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Principle of service performance: mechanical property matching – the strength of the base material in welded structures is the primary consideration. Under normal circumstances, the strength of the welded joint should be no less than that of the base material, to ensure that the structure does not fail at the welded area when subjected to loads. For example, for welded steel beams subjected to tensile loads, steel with yield strength and tensile strength that meet the design requirements should be selected. If the strength of the base material is too high, defects such as cracks are likely to occur during welding; therefore, the strength must be appropriately matched based on the specific loading conditions, such as tension, compression, bending, and torsion. Resilience is also one of the key performance characteristics. For welded structures operating under low-temperature, impact, or dynamic loading conditions, the base material must possess sufficient toughness to prevent brittle fracture. For welding structures of oil pipelines used in cold regions in the north, it is necessary to choose pipes with good low-temperature toughness to prevent the pipelines from cracking due to insufficient toughness in low-temperature environments. Hardness affects properties such as the wear resistance of welded structures. In some welded structures that require wear resistance, such as the bucket teeth of excavators, a base material with higher hardness should be selected. However, materials with high hardness may have poor weldability, and appropriate welding techniques must be employed to prevent issues such as cracks. Physical property adaptation: The coefficient of thermal expansion is an important physical property parameter. When a welded structure operates in an environment with large temperature fluctuations, if there is a significant difference in the thermal expansion coefficients between the base material and the weld metal, large thermal stresses are generated during heat cycles, leading to cracks in the welded joint. For example, when welding aluminum and steel, due to the large difference in their coefficients of thermal expansion, cracking can easily occur as a result of thermal stress after welding; therefore, it is necessary to carefully select the materials to be used or take special measures to reduce thermal stress. For some welding structures with special thermal conductivity requirements, such as heat exchangers, the thermal conductivity of the base material must match the operational requirements. Materials with high thermal conductivity facilitate rapid heat transfer, whereas for structures that require insulation, materials with lower thermal conductivity should be chosen. Weldability principle: The chemical composition is important – the chemical composition of the base material has a significant impact on weldability. Carbon content is one of the key factors affecting the weldability of steel; generally, the higher the carbon content, the worse the weldability of the steel. Because a high carbon content increases the hardening tendency of steel, it is easy to form a hard and brittle martensitic structure during welding cooling, leading to the occurrence of welding cracks. For example, when welding high-carbon steel (with a carbon content of over 0.6%), strict preheating and post-heating procedures are required to prevent cracks. Alloying elements also affect weldability. Impurity elements such as sulfur and phosphorus can form low-melting eutectics during the solidification of the weld, increasing susceptibility to hot cracks. An appropriate amount of alloying elements such as manganese can improve the toughness and heat-crack resistance of steel. When selecting the base material, the types and contents of alloying elements must be considered to ensure they are beneficial to the welding process and the quality of the welded joint. Avoid tendency to welding defects: Choose base material that is less prone to forming welding pores. Some base materials have impurities such as oil and rust on their surface, or their structure is loose, which can easily lead to the formation of pores during welding. For example, due to its porous structure and high carbon content, cast iron is prone to porosity and cracks during welding; therefore, when selecting this material, it is necessary to consider whether the welding process can effectively overcome these defects. The base material should avoid having a strong tendency to develop large laminar tears as much as possible. For certain thick-plate structures, such as the hull structures of large ships, tensile stresses acting perpendicular to the plate surface occur during welding. If the base material has poor properties in the Z-direction (i.e., the thickness direction), laminar tearing is likely to occur. At this time, steel with good Z-direction properties should be selected, or appropriate welding sequences and processes should be employed to reduce the risk of layered tearing. Economic principle – Consideration of material costs: On the premise of meeting the service performance and weldability requirements of welded structures, it is necessary to select base materials with low costs as much as possible. For example, in the welding of some building structures where the strength requirements are not particularly high, ordinary carbon structural steel can be chosen, as its cost is relatively low. Choosing expensive alloy steel will increase the manufacturing cost of the structure. Processing cost trade-off: Some base materials have low material costs, but high processing costs. For example, although some high-strength alloy steels are reasonably priced, they have poor machinability; when machining is required during the fabrication of welded structures, this increases the processing costs. Additionally, the complexity of the welding process also affects the processing cost. For some materials with stringent requirements regarding welding processes, special welding equipment as well as complex procedures such as preheating and post-heating are necessary, all of which increase the processing costs; therefore, it is essential to take into account both the cost of the material and the processing costs.