How to Select an Appropriate Joining Method
Joining is a very broad category of engineering processes. Joining, as the name implies, simply refers to the process of attaching components together. The components can be the same or different materials, and the durability of the joint can be temporary or permanent.
We encounter examples of joined components everyday in our lives: from the adhesives holding our cardboard and plastic food containers together, to the threaded caps keeping our beverages in our bottles, from the soldering & brazing enabling our electronic gadgets to function, to the welds, adhesives, and fasteners holding our cars together. Without the ability to join components, we would not be able to enjoy the technologies and luxuries we take for granted every day.
In a very general sense, there are 3 main types of joining processes, each with its own advantages and disadvantages depending on the application. A comparison of the 3 main types of joining processes can be seen in Table 1.
| PROCESS | DURABILITY (1 = Temporary, 10 = Permanent) |
EXAMPLES | STRENGTH (1 = Weak, 10 = Strong) |
|
 |
Adhesive Bonding (Adhesives) |
1-8 | Tapes, glues, sealants, etc |
1-8 |
 |
Mechanical Fastening (Fasteners) |
1-5 | Nuts & bolts, screws, clamps, pins, etc |
1-9 |
 |
Fusing / Melding |
8-10 | Welding, brazing, glass fusing, etc |
6-10 |
TABLE 1 – Examples of joining processes
Factors to consider when selecting a joining process
There are many factors to consider and questions to ask when choosing the appropriate joining process, including, but not limited to:
1) Materials to be Joined
Are the materials the same, or different? If different, what are the differences? Is one capable of accepting a weld, or being drilled, while the other is not? Is one porous while the other is not? Is one brittle while the other is flexible? Is one material much stronger than the other?
2) Durability (Permanence)
Do we want the joint to be temporary such that it can be removed or repositioned at a later date? If we want to remove or reposition, how many times can this be performed before the joint fails? If we want the joint to be permanent, do we care if the material has to be destroyed in case the joint were ever to be removed?
3) Stresses Encountered
Is the area of the joint going to undergo shear, tension, compression, etc? Do we wish for the joint to be as strong as the materials it is joining?
4) Environmental Factors
Is the joint going to be subjected to extremes of heat, cold, rain, dust, oil, etc?
5) Surface Preparation
How difficult is it to prepare the materials to be joined for the joining process? Does one or the other need to be grinded or sanded, cleaned, prepped, drilled, tapped, etc?
6) Final Appearance
Does it matter if the joint can be seen, or should it be invisible? Should the joint be seamless?
7) Maintenance / Repair
How often will the joint and / or joined components need to be maintained, repaired, replaced, etc? If maintenance is required but not performed, would a failure of the joint be catastrophic?
8) Cost & Schedule
What is the budget for joining the materials? Is there a timeframe or schedule to adhere to, and will the time it takes to join the materials fit within the schedule?
Narrowing down the process selection
Once these factors are considered and the applicable questions answered, we can then start to narrow down the type of joining process. However, we must realize that while many of these factors & questions need to be considered regardless of the joining process being used, others are unique to the specific process. Furthermore, we need to be aware that in some cases, more than one joining process will work for a given application. For instance, we can hang a picture with a “peel and stick” poster adhesive or we can hang it with nails or screws. Both would work for the task, but the final choice could be determined on strength, longevity, whether or not there is the desire to move or reposition the picture, damage of the substrate (in this case, the wall), etc.
Don’t be fooled, however, into thinking that once 1 of the 3 joining processes is chosen that the decision making process is over. There are countless choices within each of the 3 main processes:
Example 1: Let’s say we have decided we would like to join components together using Adhesive Bonding. Do we use glue, or tape? If using glue, do we use quick-drying glue, or one with a longer curing time? Do we need to be able to reposition the components? Do we use rubber cement, Elmer’s glue, Gorilla Glue, Crazy Glue, or some other type? Do we use epoxy? Can we use sealant?
Example 2: Now, we would like to use Mechanical Fasteners to join something. Should we use nuts and bolts, clamps, or pins? If using nuts and bolts, do we need to drill and tap? Hex, Allen, Phillips, or some other head interface? What material is adequate for the fasteners? If metal, what kind of metal? Do we need a coating (galvanized, etc)? Are we using metric, or US Imperial (aka “standard”, aka English) sizes? Course or fine thread? Grade 2, Grade 8, or some other Grade? What size bolt head do we want? Do we need a locknut, or would a traditional hex nut suffice? Do we need washers?
Example 3: Finally, we have decided we want to Fuse / Meld materials together. Do we braze, solder, weld, or fuse (as in the case of fusing glass, for instance)? If welding, do we use MIG, TIG, SMAW, or some other welding process? Do we need to heat treat the materials, and how will this affect the weld? How does heat treating before welding affect the materials compared to after welding?
While seeing all these variables here in print might make it seem like a daunting task to choose a joining process, it can be somewhat simplified and narrowed down by keeping 3 simple rules in mind. Remember that while there might be exceptions to the rules and overlapping applications for the processes, referring to Table 1, will help us to acknowledge and then manage the exceptions.
Summary – the basic rules
RULE 1 – For temporary joints, Adhesive Bonding or Mechanical Fastening is preferred over Fusing / Melding.
RULE 2 – Mechanical Fastening and Fusing / Melding typically create stronger, more permanent joints than Adhesive Bonding (EXCEPTION – epoxies and permanent glues such as “Crazy” Glue can create very strong, nearly permanent joints. Also, adhesives used on substrates such as stickers create a bond which will more than likely destroy the sticker upon removal).
RULE 3 – For components which require the ability to be taken apart for maintenance or replacement purposes, Mechanical Fastening is usually preferred.
As we can see, choosing the appropriate joining process for the task at hand requires due diligence on the part of the engineer in order to ensure that the application needs are met and that the joint does not fail. Communicating with the customer to define and understand the specific requirements and asking applicable questions will certainly help guide the decision making process.