Research on the introduction of iron powder components into the forming process

The raw materials used in this test is the iron powder supplied by Wotai Company. In this test, three types of iron powder are used: <75Lm (-200 mesh) atomized iron powder, <50Lm (-300 mesh) atomized iron powder and 213Lm powder The powder is mixed to form a mixed powder according to the proportion of 1B1B1. The tap density of iron powder powder was tested according to the powder metallurgy standard. The tap density of the iron powder is 41428g/cm3, which is a tracing analysis of the powder with a scanning electron microscope. The solvent is deionized water, the organic monomer is amide (AM), the crosslinking agent is N,N-methylene bisamide (MBAM), the initiator is ammonium persulfate (APS), and the dispersing agent is MET. This test gel injection molding system of various chemical reagents. Amide as monomer gel injection molding system chemical reagent variety monomer crosslinking agent initiator dispersing agent name amide (AM) N, N-methylene bisamide (MBAM) ammonium persulfate (APS) hexamethylene MET mixed iron The process flow of the powder particle tracing process test is as shown. First, add deionized water, 24g amide (AM) and 115gN, N-methylene bisamide (MBAM) in a beaker according to the requirements of powder solid content calculation, and then mix it with <75Lm (-200 mesh) atomized iron powder , <50Lm (-300 mesh) atomized iron powder and 200g each of 213Lm powder are ball milled in a ball milling tank for 1020 hours to prepare a concentrated suspension with low viscosity and high solid volume fraction; then vacuum mixing process is used to remove bubbles in the slurry , Add the initiator and mix thoroughly, then pour the concentrated suspension into the non-porous mold, cure it in a dry box for 60e for 30 minutes, dry it at room temperature, and then degrease and sinter the body at high temperature after demolding. The organic gel is At high temperature, it differentiates and evaporates, and the green body is densified and sintered to directly become a metal part that can be finished.

The precautions in the test are as follows:

(1) Disperse the powder in a deionized aqueous solution containing organic monomer (AM) and cross-linking agent (MBAM) equipped in a certain proportion to prepare a concentrated suspension with low viscosity and high solid volume fraction, AM and The appropriate adjustment of MBAM’s share can improve the boring function of the subsequent green body and prevent the green body from cracking.

(2) After the blank is demolded, it must be dried first. If the drying speed is too fast, the blank will be deformed and cracks will appear, which will affect the strength of the blank and the final product. In order to avoid the onset of deformation and cracks, the drying of the green body should be carried out in an environment with high relative humidity. Although the combination of particles can be formed in the initial stage of drying, and the size of the green body has ceased to be shortened, it is still necessary to maintain a certain drying moment. In order to improve the drying rate, after the blank shortening is stopped, the drying can be carried out in an environment with a higher temperature and a lower relative humidity.

(3) The time from participation in the initiator to the onset of gelation is generally called the lazy time, and injection molding is generally required to be completed within this time. Therefore, it is necessary to strictly control the amount of initiator based on the material system under study to ensure that the slurry has excellent fluidity during injection molding.

(4) Whether it is useful to clean the organic matter in the green body will affect the function of the final product. In this experiment, 1200e sintering with a differentiation gas atmosphere is used, and the effective cleaning of organic matter is completed by extending the middle and low temperature firing time. The detailed heating guidelines are as follows: room temperature to 400e, 1030e/h; 400700e, 150200e/h; 7001200e, 100150e/h. Result and analysis The effect of dispersing agent on the viscosity of the slurry In order to form parts with disorderly shapes, the rheological function of the slurry It is particularly important to the forming quality of the blank. There are many factors that affect the rheology of the suspension, such as dispersing agent, particle surface, shape, and the nature of the suspension medium, etc.

<2> Therefore, the above influence factors should be fully considered in the process of gel injection molding. Adding the amount of dispersant within a certain range, the viscosity of the suspension will generally continue to decrease, and eventually reach a minimum value, but when the amount of dispersant continues to be added, it will increase the amount of polyelectrolyte in the liquid phase, and excess The network structure of polyelectrolyte molecules bridging each other greatly restricts the movement of the particles, and then causes the slurry to flocculate and increase the viscosity. In the gel injection molding process, adding a proper amount of dispersing agent can reduce the viscosity of the slurry and help to prepare a slurry with low viscosity and high solid volume fraction. The separation shows the influence of different dispersant content on the viscosity of 40%, 45%, 50% solid phase volume fraction slurry. It can be seen that when the amount of dispersant is added within a certain range, the viscosity of the slurry continues to decrease, and there is a minimum viscosity value. At the same time, it can be seen that with the addition of the solid phase volume fraction, the amount of dispersion required to achieve the minimum viscosity is reduced. This phenomenon can be clarified by the Woodcock equation

<3>.   Woodcock equation: hd=13PU+5612-1 where h is the particle distance; d is the particle diameter; U is the solid phase volume fraction. With the addition of the solid phase volume fraction, the distance between the particles becomes smaller, and the squeezing effect of the particles increases, so the amount of dispersant required is reduced. According to the theory of D1L1V1O

<4>, the potential energy of the solid particles in the slurry is the sum of the attracting potential energy and the repulsive potential energy, namely: VT=VA+VR. In the formula, VA is the gravitational potential energy, and VR is the repulsive potential energy. When the repulsive potential energy VR is greater than the gravitational potential energy VA, it is expected to be in a slack and stable state. The effect mechanism of the squeezing force is rather messy. Most scholars believe that there are two main mechanisms: the steric stability mechanism of polymer macromolecules and the electric repulsion stability mechanism of the electric double layer <5>. The steric hindrance mechanism believes that, High molecular weight polymer molecules are anchored on the surface of solid particles with their insoluble groups, and their soluble groups fully expand into the medium, acting as a stable part, preventing particles from settling, and playing the effect of stabilizing the slurry. In the electric repulsion and stability mechanism of the electric double layer, it is generally believed that when solid particles are dispersed in a polar solvent, a dispersed electric double layer will be formed on the solid surface and inside the solution. When two colloidal particles with the same number of charges are close to each other in a solvent to produce an electric double layer stack, an electrostatic squeezing effect will occur, and this squeezing effect will change with the electrical distribution of the surface. Participating in the dispersion agent can greatly increase the squeezing force between the particles, and then improve the dispersion of the slurry.

The effect of different dispersant content on slurry viscosity a-solid phase volume fraction is 40%; b-solid phase volume fraction is 45%; c-solid phase volume fraction is 50%. The effect of solid phase volume fraction on slurry viscosity 212 The effect of solid phase volume fraction on slurry viscosity The effect of solid phase volume fraction on slurry viscosity is that the gel injection molding skills require the slurry to have as high a solid phase volume fraction as possible on the premise that the required viscosity for forming is satisfied. This paper discusses the relationship between slurry viscosity and solid phase volume fraction under optimal dispersion conditions, as shown. For example, Gr indicates the relative viscosity of the slurry; <indicates the volume fraction of the solid phase. Regarding the dilute slurry (<<0105), the Einstein formula under ideal conditions in fluid mechanics and the Batshelor formula considering the mutual effect of particles can be used to calculate the effect of solid phase volume fraction on viscosity<6>.  Einstein formula: Gr=1+ 215  This model is about rigid spherical particles. At low shear limit,

Verdict

(1) Within a certain range, adding an appropriate amount of dispersant can greatly reduce the viscosity of the slurry;

(2) The relationship between the volume fraction of the solid phase and the viscosity of the slurry basically fits the Quemada model at the low shear limit. With the addition of the solid phase volume fraction, the amount of dispersion required to make the viscosity of the slurry reach the lowest value shows a downward trend.

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