packed and shipped.
In this brief article, we have touched upon what we see in our work as the prevalent causes of weld failure, some of the specifics, and strategies for mitigation. The main takeaway is that robust, reliable welds in a critical application don’t occur haphazardly, and are instead the result of experience and a carefully implemented welding process that encompasses everything from part storage to work area cleanliness to tool design to machine maintenance to operator and programming skill. If you’re developing a new part for a high-tech and high-stakes industry, our engineering staff can work with you to ensure your welds meet the required quality specifications. If you’re already manufacturing a part, and are concerned about failures, we can help you in that aspect as well.