MANUFACTURING

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The present disclosure relates to the field of additive manufacturing. More particularly, the present disclosure relates to methods and equipment for the additive manufacturing of articles of manufacture.

Description of the Related Art

[0002] Additive manufacturing techniques, commonly known as 3-D printing, include powder bed fusion and other directed energy techniques, wherein selected portions of a thin layer of a fusible powder or other particulate material is spread over a heated powder bed, or over layers of powder previously spread on the powder bed and over the previously fused slices forming the partially manufactured article, are exposed to energy sufficient to fuse or react the selective portions of the powder to form a layer or a slice of an article of manufacture. In this additive manufacturing process, the article to be manufactured is configured as a plurality of individual slices of the article in a computer file, which in aggregate when sequentially formed result in a completed article. The computer file is used to position the output of, and pulse where appropriate, a laser or other energy source to direct an energy beam at the selected portions of the material to fuse together the selected portion of the material in the shape of the slice. Slices are sequentially formed one over the other, and the powder forming each slice is also fused to the material of the article of manufacture which was previously fused together, to form the finished article.

[0003] To form each slice, a layer of powder of a material used to form the article is deployed on a surface of the build chamber located adjacent to the powder bed, and then spread over the powder bed by a recoater having a wiper or "blade" which is pulled or pushed across the deployed powder and thence over the powder bed to spread the powder over the powder bed, and any portion of a partially manufactured article, to a desired thickness. The wiper is spaced a slight distance above the powder bed as it passes thereover to spread the powder at a desired thickness. After the layer of powder is spread across the powder bed, a laser selectively melts portions thereof which are then allowed to cool and fuse together, and also fuse to any previously formed portion of the part in the previously fused slices, if any, on the powder bed. This process is repeated, slice by slice, until manufacture of the article is completed.

[0004] The powder bed fusion process is commonly performed in a build chamber in an inert gas environment. During the process of fusing the powder layer with the laser, metal sparks, also known as dust or condensates, can form and settle on the powder on the powder bed including on the slice being processed, leading to defects in the finished article being manufactured. To remove this material before it can be deposited on the partially manufactured article, an inert gas, commonly argon, is flowed across the powder bed during the fusing of the powder. The argon may be supplied from a storage bottle or tank, in liquid form, which when released from the pressurized bottle, undergoes a reduction in the pressure thereof and absorbs ambient heat and changes into the gas phase.

[0005] As the argon or other inert gas passes over the powder bed, it continues to be heated by the higher temperature powder it is flowing over. This exchange of heat from the powder in the powder bed to the inert gas such as argon produces a temperature gradient across the length of the powder and partially formed article on the powder bed in the direction of the inert gas flow, as well as across the depth direction of the powder and the partially manufactured article of manufacture on the powder bed, resulting in a thermal gradient. Where the inert gas initially reaches the powder bed, a high temperature difference is present between the gas and the powder bed, and thus a large thermal gradient is present in the gas flow direction of the uppermost portions of the powder and in the depth direction thereof, including any partially manufactured article. As the gas flows across the surface of the powder, the gas is heated to a higher temperature, until it reaches, or nearly reaches, the temperature of the powder. At this point, across the powder surface and in the depth direction of the powder, a high thermal gradient region is formed from the location where the inert gas initially reaches the powder to a location on the powder where the temperature is close to the temperature of the powder, and a low thermal gradient region across the powder surface and depth of the powder, from the location where the temperature of the gas is nearly that of the powder to the end of the powder on the powder bed in the gas flow direction. The portion of the powder layer being fused in the higher thermal gradient region will have a significantly higher solidification rate than the portion of the powder having the smaller temperature gradient, causing non-uniform shrinking and hence distortion or warpage of the articles being manufactured. Likewise, internal stress can be imposed on the part by the different temperatures and temperature gradients thereon, also resulting in warping or distortion of the partially completed article. If the partially completed article warps, the next slice may not properly align on the partially completed part. If excessive warping or distortion of the article under manufacture occurs, the uppermost surface of the article being manufactured may interfere with recoating of the powder bed, and could damage or tear the re-coater blade by interfering contact therewith. If the recoater blade is damaged or torn, the subsequently spread layers of powder will be non-uniform, resulting in non-uniform depth slices being formed, or non-fused regions of powder being incorporated into the part where the laser beam cannot penetrate an excessively thick powder layer deposited as a result of a damaged recoater blade. Additionally, even if the recoater blade is not damaged, the contact of the recoater blade with the part may cause shifting of the part on the powder bed, so that the subsequent slice will be misaligned with the underlying partially formed article, requiring scrapping of the partially completed article. Hence, once warpage or distortion occurs in the article, forming a uniform powder layer will be affected and the printing process cannot continue. Warping of as little as 20 to 30pm can cause the blade to hit the part under manufacture on the bed. Additionally, more complex shapes tend to ward a greater amount when a temperature or heat gradient is imposed therein or thereon. However, warping and distortion occurring during manufacture of the article is not easily detected.

SUMMARY OF THE INVENTION

[0006] Provided herein are methods and apparatus for reducing the thermal gradient induced in the powder layer on the powder bed of a build chamber of a powder bed fusion apparatus, by reducing the difference in temperature of the powder over the length of the powder bed in the recoating direction, and thus yield more effective and reliable manufacture of articles of manufacture, and higher yield of useful articles from the apparatus. In one aspect, an inert gas is heated prior to passing inwardly of the printing area of the fusion bed build chamber, thereby reducing the high thermal gradient region across the length of the powder bed and through the depth of the powder, and thus create a more uniform temperature in and across the layer of powder being processed to form a slice of the article being manufactured. In a further aspect, the gas, once passed over the powder bed, is filtered and recirculated. Thus gas, which has been previously heated, by a heater in the gas delivery system, by heat transfer thereto as it passed over the powder bed, or both, is mixed with fresh gas and the heat absorbed by the recirculated gas is used to reduce the energy required to heat the gas freshly delivered from the liquid source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0008] Figure 1 is a schematic perspective view of a powder bed coating apparatus;

[0009] Figure 2 is a plan view of the a powder bed coating apparatus of Figure

1 ;

[0010] Figure 3 is a schematic perspective view of a powder bed coating apparatus showing a partially fabricated part being formed thereon; [0011] Figure 4 is a flow chart of the gas flow used with the powder bed coating apparatus of Figure 3;

[0012] Figure 5 is a plan view of a powder coating bed; and [0013] Figure 6 is a further plan view of the powder coating bed.

DETAILED DESCRIPTION

[0014] Provided herein is a powder bed fusion apparatus, wherein a temperature controlled gas is introduced to the build chamber of a powder bed fusion apparatus and thence passed over the powder on a powder bed and on any partially manufactured article on the powder bed, while fusing selected portions of the powder on the powder bed using an energy source. The apparatus can be used to provide a method of forming an article of manufacture, wherein a reduced thermal gradient is induced on the powder bed and thus more reliable manufacture of articles, with greater yield of useable articles, results. The heating source for the gas may be retrofitted into existing powder bed fusion apparatuses, to improve the reliability thereof and yield of useable articles of manufacture therefrom.

[0015] Referring initially to Figures 1 and 2, a mechanism for passing an inert gas over the powder bed is shown. Powder bed 10 is heated, such as by electric resistance heating thereof using a ceramic heater 18 (shown in phantom) or the like, located on a lowermost surface 16 thereof, by passing a heated fluid through passages in the powder bed 10, or by other appropriate mechanisms. The heater is configured to provide a substantially uniform temperature on the upper surface 12 of the powder bed 10. The powder bed 10 itself may be a ceramic or another material capable of withstanding the fusing temperature of the powder layers without causing warping or damage thereto. The powder bed 10 is shown schematically in Figure 1 , whereas in Figures 2 and 3, the powder bed 10 is surrounded by the base 8 of the build chamber 4, and a powder distribution region 6, a re-coater 2, an powder overflow tank 5 are also shown. The powder bed 10 is configured having a substantially planar upper surface 12, and is moveable upwardly and downwardly with respect to the upper surface 14 of the base chamber 8 by a ram 20 (Figure 1) which may be incrementally moved inwardly of the base 8 by the thickness of a fused slice of powder, and, after the article is manufactured, move upwardly to position the powder bed 10 upper surface 12 at or above the upper surface 14 of the powder bed for removal of the article therefrom.

[0016] To supply the energy to fuse the powder layers into fused slices on the powder bed 10, a laser 26 is supported from an overstructure 28 shown in dotted line in Figure 3, and the laser is directed at a lens, lenses, or a rotating polygonal mirror 32. The laser may be a continuous laser or a pulse laser, and the location of the beam 34 at the powder 24 surface of the powder 24 on the powder bed 10 is controlled to selectively fuse the uppermost layer of powder in the desired slice shape.

[0017] Powder 24 is dispensed from the powder dispense region 6 by first raising a plate or support (not shown) supporting the powder 24 in the dispense region 6. Once a portion of the powder 24 in the dispense region is located sufficiently above the upper surface 14 of the base plate 8, the re-coater 2, with a re-coater blade or blades 22 disposed extending from the powder bed facing side thereof, is moved across the powder to spread the powder over the powder bed 10, and any previously spread powder 24 and fused slice(s) of powder thereon. Any excess powder is spread into overflow tank 5. The operation of the re-coater blade and structures therefore are disclosed in US Provisional Application Serial No. 62/296,200, filed February 17, 2016, incorporated herein by reference.

[0018] As shown in Figure 3, a partially formed article 36 is shown on the powder bed 10, with unfused or unprocessed powder 24 on the particle bed 10 surrounding the partially manufactured article 36. The powder bed upper surface 12 is shown in phantom. Article 36 includes an upper surface 38 exposed from the powder 24 after the powder 24 is fused into a slice, and an embedded portion 40 which extends inwardly, and is surrounded by, the unfused powder 24, the lowermost surface of which is supported on the powder bed 10.

[0019] Referring to Figures 1 to 3, an inlet manifold 42 is located to one side of the powder bed 10 and is fluidly connected via a pipe 16 to a source 48 of inert gas such as argon. Manifold 42 is mounted to a sidewall of the overstructure 28 of the build chamber 1 and is configured to receive an inert gas which is maintained under pressure in source 48 in a liquid state and changes therefrom to a gaseous state when released from the source 48 by a selectively openable and closeable valve 49, and spread the gas flow across the width W of the powder bed 10. The manifold 42 includes a plurality of apertures 44 extending across the front face 50 thereof, configured to ensure that the inert gas which is distributed therefrom extends across the entire width W of the powder bed 10 without interruption. Although specific apertures 44 are shown, other paradigms for distributing the gas across the width of the powder bed 10, such as a screen or an elongated slot extending across the width W of the powder bed 10, are also contemplated herein. The gas moves across the powder bed 10 in the direction shown by arrows 52, where it is captured by exhaust manifold 54 which is mounted to a sidewall of the overstructure 28 of the build chamber 1 opposed to the inlet manifold 42 and which is and connected to an exhaust duct 56. Exhaust manifold 54 may have the same basic configuration as inlet manifold 42, and the exhaust duct 56 fluidly connected thereto provides a slight vacuum at the inlet(s) of the exhaust manifold 54 to help pull the inert gas across the powder bed 10 and remove the inert gas, once across the powder bed 10, from the build chamber 1 .

[0020] As shown schematically by in Figure 2, the gas leaves the source 48 and flows into the build chamber 1 through the piping 46 and inlet manifold 42, and thence flows across the powder bed 10 as shown by arrows 52, where it is captured by the exhaust manifold 54 from which it may be exhausted to atmosphere by the exhaust duct 56, or recycled as shown in Figure 4 by being directed to a recirculation filter system 68. During processing while the inert gas is flowing and being recycled, some or all of the inert gas may be lost. The portion of the inert gas recovered in the recirculation filter system 68 is redirected to the inlet piping 46 through recycle line 52 for reintroduction thereof into the build chamber 1 through the inlet manifold 42. The pressure of the inert gas passing through the recycle line 52 may be increased, such as by a pump (not shown) located upstream or downstream of the recirculation filter system 68 to ensure a pressure drop across the powder bed 10 to ensure flow of the inert gas across the powder bed.

[0021] When the inert element liquid such as liquid argon vaporizes from a liquid into a gas as a result of the drop in pressure experienced when the valve 49 connected to the liquid source 48 bottle is opened to the piping 46 ported to the inlet manifold 42, the gas enters the piping 46 at a temperature well below room temperature on the order of minus 200 to 300 degrees C. The ambient temperature surrounding the manufacturing apparatus is typically in a temperature range of 16 to 25 degrees C. The chamber itself is maintained at about 30 degrees C. The piping 46 and the inlet manifold 42 have a large thermal mass as compared to the thermal mass of the inert gas, and hence the temperature of the cold inert gas entering the piping 46 is raised to close to the ambient temperature by heat transferred from the warmer piping 46 and inlet manifold 42 into the inert gas flowing therethrough.

[0022] The temperature of the inert gas reaching the powder bed 10 is however typically less than the temperature of the powder and any partially manufactured article on the heated powder bed 10. The powder bed 10 is commonly maintained at 30 degrees C to 200 degrees C depending on the type of powder material being processed, and the gas is typically at, or slightly less than the ambient temperature of the facility or environment in which the powder bed fusion device is located as a result of the heat transferred thereinto from the piping 46 and manifold 42. As the relatively cold inert gas crosses the powder bed 10, it absorbs heat therefrom and the temperature of the inert gas is raised, while the heat transferred from the powder 24 and the partially manufactured article 36 on the powder bed 19 lowers the temperature of the powder 24 and partially manufactured article 36 on the powder bed 10 primarily at the gas exposed surfaces thereof and immediately therebelow, which results in the powder 24 and partially manufactured article 36 on the powder bed 10 having at least two different thermal gradients; one across its length L (the gas flow path between inlet manifold 42 and exhaust manifold 22); and one in the depth direction of the powder 24 and the partially manufactured article 36 on the powder bed 10, thereby creating a high temperature gradient region 60 where the gas begins crossing the powder bed 10, and a low temperature gradient region 62 adjacent to the exhaust manifold 54 end of the powder bed 10. A transition from the high thermal gradient portion 30 to the low thermal gradient portion 32 occurs where the temperature of the gas and the powder over which it is flowing come to within about 5 deg. C of each other. In the prior art, the temperature of the gas entering the build chamber 1 becomes relatively constant, and the relative length of the resulting high temperature gradient portion 30 is primarily a function of the temperature of the powder bed 10 and the upper surface 38 of the article 36, as a large temperature difference between the powder bed 10 and the temperature of the gas as it leaves the inlet manifold results in a longer high temperature gradient region 60, and a small temperature difference results in a smaller high temperature gradient portion 60 and a larger low temperature gradient portion 62.

[0023] Because the powder bed 10 is heated to a relatively uniform temperature across its lower surface by the heated powder bed 10, the portions of a partially manufactured article 36 closest to the powder bed 10 will have a relativity uniform temperature, varying by less than 2 or 3 deg. C. As the powder layers are laser processed, the processed slice region of the powder bed 10 will have a significantly higher temperature than the adjacent, unfused, powder 24 immediately after being fused. Also the temperature at the just fused slice surface 38 of the article will be higher than the temperature of the powder bed 10. Thus, the next layer of dispensed powder 24 overlying the uppermost surface 38 of the article being manufactured will have a higher temperature than the adjacent powder 24 region once dispensed. When cold inert gas is used to sweep particulates out of the build chamber 1 , a significant temperature gradient will develop in the powder 24, and in the partially manufactured article 36, along the inert gas flow direction of the powder bed 10 and in the depth direction of the partially manufactured article 36, and the temperature of the upper surface 38 of the article 36 being manufactured directly below the very thin layer of powder 24 to be processed into the next slice will experience a significant temperature difference in the gas flow direction, where the temperature is lowest at the inlet end of the article and highest at the exhaust end of the article. Because the articles being manufactured are commonly metals, or alloys thereof, possessing high thermal conductivity, the thermal gradient in the depth direction of the article will be greatest closest to the inlet gas manifold 42, and be lowest closest the outlet manifold 54. Thus, in the gas flow direction, the thermal gradient will be significant across the upper surface of the partially manufactured article. However, as the powder 24 which surrounds the article 36 is not significantly compacted, significant air or inert gas voids occur therein, such that the thermal gradient caused by the cold gas flow in the non-processed powder is significant only near the uppermost portion thereof and most significant over the partially manufactured article. The portion of the slice being fused in the high thermal gradient region 60 will experience a significantly higher solidification rate of the fused melted powder than portions of the slice being formed in the low thermal gradient region 62, as well as a temperature difference as compared to the low thermal gradient region 62, which causes distortion or warpage in the article of manufacture being manufactured on the powder bed 10 can occur. Where multiple articles are being prepared simultaneously, those closest to the gas inlet will warp to a greater extent than those spaced from the gas inlet. When the article 36 being manufactured warps or distorts significantly, sharp edges or the interior regions of the upper surface 38 of the partially manufactured article 36 can extend above the plane of the just fused powder layer by a distance greater than the depth of the next layer of powder to be deposited, which is typically dispensed at a thickness of between 10 pm and 50 pm. If the part extends a sufficient distance more than the thickness of the dispensed powder, the re-coater blade 22 will contact the partially manufactured article 36. The contact with the article 36 can cause damage to the re-coater blade 22, such as a tear or wear of the blade, leading to an uneven thickness of the next dispensed powder layer, and the partially manufactured article 36 may become shifted on the bed, resulting in misalignment of the following processed slice with the underlying partially manufactured article 36. If the position of the partially manufactured article 36 is shifted on the powder bed 10, the partially manufactured article 36 must be scrapped. If the recoater blade becomes damaged, it must be replaced, which requires the apparatus be shut down and manufacturing time is lost. This allows the partially manufactured article to cool, and shrink, and when manufacturing is resumed, a line will occur on the article at the location where manufacturing was stopped, and then restarted using a new coating blade. However, damage to the re-coater blade 22 and shifting of the article 36 on the powder bed, is often not detected immediately, and a partially manufactured article 36 will continued to be manufactured, resulting in further cost of material and build chamber 1 utilization, as well as potentially further damage and cost to the machine.

[0024] Referring now to Figure 4, to ameliorate the consequence of the presence of high and low thermal gradient regions 60, 62, a secondary heater 70 is located between the gas source 48 and the inlet manifold 42 of the build chamber 1 to preheat the inert gas before it reaches the inlet manifold 42 of the build chamber 1 , and a recirculation filter system 68 is provided for recovery of the inert gas from the build chamber 1 after it has passed over the powder bed 10 and recovery of the sensible heat therein for reuse. The heater 70 may be used to heat the inert gas to near to or to the temperature of the upper surface 38 of the partially manufactured article 36, or to raise the gas temperature sufficiently to shift the transition 64 from the high thermal gradient region 60 to low thermal gradient region 62 in the direction of the inlet far enough so that the article 66 or articles being manufactured are present only in the low thermal gradient zone 62, or, the article overlies both regions 60, 62, but the article 36 does not warp or distort sufficiently to disrupt the manufacture thereof. Figures 5 and 6 show the shift in the location of the transition region, in Figure 5, the inert gas is not preheated, in Figure 6, it is preheated.

[0025] In use, inert gas is initially flowed from the inert gas source 48 through the heater 70 within which it is preheated to a temperature of on the order of the temperature, or close to the temperature, of the powder on the powder bed 10 of the build chamber 1 , and then into piping 46 feeding the inlet manifold 42. A temperature sensor 72, such as a thermocouple or other temperature sensing device, is coupled to a controller 74 by a cable 56 or other communication device or system, including wireless communication devices or systems. The sensed temperature of the inert gas leaving the heater 70 is used by the controller 74 to the control heating mechanism within the heater 70 to, when needed, change the output temperature of the inert gas leaving the heater 70. The heating mechanism may be configured as an electric resistance heater wrapped around the inert gas flow passages extending through the heater 70, a fluid heating system wherein a temperature controlled heated fluid is passed through channels surrounding the inert gas passages through the heater 70, a fired fuel system wherein the inert gas passes through a heated plenum heated by combusting fuel, or other appropriate heating systems by which the temperature of the inert gas may be controlled and the pressure drop of the inert gas passing through the heater 70 is not excessive.

[0026] The inert gas, after passing across the powder bed 10 in the build chamber 1 and into the exhaust manifold 54, passes through the exhaust duct 56 and into the recirculation filter system 68. Recirculation filter system 68 includes, in line with the inert gas flow therethrough, one or more particulate filters through which the portion of the inert gas recovered from the build chamber 1 is passed to remove metal sparks, condensates or other contaminants therein, and a compressor or pump configured to boost the pressure of the recirculated gas passing through the recirculation filter system recycle pipe 82 and into the heater 70, and to create a lower pressure region in the exhaust manifold 54 and exhaust duct 56 with respect to the build chamber to pull the inert gas from the build chamber 1 and thence through the exhaust manifold 54 and exhaust duct 56.

[0027] The temperature of the portion of the inert gas recovered from the build chamber 1 and passing through the exhaust duct 56 is monitored by an outlet temperature sensor 86 coupled to the exhaust duct 56. As with temperature sensor 72, outlet temperature sensor 86 may be a thermocouple attached to the exhaust duct 56 to inferentially determine the outlet temperature of the recovered inert gas, a thermocouple located within the exhaust duct 56 to more directly monitor the temperature of the recovered inert gas passing therethrough, or another type of temperature sensing device. The measured outlet temperature is sent to the controller 74 via a cable 88 or other connection, which may include a wireless connection. The controller 74 is coupled to heater 70 such as by cable 90 or wireless connection, and the controller 74 monitors the outlet temperature of the gas passing from the heater 70, as well as infers or directly monitors the heat input to the inert gas which is provided by both the warmer gas entering the input gas stream through the recirculation filter system 68 and the heater 70, and adjusts the quantity of heat provided to the inert gas by the heater 70 as it passes therethrough to establish a desired outlet temperature of the gas as it leaves the heater 70.

[0028] During processing, when the build chamber 1 is initially started, the temperature of the build chamber inlet 42 and exhaust duct 54 are at ambient room temperature, and the powder bed 10 is heated to the processing temperature desired for the material of the powder 24 before powder 24 is dispensed onto the powder bed 10. The heater 70 is activated, such that the heat exchanger in the heater is brought to the desired inert gas temperature for introduction to the inlet piping 46 to the build chamber . As discussed previously, the temperature of the powder bed 10 on which the powder is positioned is maintained at 30 degrees C to 200 degrees C, and the temperature of the inlet manifold 42 and exhaust manifold 54 are initially at ambient room temperature. As the inert gas is passed through the heater 70 the temperature thereof is raised to a heater output setpoint temperature, and the gas then flows to the inlet piping 46 and into the build chamber 1 through the inlet manifold 42. As the gas passes through the inlet piping 46, the temperature of the inlet piping 46 may be less than the inert gas temperature passing therethrough, and the inlet piping 46 will increase in temperature and the inert gas will decrease in temperature as it passes therethrough. Hence, initially, the heater 70 setpoint temperature may be set above the powder bed 10 temperature, or above a desired steady state temperature of the gas leaving the heater 70, until the temperature of the inert gas at the build chamber inlet manifold 42 reaches the desired temperature. Alternatively, the inlet manifold may also be heated such as by an electrical resistance type heater.

[0029] Once a steady state gas temperature is established at the build chamber inlet manifold 42, the heater 70 is maintained at a specific setpoint temperature, typically between 30 degrees C to 200 degrees C, so that the inert gas entering the build chamber 1 is the same temperature as, or a desired constant or nearly constant difference in temperature from, the powder bed 10, or alternatively, of the temperature of the powder 24 at the top of the layer of powder 24 and over the partially manufactured article 36 on the powder bed 10. By fixing the temperature of the inert gas entering the build chamber 1 with respect to the temperature of the powder 24 over the partially manufactured article 36, or to the temperature of the powder bed 10, the temperature of the powder 24 across the length of the powder 24 on the partially manufactured article 36 in the inert gas flow direction may be maintained at a uniform, or nearly uniform temperature, and thus reduce the temperature gradient present in the partially manufactured article in the gas flow direction 24. Likewise, by matching the temperature of the inert gas to the temperature of the uppermost portion of powder 24 on the powder bed 10, the temperature gradient across the depth of the powder 24 will be uniform over the length of the powder bed 10. If the inert gas temperature is maintained at or close to that of the powder bed 10 below the powder 24, again the temperature gradient in both the length and depth directions of the powder 24 will be uniform or near to uniform. As a result, thermal gradient induced warping or distortion and the resulting reduced yield of useable articles of manufacture and potential damage to the recoater are significantly ameliorated. By monitoring the change in the inert gas temperature between the temperature sensor 72 at the inlet to the build chamber 1 and the gas outlet temperature, the resulting temperature change can be maintained at a steady state by adjusting the temperature of the gas leaving the heater 70. If the difference in temperature at the build chamber inlet manifold 42 and outlet manifold 54 exceeds a desired temperature, the temperature of the heat exchanger in the heater 70 is increased, to increase the heat transferred into the gas flowing through the heater 70 until a desired temperature difference between the build chamber 1 inlet and exhaust temperatures is achieved. If the inert gas temperature overshoots the desired exhaust temperature and thus the desired difference between the inlet and exhaust temperatures is too low, the temperature of the heat exchanger in the heater 70, or the flow rate of the gas, or both, are decreased. By monitoring the difference in the inert gas temperature between the inlet and exhaust of the build chamber 1 and adjusting the heater 70 heat exchanger temperature based on that difference, a steady state temperature change between the build chamber 1 inlet and exhaust temperatures can be achieved, even without setting an inlet temperature based on the temperature of the powder bed 10. However, the time required to bring the system to a steady state, where the difference between the inert gas inlet and exhaust temperature is within a desired range where the temperature of the gas leaving the heating system is not chosen based on the powder bed 10 temperature will be long, leading to long down time of the build chamber while the temperature stabilizes. Therefore, the initial temperature of the inert gas leaving the heater 70 is chosen based on the temperature of the powder bed 10, the temperature of the article being manufactured, or the inferred temperature of the powder on the partially manufactured article which is exposed directly to the flow of the inert gas. Thus, the down time of the build chamber during the period in which the temperature at the inlet and exhaust achieve a steady state temperature change is minimized. Preferably, the gas temperature difference between the build chamber inlet and outlet is less than 5 degrees C, and optimally 0 degrees C. Further, the temperature at the inlet is preferably within 10 degrees C of the temperature of the powder bed 10, more preferably less than 5 degrees C different that the powder bed 10 temperature, and most preferably the same as the powder bed 10 temperature.

[0030] In many cases, the article or articles being manufactured do not cover the entire surface of the powder bed 10, or the fusing temperature of the powder exceeds the gas temperature which can be achieved by the heater 70. Thus, the temperature of the inert gas entering the build chamber 1 need only be sufficiently close to the temperature of the powder on the powder bed or over the partially manufactured article of manufacture, such that the gas is sufficiently heated, before reaching the location of the partially manufactured article on the powder bed 10, so that the transition from the high thermal gradient portion 60 to the low thermal gradient portion 62 occurs before the inert gas passes over the partially manufactured article, or the thermal gradient in the high thermal gradient region 60 is not sufficiently great to cause meaningful warpage or distortion of the partially manufactured article 36. Thus, the energy required to heat the inert gas may be reduced, lowering the cost of operation of the build chamber 1.

[0031] The underlying composition of the article being manufactured and the material used for the manufacture of the article also effect when a low temperature gradient is achieved. The designs of some articles are more forgiving than other articles in terms of thermal stresses therein causing the article to warp of distort during manufacture as a result of the thermal stress. Different materials have different coefficients of thermal expansion, and thus the temperature difference across those with lower coefficients of thermal expansion can be greater, and thus the thermal gradient may be greater and still be considered a low thermal gradient. Thus, a low thermal gradient, and a transition from a high to a low thermal gradient occurs, when the actual thermal gradient is below that which will cause warping or distortion of the part sufficient to either:

[0032] Dislocate the top surface of the partially manufactured article to result in an unintended shift line appearing in the part; or

[0033] Cause the upper surface of the partially manufactured article to come close to contacting, or actually contacting, the blade of the re-coater.

[0034] By preheating the inert gas before it enters the build chamber 1 , the temperature gradient across the length of the powder bed 10 in the gas flow direction, and in the depth direction of the partially manufactured article, and the associated thermal gradient across the length of the powder 24 on the powder bed 10 and the depth of the partially manufactured article is controlled, and if desired substantially eliminated, substantially ameliorating thermal gradient induced warping of the article being manufactured and the incidence of scrapping of partially built articles and the potential damage to the recoater resulting from the warpage or distortion.

[0035] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.