There are several different techniques that can be used to weld metals, each of which offers its own advantages that make them suitable for different applications and materials. These methods include arc welding, friction welding, electron beam welding, laser welding, forge welding, ultrasonic welding, and others.
Each of these welding techniques impacts the strength, quality and characteristics of the final join. For example, high heat input methods (such as metal inert gas – or MIG - welding) provide deeper penetration but can also cause more distortion, whereas lower heat input methods (such as tungsten inert gas - or TIG - welding) create less distortion and greater control but require greater skill on the part of the welder. The final appearance of the weld can also be affected by the welding method used, with TIG welding offering more aesthetically-pleasing joins than methods like stick welding. However, this tends to create a slower process than with high-speed techniques like MIG or flux-cored welding. This creates a choice between speed of welding and the associated reduction in costs and the final aesthetic appeal of the weld.
Soldering offers a flexible, efficient and accessible method of metal joining that doesn’t melt the base materials. Instead, soldering uses a filler material, called a solder, which melts at relatively low temperatures to form a bond between the work pieces to be joined.
This technique requires both work pieces to be clean and free from dirt, grease or oxidation as these can negatively impact the quality of the join. The next step is to apply a layer of flux across all areas to be soldered, which prevents oxide formation during heating. With the layer of flux applied, a heat source such as a soldering iron is used to heat the work pieces so that they melt the solder so that it flows into the joint. Once the solder has been applied you should remove the heat source and allow the joint to cool naturally before cleaning off any remaining flux residue.
Although the process sounds relatively simple, there are a number of errors in the application of the process that can cause weak joins or even failures. These include inadequate cleaning, not applying enough heat to the work pieces to melt the solder enough to create a strong connection, overheating the solder and damaging components, using too much solder or moving the joint during cooling.
Advances in adhesive technologies have allowed adhesives to grow in importance for metal joining in recent years. Metal bonding adhesives offer a number of benefits, even when compared to traditional joining methods such as welding and mechanical fastening. These benefits include cost reductions and simplified or improved part designs. Because adhesives distribute loads across a whole joint there are fewer points of localised high stress. The main types of load or force that adhesives have to be able to resist include:
- Cleavage and Peel Stress: This is a force that causes a flexible joint to bend, which creates a stress concentration at the edge of the bond
- Shear Stress: This force causes layers or parts to slide against each other in opposite directions
- Tensile Stress: This force stretches and elongates a material
Because adhesives do not require holes to be drilled through the materials to be joined (as is the case with mechanical joining), this type of bonding doesn’t reduce the material strength or create specific areas of stress in the parts. Instead, adhesives preserve the original strength of the materials while also creating a seal that can protect against environmental conditions like chemicals or moisture.
Common types of adhesive used for metal joining include:
- Acrylic Adhesives: These fast-curing adhesives provide high-strength bonds that can resist harsh environmental conditions
- Cyanoacrylate Adhesives: Commonly called ‘super glue,’ these adhesives rapidly bond a range of different materials to create high strength but inflexible joints
- Epoxy Adhesives: Curing at room temperature, these adhesives provide strong resistance to chemicals, fatigue and high temperatures, making them known for their flexible and durable bonds
To choose the best adhesive for metal joining it is necessary to consider the conditions for the final application, the metals that are being joined and the required joint properties. The cure time is also important as it will determine the productivity of your metal joining work.
Mechanical joining methods, such as bolting, can provide an adaptable yet robust metal joining solution that is used in sectors including automotive, construction and heavy engineering. Mechanical joining is a simple method of securing two or more pieces of metal together, using nuts and bolts, screws, and other temporary solutions. With proper application of the fasteners, mechanical fastening can deliver strong joins that can withstand substantial stresses.
Bolts can be paired with washers or spring washers to help prevent them from loosening over time or to better distribute the load. Mechanical fastening is a simple and fast process that doesn’t require specialised skills or equipment and allows parts to be disassembled as required without damaging the base materials. Because there is no need to use heat to join the parts together there is no risk of thermal distortion or metallurgical changes in the metal.
Despite these advantages, it is important to properly align the holes in the metal parts to ensure a proper fit and to prevent joint weakness and potential failure. If the fastening is too loose it can cause the parts to slip and the joints to loosen, however, if it is too tight the fastening can break or damage the metal parts themselves. Mechanical fastening will also increase the weight of the finished parts when compared to adhesive bonding, for example.
Factors that influence which method is best for joining metals include:
- Type of Metal: Some metals react better to some techniques than others as a result of their melting points, ductility or reactivity
- Shape and Size of Work Pieces: Larger components are more difficult to uniformly heat when brazing or welding
- Required Joint Strength: Welding typically provides the strongest bond strength, followed by brazing. Soldering and adhesive bonding tend to offer lower mechanical strengths
- Environmental Conditions: The operating conditions for the joint can determine which joining method will be most suitable, having considered factors including temperature, pressure, chemical exposure, and more.
- Time and Cost: The time and cost required to make the joint can also influence which technique is preferable. Adhesive bonding, for example, tends to be cheaper and faster than welding, but it doesn’t tend to deliver the same bond strength.
Metal joining is an important part of industries ranging from aerospace and automotive through to construction, manufacturing, and beyond. The different methods for joining metals, whether welding, brazing, soldering, adhesive bonding or mechanical fastening, has its own benefits and drawbacks. Understanding these factors allows you to choose the most suitable joining method for your given application, as well as determining the strength, durability, aesthetic appeal and cost of your finished product or products.
1. How can I join broken metal parts?
Broken metal parts can be repaired using different joining techniques, depending on the metals themselves, the material thickness and the required strength of the join. These include welding, brazing, soldering and adhesive bonding.
2. What is metal joining?
Metal joining is an engineering process that combines two or more metal pieces to form a single item.
3. How do welding and soldering differ?
The main difference between welding and soldering is the temperatures involved, although there are also differences in the processes themselves. Welding uses the application of heat to work pieces and often filler materials to form a joint. Soldering uses heat to melt the filler metal that bonds the work pieces together.
4. What factors should be considered when joining metals?
Factors to consider when joining metals include the types of metal being joined, their melting points, the required service conditions and applications for the metal parts, and the required strength of the join. Of course, relevant safety standards need to be taken into account too.