The Frictional Behavior And Mechanism Of Densificati On Of Iron Based Powders In High Velocity Powder Compaction

At present, China’s manufacturing industry is growing fast. The quality standards of powder metallurgy productsare improving and the overall demand for powder metallurgy components is also increasing significantly. With the advantages of short process, low cost and high density, the cutting-edge high velocity compaction (HVC) technology is of great significance to powder metallurgy industry.This paper analyzed the technical characteristics of existing HVC devices and testers both at home and abroad, compared advantages and disadvantages of different HVC devices and initially designed an HVC tester in which metal powder is compacted by force from gravity. To study iron-base powder, this paper used finite element numerical simulation to do systematic research on the forming laws, fiction behavior and mechanism of powder densification through experiment, from both macroscopic and microscopic angles. The results are showed below:An impacting device using gravitational potential as the energy storage mode was designed and manufactured. A permanent magnetic sucking disc was used in the device to lift the impact module and adjust the height of impact hammer, getting the corresponding impact velocity and impact force. The powder was compacted under the hammer gravity to form powder metallurgy products. The compacting hammers weighed 50 kg and 100 kg were made to realize three weights of 50 kg,100 kg and 150 kg, thus to meet the requirements of different tests. Taking gravitational potential energy as its driving force can simplify the device’s overall structure, enhance its safety, reduce its environmental pollution, lower production cost, and facilitate processing and production. Data showed that, compared with its imported counterparts, price of this device is reduced by 93%. It removed the disadvantages of traditional hydraulic drives. Theoretically, the device can reach a velocity 9.49 m/s and its maximum impact energy can reach 5145 J. Tests showed that all index of the device can meet designed requirements.Secondly, based on the developed HVC device and the theoretical calculation, a test system was designed with fast data-collecting sensors, accurate experimental data output and well-matched hardware platform. To test feasibility of the HVC device in compacting metal powder, and learn the relationship between compacting rates, height, compacting hammer weight and compact density, data concerning HVC device was collected for analysis. To test the effectiveness of the HVC device, we used a self-developed HVC device to compact low-diffusion alloy steel powder. If a compacting hammer weighted 50 kg was dropped from a height of 2 meters to compact a sample weighted 8.09 grams, the green density of the compacted sample would be 7.45 g/cm3. All testing results can meet the designed requirements.Furthermore, relationship between compacting height, velocity, energy, force and green density in metal powder HVC was studied; fiction coefficients of powder of different natures were tested under varied pressing force to learn the relationship between fiction coefficient, pressing force and green density. The test result will give reference to and calibration standard for later simulation analysis; Analysis of striation on the pressed compact showed that greater pressing force brings in higher density of compacted sample; when the product density is lower, the friction coefficient between the product and the mold will increase with enhancement of the pressing force. In this case, the sample in the friction test shows greater striation on its fiction surface and metal spalling on the sample surface is evident. If product density increases to 0.803cm/m3, the friction coefficient will decrease in nearly linear trend. In this circumstance, striation on the sample surface would be reduced and shallower, and the friction coefficient peaks. If the pressing force was further increased, the sample density would be increased accordingly. At this time, the sample surface was smooth and free of evident striation and the friction coefficient was decreased significantly.Finally, discrete finite element modeling was used to establish two-dimensional powder particle model and random distribution particle model. Material properties of the model was defined; Contact algorithms used in powder HVC was discussed; key problems, such as convergence property of solutions, solutions to penetration and unification of unit were simulated. The change law of powder density tested in HVC was discussed from microscopic view and a simulation was performed to verify the change law. Compacting velocity, hammer weight and surface fiction were analyzed to learn their relationship with density, transformation law, anelasticity and movement law of powder particles. Individual particle behavior and powder’s collective macroscopic behavior were correlated; Comparison of formation features of HVC and static powder compaction showed that, with identical average pressing forces, HVC will bring in higher and more uniformly distributed density. HVC will better compact parts with greater ratio of height to diameter. Validity of the numerical simulation model and practicability of the platform were verified by comparing test results. Above conclusions have a great significance to improving powder metallurgy process and improving performance of products produced with powder metallurgy products.