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1.1 Basic Situation
Casting is a fundamental process in mechanical manufacturing, but it often leads to various defects during production, especially in complex thin-walled components like cylinder blocks of diesel and gasoline engines in the automotive industry. These parts tend to have a higher scrap rate due to their intricate structures. Additionally, cast iron components in various machinery are highly prone to damage during operation. To date, there is still no comprehensive theory or method available globally to fully address these issues.
1.2 International Developments
Globally, cold welding of cast iron—where preheating is performed before welding—has been practiced for decades. In the 1940s, the United States introduced a pure nickel cast iron electrode that is still in use today. Under certain conditions, nickel-based powder spray welding can also be applied. For instance, the 308 wire developed in the U.S. (similar to the present invention) has a fusion zone hardness of HB ≤ 300, whereas the present invention achieves HB ≤ 240. Moreover, continuous welding was not possible at the time due to issues such as slag inclusion and cracking.
1.3 Domestic Status
In the 1980s, the State Planning Commission promoted the "low carbon steel electrode" developed by Professor Chen Zhongsheng from Beijing 211 Factory during the "Seventh Five-Year Plan" and "Seventh Eight-Year Plan" periods. However, this technology failed to effectively solve the fundamental problem of cast iron welding. In the 1990s, Chongqing University researched spray welding, which was recommended by the Sichuan Provincial Planning Commission and saw limited application in diesel engine manufacturers. Despite its potential, several challenges emerged during practical use:
1. Controlling the welding temperature was difficult. The gas flame was used to heat the workpiece manually, making it hard to achieve the right temperature. Too low a temperature led to weak welds, while too high a temperature caused excessive hardness.
2. Pre-weld preparation was labor-intensive, requiring thorough cleaning of the area to be welded.
3. The welding process required precise operation; any mistake could lead to loss of the welding material.
4. The technique had strict requirements for the size and thickness of the welding area, making it suitable only for large, shallow surfaces.
5. Welders faced poor working conditions, with high temperatures in summer and long preheating times in winter.
Overall, although spray welding can achieve machining and reduce thermal cracking under certain conditions, it has significant drawbacks. Hot cracks still occur frequently in cast iron welds, with a probability of over 80%, limiting its widespread use.
1.4 Key Innovations in the Invention
(1) Solving the White Mouth Problem Without Post-Weld Heat Treatment
Currently, international and domestic methods for repairing cast iron parts have not resolved the white mouth issue. Even the most advanced 308 welding wire from the U.S. can only reach a thickness of 0.1 mm, and white mouth becomes noticeable. The invention uses a special welding wire combined with COâ‚‚ shielding semi-automatic cold welding technology. This method results in a high current density and rapid cooling, creating an intermittent white layer in the metallographic structure, typically less than 0.07 mm. This has minimal impact on cutting tools and eliminates the need for heat treatment after welding.
(2) Eliminating Pre-Weld Heating and Preventing Cracking
During welding repair, localized rapid heating is inevitable, leading to uneven temperature distribution. When thermal stress exceeds the tensile strength of the cast iron, cracks are likely to form. The invention’s welding wire is designed with a yield limit as low as 200 MPa, and can reach up to 400–500 MPa. This allows the weld to deform and relieve stress when the temperature exceeds 200 MPa, effectively preventing hot cracks.