In the first part of this series, we examined five basic principles of distortion control that can be managed by the designer. Now, I will add three more concepts that, although not overly complicated, might require a bit more thought to understand and implement.
These related distortion control principles require an understanding of longitudinal shrinkage, and how it causes distortion. Consider the weldment in Figure 1.
In this simple tee joint, the fillet welds are located below the neutral axis of the cross section. As the hot, expanded weld and base metal cool, they shrink and will induce a longitudinal curvature or camber to the assembly. The amount of deflection can be estimated from the following relationship:
This relationship allows us to predict how much distortion will occur, and it provides some insight into how to minimize it. To reduce distortion, the factors in the numerator can be decreased, and those in the denominator increased. For example, the total cross-sectional area of the welds (Aω) can be reduced, as was extensively discussed in the first part of this series.
The function of the part being fabricated usually determines the length (L) and moment of inertia (I) of the weldment. However, because the variable of the length is a squared function, that variable is particularly important and special consideration for it is warranted when long weldments are called for. Furthermore, weldments that have shallow depths, and hence lower values of inertia (I), should be evaluated carefully.
♦ Place the weld(s) on or near the neutral axis.
When the dimension "d" in the equation is zero, the predicted distortion due to longitudinal shrinkage also will be zero.
The designer has several ways to control the "d" dimension — that is the distance from the center of gravity of the various welds to the neutral axis of the section.
The top illustration in Figure 2 that shows one tee section added to a flat plate to form an I-shape provides the first example. The neutral axis (N.A.) is half the depth of the section, and the welds are displaced from this location by the distance "d". As these welds shrink longitudinally, a positive camber will be introduced along the length.
However, when the designer selects two tee sections and places them stem-to-stem, as in the lower illustration in Figure 2, the welds will be placed on the neutral axis, and the distance "d" becomes zero. While the welds will shrink longitudinally, the assembly will remain essentially straight because the welds are placed on the neutral axis.
♦ Balance welds about the neutral axis.
If one is unable to place the welds on the neutral axis, the next best option is to balance the welds around the neutral axis. Since "d" is the distance from the center of gravity of the total weld area "Aω" to the neutral axis, it is easy to achieve a balance of the welds on symmetrical sections, as shown in Figure 3.
Ideally, the fabricator would make the welds simultaneously, but this is usually impractical. Instead, the welder either can rigidly fixture the assembly or tack weld it, then weld each seam individually.
♦ Use a welding sequence that minimizes distortion.
Sometimes the designer can minimize or eliminate longitudinal sweep or camber by stipulating a carefully planned and properly executed welding sequence. Complicated, non-symmetrical assemblies often can be broken down into subassemblies for which the weld center of gravity and the section neutral axis can be made to be concurrent. In the first sequence, the designer calls for tack welding the three members, then completing the four welds. Consider Figure 4. Because the shape of the part being made is non-symmetrical, the center of gravity for the welds is displaced from the neutral axis of the shape by 0.442 in. (i.e., 1.5 in. -1.058 in.)
In the second sequence, the designer specifies that the tee section be fabricated first. Notice that for this particular geometry, the weld center of gravity and the section neutral axis are concurrent. While the part might shrink longitudinally, it should remain straight.
Next, the designer directs that the tee section be added to the bottom flange. Again, the weld center of gravity and the section neutral axis are concurrent, and no sweep or camber will occur.
In the third and final part of this series, we'll move from the drawing board onto the shop floor and examine what we can do there to control distortion.
Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments, and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.