Welding is a non-detachable connection. Two metal parts that need to be joined are locally heated at the joint and filled with molten metal, or they are fused together by applying pressure or other methods. The fused joint is called a weld.
1. Advantages of welded structures (lightweight, reliable connection, simple process and equipment, etc.)
1) Compared with cast structures, welded structures are lighter and have greater freedom in structural design. Since no mold is required, the manufacturing cycle is shorter and the cost is lower. This advantage is even more prominent when producing small batches.
2) Compared with riveted and bolted structures, this structure has no gaps, which facilitates corrosion prevention. In addition, since no accessories are required, it also has the advantage of being lightweight.
Welded structures have very high requirements for welding quality, and ensuring welding quality is the key to using welded structures.
2. Three ways to ensure or improve welding quality
1) Materials: The most important factor in material selection is weldability. Carbon steel has a carbon mass fraction of less than 0.22%, which gives it good weldability.
2) Process: The process includes pretreatment, posttreatment and welding process, among which welding skills are the decisive factor.
3) Structure: The main factors affecting the structure are the load-bearing form and size of the weld, and whether it is conducive to the implementation of the welding process.
3. Design Guidelines for Welded Components
1) Geometric continuity criterion:
The strength of the weld and its affected zone, especially its dynamic load strength, is generally lower than that of the surrounding material, and it often has internal stress. Therefore, welds should be placed in areas with lower stress levels as much as possible.
For example: avoid placing welds at points of abrupt geometric changes (because stress concentrates there);
Sometimes, the geometric continuity of the two sides to be joined by a weld cannot be guaranteed, often due to differences in plate thickness. To address this, a transition structure should be incorporated into the structural design to mitigate the abrupt change in geometric shape.
Engineering application examples:
(1) When welding plates of different thicknesses, a transition structure is left to reduce the abrupt change in geometry;
(2) When welding pressure vessels (where curvature changes abruptly, stress concentration is severe, and it is also a high-stress area where welds should not be placed), the head is bent at a certain distance from the weld to prevent curvature changes at the weld.
(3) When welding pipes of different thicknesses, try to stagger the welds and avoid circumferential welds.
2) Guidelines for avoiding weld overlap:
Welding complex structures often involves multiple overlapping welds. The intersections of these welds exhibit high rigidity, leading to severe structural warping and increased internal stress. Repeated overheating degrades material properties and increases the likelihood of cracking. All of these factors contribute to a decrease in the load-bearing capacity of the welded structure; therefore, overlapping welds should be avoided.
There are three structural measures to prevent weld overlap:
(1) Add auxiliary structures;
(2) Removal of part;
(3) The welds are staggered.
3) Weld root preferential pressure criterion:
When a welded component is subjected to bending, i.e., one side is under tension and the other side is under compression, the root of the weld should be placed on the side under compression. This is because the root of the weld is prone to notch formation, and its tensile load-bearing capacity is lower than its compressive load-bearing capacity. For welded components with changing load directions, double-sided welding can be used.
4) Avoid riveted structures:
In actual engineering practice, many welded structures are derived from riveted structures. These structures usually use lining plates, lap joints, etc., which seem to enhance the load-bearing capacity, but in fact they do not.
5) Avoid sharp corners:
Welding at sharp corners is difficult to position, and the welding quality is hard to guarantee. Moreover, the hot melt is too small, and the sharp corner is easily melted. This type of welding structure should not be used.
6) Convenient welding process guidelines:
The design of welded structures must not only meet the functional requirements of the structure itself, but also the requirements of the welding process. The welding process includes not only the welding itself, but also the pre- and post-treatment and inspection.
(1) There must be a sufficiently large operating space;
(2) Welding should be easy to position;
(3) Welded parts should be easy to inspect;
(4) Spot welding requires a sufficiently large platform to prevent the electrode from sticking to the adjacent plate.
7) Butt welding priority criteria:
There are various types of welds, among which butt welds have the highest strength and are preferred under heavy loads, especially under dynamic loads. However, butt welds are not always preferred. For example, although fillet welds are not as strong as butt welds, they do not require pretreatment, saving costs, and can be preferred when the load is not heavy.
8) Flexibility criteria for the welding zone:
During welding, the weld zone is subjected to high temperatures, causing thermal deformation of the components. The two components are joined together under these conditions. When the welding heat dissipates, the resulting thermal deformation cannot be completely eliminated due to new restrictions on free deformation, leading to residual deformation and internal stress. A common method to eliminate this internal stress is to heat-treat the welded components. While heat treatment can effectively reduce or eliminate internal stress and improve weld quality, it also increases the manufacturing cost of the components. For larger components, heat treatment is also more difficult. Targeted structural design can also reduce internal stress.
The fundamental reason for the stress generated in the welding zone is that the free deformation of the structure is restricted to a certain extent. The greater the rigidity around the welding zone, the greater this restriction. Therefore, whenever possible, the rigidity of the components around the welding zone should be reduced as much as possible, which fundamentally reduces the generation of internal stress.
9) Minimum welding quantity criterion:
The best welded structure is one with the fewest welds; the number and amount of welds should be minimized. The mechanical properties of welds are generally inferior to those of the base metal. A large number of welds means a large heat-affected zone, resulting in greater internal stress and thermal deformation, thus increasing the requirements for heat treatment and structural correction.
