Effect of Die Wall Lubrication on Warm Compaction Powder Metallurgy

Die wall lubrication was applied on warm compaction powder metallurgy in hope to reduce the concentration level of the admixed lubricant since lubricant is harmful to the mechanical property of the sintered materials. Iron-based samples were prepared by die wall lubricated warm compaction at 135 ℃ and 175 ℃, using polytetrafluoroethylene (PTFE) emulsion as die wall lubricant. A compacting pressure of 700 MPa and 550 MPa were used. The admixed lubricant concentration ranging from 0 to 0.6 wt.% was used in this study. Compared with non-die wall lubricated samples, the die wall lubricated samples have higher green densities. Results show that in addition to the decrease in ejection forces, green density of the compacts increased linearly with the decrease in admixed lubricant content. Mechanical property of the sintered compacts increase sharply when the admixed lubricant concentration reduced to 0.125 wt.% or less. Ejection force data indicated that samples with die wall lubrication show lower ejection forces when compared with samples without die wall lubrication. No scoring was observed in all experiments even for samples contain no admixed lubricant. Our results indicated that under experimental condition used in this study, no matter at which compaction pressure, compaction temperature, graphite and lubricant contents in the powder the die wall lubricated warm compaction would give the highest green density and lowest ejection force. It can be concluded that combination of die wall lubrication and warm compaction can provide P/M products with higher density and better quality. It is a feasible way to produce high performance P/M parts if suitable die wall lubrication system was applied.

Densification Mechanism of Warm Compaction and Powder Mixture Designing Rules

By phenomenological analysis of warm compaction, it is found that, compared with the contribution of particle plastical deformation to densification of powder compact,the particle rearrangement is a dominant densification mechanism for powder warm compaction, and the plastical deformation of particles plays an important role in offering accommodating deformation for particle rearrangement and densifying powder compact at the final stage of pressing.In order to attain density gain as high as possible during warm compaction, six rules for designing warm compacting powder mixtures were proposed in detail.

Magnetic properties and thermal stability of anisotropic bonded Nd-Fe-B magnets by warm compaction

Anisotropic bonded magnets were prepared by warm compaction using anisotropic Nd-Fe-B powder. The forming process, magnetic properties, and temperature stability were studied. The results indicate that the optimal temperature of the process, which was decided by the vis-cosity of the binders, was 110°C. With increasing pressure, the density of the magnets increased. When the pressure was above 700 MPa, the powder particles were destroyed and the magnetic properties decreased. The magnetic properties of the anisotropic bonded magnets were as follows： remanence Br=0.98 T, intrinsic coercivity iHc=1361 kA/m, and maximum energy product BHmax=166 kJ/m3. The magnets had excellent thermal stability because of the high coercivity and good squareness of demagnetization curves. The flux density of the magnets was 35% higher than that of isotropic bonded Nd-Fe-B magnets at 120°C for 1000 h. The flux density of the bonded magnets showed little change with regard to temperature.

Die wall lubricated warm compaction behavior of non-lubricant admixed iron powders

The phenomena of die wall lubricated warm compaction of non-lubricant admixed iron powders were researched, and its mechanism of densification was discussed. Water atomized powder obtained from the Wuhan Iron and Steel Corporation was used. With compacting and sintering, compared with cold compaction, the density of warm compacted samples increases by 0.07 - 0. 22 g/cm^3 at the same pressed pressure. The maximum achievable green density of warm compacted samples is 7.12 g/cm^3 at 120℃, and the maximum sintered density is 7.18 g/cm^3 at 80℃. Compared with cold compaction, the ejection force of warm compaction is smaller; the maximum discrep- ancy is about 7 kN. The warm compacted mechanism of densification of iron powders can be obtained： heating the powder contributes to improving plastic deformation of powder particles, and accelerating the mutual filling and rearrangement of powder particles.

Research of warm compaction technology on nylon bonded Nd-Fe-B magnets

Warm compaction and room temperature compaction were applied to prepare bonded Nd-Fe-B magnets. The results indicated that the density of magnet was determined by the compaction pressure and warm compaction temperature, whereas, the thermosetting temperature could hardly affect the density of magnet. The mechanical properties of magnets were the best when the thermosetting temperature was 200 ℃. The Br, Hob, and （BH）max of warm compaction magnet were higher than those of room compaction. When the warm compaction temperature and thermosetting temperature were invariable, the density of magnet increased with the increase of compaction pressure, which resulted in the increase of Br, Hcb, and （BH）max of magnet and the decrease of Hcj of magnet. When the warm compaction temperature and compaction pressure were invariable, the magnetic properties of magnets decreased with the increase of thermosetting temperature. The magnetic properties of warm compaction molding magnets were better than those of injection molding magnets.

Lubrication effectiveness of composite lubricants during P/M electrostatic die wall lubrication and warm compaction

The lubrication effectiveness of the composite lubricants, 50wt% ethylene bis-stearamide （EBS） wax ＋50wt% graphite and 50wt% EBS wax ＋ 50wt% BN, during the powder metallurgy （P/M） electrostatic die wall lubrication and warm compaction was studied. The results show that the combination of 50wt% EBS wax and 50wt% graphite has excellent lubrication performance, resulting in fairly high green densities, but the mixture of 50wt% EBS wax and 50wt% BN has less beneficial effect. In addition, corresponding die temperatures should be applied when different die wall lubricants are used to achieve the highest green densities.

Phenomenological Modeling of Warm Compaction and Experimental Verification

A phenomenological modeling approach to establishing the warm compaction equation and curves by modifying the regression equation of the room-temperature compaction curve is presented. An enhanced factor of compacting pressure is introduced into the equation in order to reveal the effects of powder/die temperature and filling height of powders on green density. Compaction curves yielded from this equation are consistent with the experimental data of ATOMET grade iron powders. The curves show that the powder/ die temperature should reduce as the filling heights of powders increase and that in some cases warm compaction can not give rise to a higher green density.

Effects of warm compaction on mechanical properties of sintered P/M steels

The green and sintered densities,and tensile strength of sintered P/M steels produced by cold compaction,warm compaction,warm compaction combined with die wall lubrication(DWL)were measured under various compaction pressures using polytetrafluoroethylene(PTFE)emulsion as the die wall lubricant.The effects of warm compaction on the mechanical properties were studied.The tensile fracture behaviors of cold compaction and warm compaction were studied using scanning electron microscope(SEM).The results show that the density of sintered P/M steel prepared by warm compaction or warm compaction with DWL is higher than that by cold compaction under all compaction pressures.Meanwhile,the highest tensile strength is obtained by combination of warm compaction and die wall lubrication under all compaction pressures.The SEM results show that the fracture modes of the sintered samples prepared by cold compaction and warm compaction at 700 MPa are the mixed mode of ductile fracture and brittle fracture,and obvious dimples can be found in some regions.The fracture of sample prepared by cold compaction is uneven and has irregular and big pores,but that by warm compaction is relatively even and the pores are round mostly,and the samples have many obvious dimples on the whole fracture surface.

Warm Compaction Process of P/M Steels by Orthogonal Testing Method

In order to achieve higher density of P/M steels using the die wall lubrication compacting method or powder lubricant in warm compaction process, the influence of different process parameters on the green density of warm compacted samples was studied. According to the orthogonal test method, the authors systematically study the influence of the different compaction pressure, condition of lubrication and compaction temperature on the green density of the sample in the warm compaction process, and put forward the optimal process parameter of warm compaction experiment. It is found that, a high compaction pressure （≥700 MPa）, die wall lubrication combined with a small amount of internal lubricants, and fitting compaction temperature by different condition of lubrication, are the optimal parameters in warm compaction process.

Predicting Equation for Adjusting Glass Temperature of Binder Used for P/M Warm Compaction Processing

Based on considering both negative effect of compacting pressure on glass temperature (Tg) and positiveeffect of plasticizer added into organic binder, a mathematical equation for adjusting Tg of binder used for warmcompaction processing is given. The equation shows that mass fraction of plasticizer need for lowering Tg is function of compacting plessure. The rationality of the equation for warm compaction processing is also discussed.

Key parameters for low temperature warm compaction of high density iron-based P/M materials

In order to reduce powder temperature to lower than 100℃ in warm compaction by changing polymer lubricant design, powder flowability, warm compacting behavior, lubricating mode as well as ultimate tensile strength after sinter-hardening and tempering were investigated systematically. By means of low temperature warm pressing and sintered hardening technique, samples with the sintered densities of 7.407.45g/cm3 and the strengths of 950 1390MPa are achieved as the early compacting pressure is 686735MPa.

Possibility of utilizing water-atomized Fe-Ni-Mo steel powder as base materials for warm compaction process

Water atomized Fe Ni Mo steel powder, was utilized as base powder for designing powder mixtures for warm pressing. The warm pressing and sintering behaviours of the powder mixtures were studied. The results show that, compared with the pressing at room temperature, the green density gain by warm pressing is within a range of 0.10 0.19 g/cm 3 and reduction in spring back is 30% 40% of the ambient, and maximum green density of 7.32 g/cm 3 at 735 MPa is obtained as the graphite mass fraction is 0.8%. It was found that sintered densities of the compacts were reduced slightly due to releasing of elastic stress stored in the compacts during sintering. The warm pressing of steel powders gives evidence for substituting the traditional double pressing and double sintering process.

Structure of Grading a Resistor-Heated System of Warm Compaction in Powder Metallurgy

We present the scheme of the structure of grading a resistor-heated system ofwarm compaction in powder metallurgy. The structure has the first heater and the second heater thatare heated by electrical tubes. Powder is heated in turn in the first heater and the second heater,where there is the mass fluidity of powder under gravity. The dimensions of the first heater andthe second heater were calculated from the Fourier equation of heat conduction, and the boundarycondition was constant temperature. The drawings of the first heater, the second heater and thepowder-delivering device were given. The structure of the heat equipment is simple and easy tomanufacture. Finally, an exact warm compaction press system HGWY- II was developed for the heatingsystem.

Anisotropic NdFeB/SmCoCuFeZr composite bonded magnet prepared by warm compaction process

Anisotropic NdFeB/SmCoCuFeZr composite bonded magnets were prepared by warm compaction process. The effects of adding SmCoCuFeZr magnetic powder on the properties of anisotropic bonded NdFeB magnet were investigated in this work. The results show that, both magnetic properties and temperature stability of the bonded magnet can be improved by adding fine SmCoCuFeZr magnetic powder. In the present study, the optimal content of SmCoCuFeZr magnetic powder was about 20 wt.%, in this case, the Br, Hcj, and（BH）maxof the NdFeB/SmCoCuFeZr composite magnet achieved 0.943 T, 1250 kA/m, and168 kJ/m^3, respectively.