Research On Coupled Pressure-Gas Atomization Technique And AS-prepared Metal Powder Properties

Fine spherical metal powders are important raw materials for advanced manufacturing technologies,such as powder metallurgy(PM),thermal spraying and additive manufacturing(3D printing).Atomization of metal melts is the principal method to prepare fine spherical metal powders.In this thesis,a technique called Coupled Pressure-Gas atomization(PGA)is introduced and verified to enhance the fine powder yield and reduce the consumption of inert gas,based on the principle of coupled multi-energy atomization and low-dimensional melt atomization.The aluminium-alloy and the die-steel powder for the selective laser melting(SLM)process and the high temperature aluminium alloy powder for the powder metallurgy(PM)process have been prepared by this technique.During the PGA process,because of the over-pressure imposed on the molten metal in the tundish,the molten metal can go through the melt-guiding-nozzle(MGN)with a diameter of less than 3 mm to form a very thin molten metal jet which has high specific surface energy and instability.In this case,the efficiency of gas atomization could be improved,and thus a high yield of fine powder could be obtained.In addition,due to the low dimension of jet flow,the molten metal can be atomized into small droplets easily using lower atomization pressure and less inert gas consumption.The capillary resistance that caused by the molten metal goes through the MGN was analyzed by varying the MGN inner-diameter and the solid/liquid contact angle.In the case of a constant contact angle,the capillary resistance increased with the decrease of MGN inner-diameter.For example,the capillary resistance changed from 435 Pa to 3480 Pa while the MGN inner-diameter decrease from 4 mm to 0.5 mm when the contact angle is a given value(0 = 120°).In the case of a constant MGN inner-diameter,the capillary resistance is proportional to the change in the contact angle.When the molten cast-iron flowed through a MGN with an inner-diameter of 1 mm,the capillary resistance changed from 798 Pa to 4530 Pa with an increasing contact angle from 100° to 170°.Therefore,an over-pressure imposed on the molten metal is necessary to overcome the capillary resistance,thus promoting the molten metal through the MGN with a small MGN inner diameter.AlSi10Mg alloy powder used in selective laser melting(SLM)was produced by the PGA process under the following process parameters:inner-diameter 2mm,gas atomization pressure 2.0±0.3MPa and over-pressure on the molten metal 0.3±0.05atm.In this case,a powder yield of>40%could be reached for the particle size within the range of less than 53μm.Compared with those from domestic market and imported from abroad,it was found that the powder produced by the PGA process processed higher degrees of sphericity,fewer satellite particles and more fine particle size.Some samples were prepared by selective laser melting process using different powders which were produced by the PGA process and imported from abroad respectively,and tensile properties of samples were analyzed.It was found that the properties of the samples made of the PGA-prepared powders were better than those made of imported powders.Die steel powders produced by the PGA process were processed into Charpy impact test specimens by the SLM process.The comparison indicated that the SLM-processed specimens possess the lowest impact toughness but have achieved the need of die steel components.The PGA technique can deal with the clogging problem when a highly viscous melt is atomized.Using the PGA technique can produce high temperature Al-alloy powders,which contain 18-20wt%Si and 5-6wt%Fe.At a melting temperature of 850 ℃,the viscous melt,driven by an over-pressure of 0.4 ± 0.05 atm,can go through a MGN with the inner-diameter in a range of 4 mm-2 mm.With a decreasing MGN inner-diameter,the fine powder yield increases,the particle size distribution becomes narrow,and the volume median diameter decreases.In the case of a MGN with the inner-diameter of 2 mm,the fine powder yield(<100 ±m)can reach 80%,and the volume median diameter(d50,3)is about 55 μm.