Joining is one of the basic categories of manufacturing processes by which two or more parts are joined to create a single assembly. In contrast to forming or material removal processes, joining allows multiple pieces, possibly of dissimilar material, to be brought together to create sophisticated and functioning products. Joining is a fundamental process in the automotive, aerospace, electronics, and heavy machinery sectors.

Joining processes can be grouped at a fundamental level as mechanical fastening, fusion welding, solid-state welding, brazing, soldering, and adhesive bonding. The choice of an appropriate method will typically be determined by the base materials employed, needed joint strength, temperature sensitivity, disassembly requirements, and cost.

Process Categories

Mechanical Fastening

Mechanical fastening with hardware, such as riveting and bolting, physically connects components without distorting the base materials. They are simple to dismantle but may have stress concentrations and expose both sides of the assembly.

Welding

Welding unites materials, typically metals, with heat (and in some instances, pressure), melting the pieces and fusing them after they have cooled. Fusion welding processes (MIG, TIG, and laser beam) utilize specific filament materials for specific applications like ER70S-6 – Carbon steel wire with deoxidizers (MIG), ER4043 – for general-purpose applications (TIG), etc., to produce tough, permanent welds but are likely to produce thermal distortion. Solid-state welding, like friction stir welding, entails no melting and can be employed in temperature-sensitive applications.

Brazing & Soldering

Soldering and brazing are metal joining operations wherein filler metals of lower melting point than the base metals to be joined are employed. The most commonly used base materials are copper, brass, steel, stainless steel, and aluminum.

In brazing, the filler metals are typically copper-alloyed metals (e.g., Cu-Zn, Cu-Ag, or Cu-P) and the process is above 450°C. Brazing forms strong leak-tight joints and is applied in aerospace, automotive, and HVAC industries. A flux is generally needed to avoid oxidation, enhance wetting, and permit flow of the filler metal.

In soldering, the filler metals are usually tin-based alloys (like Sn-Pb, Sn-Ag, or Sn-Cu) with melting points lower than 450°C. Soldering is especially well suited to electronics and electrical assemblies since it induces minimal thermal distortion and is of high accuracy. Flux is also critical in soldering to remove oxides and facilitate good electrical and mechanical contact.

Adhesive Bonding

Adhesive bonding is the establishment between surfaces of engineered polymers or resins. Although typically of lower mechanical properties than welding, adhesives can bond dissimilar materials, minimize concentrations of stress, and absorb vibration, and thus are particularly suited to composite structures and light-weight assemblies.

Material Compatibility

Different levels of compatibility exist for each joining process with different materials depending on their thermal properties, chemical activity, and heat resistance. For example, fusion welding requires high heat resistance and good weldability, and therefore is applied to metals such as steel and aluminum. Nevertheless, adhesive bonding is also capable of joining incompatible or heat-conductive materials such as plastic and composites without warping them because of heat. Understanding of material strength and limitations of each joining process is required in determining the right process for an application. A summary of typical materials that are easily joined by each process is shown in the table below:

There are limitations and strengths with each joining process. Joint geometry, load carrying, environmental exposure, and service conditions need to be considered by designers when choosing a joining process. Knowledgeable decisions make manufacturability, performance, and product life cycle reliability better.

Joining Process Comparison

Joining Process Capabilities

Advantages: Wide material compatibility, variety of strength levels, and scalable

Disadvantages: Process selection is highly dependent on geometry, cost, and thermal exposure

Applications: Aerospace panels, automotive bodies, electronics, HVAC tubing, structural assemblies