Aiming at the research and application of preparation technology of high-performance nodular cast iron crankshaft, an exploratory study was made from two aspects: numerical simulation and ultrasonic treatment of melt. The temperature field of nodular cast iron of crankshaft is analyzed by finite element numerical calculation, the location of shrinkage defects is correctly predicted, and the influence of casting technology on the temperature field of castings is explored; The ultrasonic melt treatment equipment and ultrasonic probe are designed, and the ultrasonic derivation efficiency of the probe is tested by water simulation to complete the ultrasonic melt treatment experiment. The microstructure of nodular cast iron is refined and the mechanical properties are improved successfully. The specific conclusions are as follows:
(1) The finite element analysis software ProCAST is used to simulate and calculate the actual casting process of a certain type of crankshaft in the factory. The pouring temperature is 1420 ° C and the thickness of the sand shell is 6mm. It takes 13s to complete the mold filling. When the surface temperature of the nodular cast iron crankshaft drops to 600-700 ° C, it takes 800s. It is judged that the cooling is the slowest at the corner of the nodular cast iron crankshaft and the thicker part of the axis, which is the hot spot area, It is easy to form shrinkage porosity and shrinkage cavity.
(2) At 1450 ° C and 1400 ° C, the cast iron surface needs to solidify rapidly at 1450 ° C and 1400 ° C, respectively. The former edge solidification speed is too fast, which is not conducive to feeding the hot spot. When the pouring temperature is 1450 ° C, the hot spot range is similar to that under 1420 ° conditions, but the overall solidification is slow and reduces energy efficiency. To sum up, the 1420 ° C used in the current factory casting process is the most suitable.
(3) Change the thickness of sand shell in iron mold sand coating process, and calculate the temperature field of castings under the conditions of 8mm, 6mm and 4mm respectively. When the thickness of sand shell is 4mm, it takes only 80s for the surface of nodular cast iron crankshaft to solidify completely, and the sand shell does not reduce the cooling rate; When the thickness of sand shell is 6mm and 8mm, there is little difference in the change of temperature field. The complete solidification of the surface takes 130s, which can slow down the solidification time and fully feed. Therefore, the sand shell with the thickness of 6mm-8mm is more suitable for this type of nodular cast iron crankshaft.
(4) The length of the ultrasonic probe is calculated according to the propagation speed of the sound wave in the material, and the experimental correction is carried out to minimize the attenuation of the ultrasonic wave in the probe. The graphite probe used in the experiment is 340mm and 80mm long, the titanium alloy probe is 118mm, and the stainless steel probe is 125mm and 259mm.
(5) Through water simulation, the energy efficiency differences of probes with different materials and sizes such as graphite, titanium alloy and stainless steel for ultrasonic introduction of solution are analyzed. The ultrasonic conversion efficiency of titanium alloy 20 mm below the water surface is 14% higher than that of 10 mm. At the same time, the ultrasonic conversion efficiency of graphite probe with one and a half wavelengths is 50% higher than that of titanium alloy probe, and the ultrasonic conversion efficiency of graphite probe with diameter of 36 mm is 13.7% higher than that of graphite probe with diameter of 30 mm. By observing the breakdown degree of ultrasonic on aluminum foil paper, it is concluded that the longer the ultrasonic action time is, the greater the depth under the water surface is, and the more obvious the output effect of ultrasonic is.
(6) In the laboratory experiment, when ultrasonic was applied for 2min in the spheroidizing process of nodular cast iron and about 18S in the solidification process, the ferrite content in nodular cast iron was only about 2%, while that without ultrasonic treatment was about 13%. The tensile strength of the sample after ultrasonic treatment changed with the ultrasonic action time, mainly in the range of 800-870mpa, while that of the sample without ultrasonic treatment was 793mpa, The impact energy and elongation after fracture of the former are also about 10% higher than the latter, which has better comprehensive mechanical properties.
(7) Under the same experimental conditions, the time of ultrasonic application in the spheroidizing process of nodular cast iron was increased from 1min to 2min, the number of graphite balls was increased from 41 to 63, and the ferrite content was reduced from 10.60% to 2.254%. The tensile strength, impact toughness and hardness of the former were 822mpa, 35.7j and 249hb respectively, and the latter were 864mpa, 37.4j and 270hb respectively, which were increased by about 5%.
(8) In the factory experiment, the method of continuous application of ultrasonic wave is adopted, the ultrasonic output power is low, and the effect on the tissue is not obvious. The ultrasonic wave is applied in pulse mode, and the ultrasonic output power is increased from 200W to 400-500w. The impact force of cavitation effect can strongly scour the dendrite and increase the nucleation rate. Therefore, it can refine the structure of nodular cast iron. It is observed that the structure of nodular cast iron is refined, the number of graphite balls increases and the size is uniform; The tensile strength of the sample after ultrasonic treatment changes with the ultrasonic action time, mainly in the range of 800-870mpa, while the tensile strength of the sample without ultrasonic treatment is 793mpa. The impact energy and elongation after fracture of the former are about 10% higher than that of the latter, which has better comprehensive mechanical properties.