Weld Deposition Optimization Through Design
When designing a product, we are constantly looking for ways to make the design more efficient. This includes thinking critically about weld deposition efficiency. I have written before on why reducing the number of welds creates a more optimized design that saves time, money and reduces the amount of heat generated. Experienced welders know how to optimize welds for greater efficiency. I outline two tips below that have been shared from expert welders and should be considered when determining how many welds to use in design.
Related Content: How to Spec a Weld
Weld Design Tip #1
The first thing I would like to highlight is joint selection. If you are taking your joints out of the CSA W59 pre-qualified section then you have a really nice side-by-side comparison of the different weld joints. However, the one piece I would like to highlight is the price you pay for only welding a joint from one side. I am going to look at three similar style joints. These include:
1) the single V-groove with a backing strip
2) the single V-groove with a landing
3) the double V-groove
I chose these welds to use as examples because V-grooves and bevels are some of the simplest joints to produce with a variety of methods of cutting the bevel ranging in cost and degree of automation. For this example, I will also be using CSA W59 specifications of pre-qualified joints and a material thickness of ½” and a GMAW-P process.
The Single V-Groove with a Backing Strip
The first groove is the V-groove with backing strip. This is a common joint for single-sided welding if access to the back of the weld gets blocked during construction. At Sparta Engineering, we specify this style of weld and its cousin the butt joint with backing strip many times on floors or bottom of structures where it isn’t practical to address the back side of the weld. This weld requires the highest volume to fill of the three welds but that cost is offset with the fact that you don’t need to move the welder or the object being welded.
The Single V-Groove with a Landing
The second joint is a single sided V-groove with a cap on the back of the weld. This has significantly less weld then the one-sided V-groove. The example I am using requires roughly half the weld by volume than the single-sided version with the backing strip. The reduction in the weld volume means less cost and less heat during the welding procedure. This option does require the welder to gauge to sound metal and back fill the opposite side which comes at a cost.
The Double V-Groove
The final joint I want to look at is the double V-groove. This groove has the absolute smallest amount weld volume but trades this off with the cost of having to prep both sides of the parts. The joint used here with ½” plate and GMAW-P procedures, requires half the weld volume from the v-groove with landing used in example 2, and a one quarter of the weld volume of the joint used in example 2.
Obviously there is more to consider than purely weld volume when choosing one weld over the other. In my experience, the volume of weld doesn’t get as much focus as other cost factors. Cutting weld volume down 75% just by joint selection could also be a significant savings depending on the situation.
Weld Design Tip #2
The other optimization tip I would like to share with you is fit-up. Quality of fit-up between two parts has a huge impact on cost. I would like to share with you an engineer’s perspective on this problem. So once again using CSA W59 as a basis for specifying welds, let’s look at a ¼” fillet weld between two ¼” plates.
The welded joint might look like this:
Now when an engineer does any calculations he will assume that the legs of the fillet are 0.25” each as specified and the throat of the weld is 0.177” long. This is important for an engineer as the weakest shear plane of the weld is through this area. This line is always measured from the corner of the base material to the effective edge of the weld as shown above. That is why it is important for a welder to ensure his fillet has an effective throat of at least the theoretical throat that the engineer is expecting. This weld with proper fit-up and as per engineer’s specifications has a total area of 0.031in^3.
Now let’s introduce a fit-up error from a slightly inaccurate cut part during fabrication. I will do this by showing a 1/16” gap at the root of the joint. Now remember the engineer who has done the calculations is expecting a throat depth of 0.177” as measured from the corner of the base material in the joint. We are relying on the welder to adjust for the error in manufacturing. The resulting weld needs to grow in order to maintain the engineer’s theory. The resulting weld should looks like this:
So as you can see, each leg of the weld grew in order to maintain the engineer’s throat depth and the total area of the cross section increase from 0.031in^2 to 0.048in^2. This is a 50% increase in weld volume from only 1/16th of a gap in the root. This compounds as the gap grows larger but of course there are maximum manufacturing tolerances that need to be met as well and the gap can only get so big before some sort of filler needs to be used. Conversely, if the welder just puts the same ¼” fillet down that he would have if there was no gap there would be a 33% reduction in strength from what the engineer was expecting.
So…how do you optimize a weld for production?
The answer is a fairly complicated procedure that routinely isn’t given enough thought. I covered two of the factors least considered but there are many more that need to be taken into account to complete a project. From the engineering perspective, there are so many different ways to solve the problem and many different factors to consider that it is often easier and less risky to specify a blanket solid complete joint penetration weld. This is of course is the least economical way to manufacture something. In my experience, I have had the most success when there has been lots of communication between fabricators and engineers. The more feedback and suggestions the engineer gets from the welders, the more he can optimize the process. In addition, if the welders have a say in the process, they will be more likely to follow the procedure and therefor reduce the risk of the product not getting welded correctly.