Abstract
As a new technology of producing high quality coating, HVOF spray is putted in an important position all over the world, and the spraying powder is a key of the thermal spraying technology. The technique for the preparation of thermal spraying powder consisting of ball milling, spray-drying granulation and sintering treatment which have improved rapidly in these years is an ideal process to meet the quality needed in HVOF spray powder.
Spray-drying granulation process is researched deeply in this paper by means of theoretical analysis and laboratory experiments with iron powder as the raw material.Firstly, the influence of the ball milling process was studied, and the optimized parameters were chosen to prepare the right sized and homogeneous particles by ball milling the original iron powder. The research results indicated that ball milling 8 hours, the particle size decreased from 20.01μm to 14.71μm as the ball number ratio raised from 1:2 to 1:4,and extend the ball milling time, the powder size decrease by a parabola curve, when the ratio of ball to powder increased from 10:1 to 20:1, the powder size decrease from 4.85μm to 1.69μm in 16 hours.
Secondly, spherical particles for HVOF which have good flowing quality and high loose density were getting by the technique of centrifugal spray-drying. During the process of spray-drying, through changing the related parameters such as the rotate speed of atomizing disk, solid content and drying temperature, we can get optimal feed powder for HVOF. The experiments show that as the speed of atomizing disk faster, feed speed slower, drying temperature higher, the granulation particle will be finer. Flowing quality and loose density of the powder are better with the solid content increasing from 40 wt% to 60wt%. To sum up, when the solid content is 50wt%, the rotate speed of atomizing disk is 14000r/min and drying temperature is 270℃ , we can get ideal granulation particle.
With the purpose of densification and higher intensity of the powder. The spherical particle was sintering 2 hours at 900℃ after spray granulation. Through analyzing the test data, we find that the spherical particle’s flowing quality and dimension decreased while the loose density higher.
Finally, by means of the testing of spray coating, we can study the sprayability of spray powder, find existing problem of precursor preparation and subsequent research plans were put forward in the end.
Keywords: Ball milling; Spray-drying granulation; Sintering; Powder preparation; High Velocity Oxygen Fuel spray
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