Process to increase yield of fines in gas atomized metal powder using melt overpressure

Claim 1

A method for producing powder from a metal or metal alloy comprising the steps of: delivering a metal or metal alloy melt from a crucible means having a 1-15 mm diameter melt delivery orifice in the bottom thereof to an atomizing zone through the melt delivery orifice as a generally vertically oriented melt stream at a melt mass flow rate M; providing an overpressure zone above the melt contained in the crucible means; directing one or more streams of atomizing gas at a total atomizing gas mass flow rate G and an atomizing gas velocity greater than or equal to 100 m/sec from an annular gas orifice means concentric with the melt orifice toward the melt stream, so that the gas streams converge to generally define a cone the apex of which coincides with the melt stream axis, and the gas streams impinge upon the melt stream at the atomizing zone at an average impingement angle of 20°-32.5° from the vertical to atomize the melt, wherein the apex of the gas stream cone is less than 50 mm from the melt orifice and 11-24 mm from the gas orifice means; establishing an initial pressure level in the overpressure zone and controlling the pressure level above the melt during atomization to establish and maintain a predetermined pressure P m of the melt measured at the melt delivery orifice and a predetermined melt mass flow rate M, so that P m is greater than P g , where P g is the pressure of the atomizing gas measured at the melt delivery orifice, and so that the ratio M/G is less than or equal to 0.10; and solidifying the atomized melt to produce a metal or metal alloy powder.

Claim 2

A method for producing powder from a metal or metal alloy comprising the steps of: delivering a metal or metal alloy melt from a crucible means having a 1-15 mm diameter melt delivery orifice in the bottom thereof to an atomizing zone through the melt delivery orifice as a generally vertically oriented melt stream at a melt mass flow rate M; providing an overpressure zone above the melt contained in the crucible means; directing one or more streams of atomizing gas at a total atomizing gas mass flow rate G and an atomizing gas velocity greater than or equal to 100 m/sec from an annular gas orifice means concentric with the melt orifice toward the melt stream, so that the gas streams converge to generally define a cone the apex of which coincides with the melt stream axis, and the gas streams impinge upon the melt stream at the atomizing zone at an average impingement angle of 20°-32.5° from the vertical to atomize the melt, wherein the apex of the gas stream cone is less than 50 mm from the melt orifice and 11-24 mm from the gas orifice means; establishing an initial pressure level in the overpressure zone and controlling the pressure level above the melt during atomization to establish and maintain a predetermined pressure P m of the melt measured at the melt delivery orifice and a predetermined melt mass flow rate M, so that P m is greater than P g , where P g is the pressure of the atomizing gas measured at the melt delivery orifice, and so that the ratio M/G is less than or equal to 0.10; and solidifying the atomized melt to produce a metal or metal alloy powder.