WO2002092264A1

WO2002092264A1 - Metal powder blend and method for producing hardenable products by free form fabrication

Info

Publication number: WO2002092264A1
Application number: PCT/SE2002/000863
Authority: WO
Inventors: Allan Hede; Lena Thorsson; Björn EKLUND
Original assignee: Iuc Karlskoga Ab
Priority date: 2001-05-11
Filing date: 2002-05-06
Publication date: 2002-11-21
Also published as: SE520974C2; SE0101654L; SE0101654D0

Abstract

The invention relates to a metallic powder blend and to a method for producing strong and durable products by free form forming via CAD-controlled layer by layer bonding of the power particles mutually and with the previous layer. The characteristic feature of the invention is that the pulverous blend comprises at least one powder of a second metal alloy with a different, i.e. lower, melting point than the first alloy. The product is given its final hardness and durability by a subsequent precipitation hardening. The precipitation hardening metal alloy is either iron- or copper-based.

Description

Metal powder blend and method for producing hardenable products by free form fabrication

The present invention relates to a new type of metallic powder blend for freeform fabrication (FFF) of components and tools, and a method for utilising the said metallic powder blend.

The freeform fabrication technique involves a layer-by-layer fabrication of the desired part from a powder material direct from CAD data without any prior preparation. This technique thus means that the powder particles in a powder layer are bonded to each other and to the adjacent particles in a previously processed powder layer and in all areas of the latter powder layer where solid material is desired, but are left unbonded in all other areas of the layer and this residue thus can be subsequently poured off as free powder. This technique can be used for fabrication of products from metallic powder, ceramic powder and plastic powder. However, the present invention relates only to the use of the FFF method for fabrication of different types of product from metallic powder. During the build-up of the various layers in accordance with the FFF technique using a metallic powder, the powder particles are bonded to each other by a metallurgical bonding whereby the desired body shall consist of solid matter. 'Metallurgical bonding' herein denotes a sintering or coalescence of the powder particles.

A number of different processes for FFF of principally finished products from a build-up of successive layers of metallic powder that are sintered or coalesced are known per se. According to one group of such processes the procedure is based on successively applied layers of powder that are selectively sintered or coalesced internally and with the previous powder layer by a CAD-controlled laser or electron beam within the areas of each powder layer which, in combination, will form the desired product. According to another variant the powder is directly applied to the designated area in the focus of the CAD-controlled laser beam.

FFF thus constitutes a collective denomination for all these methods involving a successive build-up of a part directly from CAD data without any prior preparation. The build-up thus occurs freely without the use of tools. Extremely complex parts with ,for example, internal channels or cavities of irregular shape and position, can be fabricated using these methods. Most FFF methods are based on a three-dimensional CAD model being subdivided into thin layers, after which these layers are built up one-by-one so that the part in question gradually evolves. As already indicated, there are a number of different methods One of these is the SLS (selective laser sintering) method, which is most applicable for the fabrication of metal components and tools when close tolerances and high quality surfaces are desirable.

The present invention was conceived in the first instance for the SLS method described in more detail below, and in connection with which it has been primarily tested, but when these principles are more generally applicable variants can be used with all closely related freeform methods that are based on a layer-by-layer sintering or coalescence of a raw material in the form of a metallic powder.

The SLS method is based on a layer-by-layer build-up of a component or tool using a laser that selectively sinters (fuses together) pulverous material that is metallic, ceramic or polymeric. Regarding metallic powder, either polymer coated powder or metallic powder blends are used. The final result, provided the correct powder and the right machine parameters are used, is a component (or tool) with close tolerances and a fine surface finish. The sorts of metallic powder that are currently available for this method are either bronze powder or steel powder based. Thus a powder blend currently marketed by one SLS machine manufacturer contains nickel powder + bronze powder + copper phosphide powder. No method or powder, however, produces a completely dense material (5-30% porosities) using the SLS method. The same also applies to all the other currently known freeform methods. If dense material is required it can only be achieved by infiltration with a low-melting metal or plastic. The hardness and strength of the finished product will, however, be low compared with what can be achieved by conventional manufacture of components and tools via ingot plus forging or hot isostatic pressing of powder. This means that until now the technique could only be used to produce prototypes or components and tools for short duration use. If a better powder was available that enabled dense material and a hardness (strength) on a par with conventionally manufactured material without degradation of tolerances and surface finish, the market for this technique would increase dramatically. For example, it would then be possible to series manufacture tools for plastic moulding with more complex shapes and with cooling/coolant channels optimally located. Among other things, the latter would create opportunities for an increased rate of production. Ideas for material specific improvements

How shall one then design an appropriate metallic powder (metallic powder blend) in order to use the SLS method or one of the other currently known freeform fabrication methods to manufacture production tools for plastic moulding as per the above with high hardness (strength), close tolerances and good surface finish? The use of tool steel powder (e.g. SS 2242 or 2550) to achieve this seems a likely candidate. Trials with such a powder, however, have so far not been successful. When used with laser sintering such a powder gives a martensitic layer with high hardness and high internal stresses. This causes deformation changes and often fissuring as well. The high hardness also makes it difficult (impossible) to smooth the layer deposited with a scraper before application of the next layer. The final result is a tool with poor tolerances and poor (rough) surfaces that require extensive and time- consuming subsequent machining. Moreover, there is a major risk of fissuring.

Fundamental principle of the present invention

As claimed in the present invention it is proposed to use a powder from a precipitation hardening alloy instead of the above-mentioned theoretically more immediate tool steel powders with their hard, brittle martensite from laser sintering. A precipitation hardening powder would instead give a soft material directly after laser sintering, resulting in a tool with low internal stresses and good surface finish. The desired hardness could then instead be achieved by ageing (precipitation hardening) of the tool after laser sintering. There is a plurality of alloying systems with iron as substrate that are precipitation hardening, all of which fall within the scope of the present invention.

However, the most interesting alloying type for the present field of application is maraging steel which, via ageing, can achieve a strength and hardness on a par with that of tool steel. Maraging steel is characterised from the analysis aspect by C< 0.05% by weight, Ni 16-20% by weight, Mo 3-6% by weight, Ti 0.2-1.0% by weight and Al 0.05-0.2% by weight, as well as for this type of steel the customary (= low) contents of Si, Mn, Cr, etc elements.

The maraging steel tested within the scope of the present invention, and which has given surprisingly good results, is designated 18NiMar250. It is considered, however, that other maraging steels could give equally good results. Other precipitation hardening steels that are particularly relevant to the present invention are the stainless precipitation hardening steels that comprise a relatively large number of steels with Cr > 12% by weight, Ni > 2% by weight, as well as any or some of the precipitation hardening elements Cu, Mo, Ti or Al. One steel that is of special interest for the present invention is the well-known 17-4 PH. The precipitation hardening in this steel is based on Cu, which from a technical process point of view is easier to use than Ti or Al.

The present invention also offers a solution to another previously observed problem, namely the fact that with previously known metallic pulverous material used to fabricate products via the freeform fabrication technique the products produced have often had an unsatisfactorily rough surface structure. We have, in fact, found that it is possible to fabricate metal pieces with a significantly finer finished surface structure and a denser material if, instead of using a uniform pulverous material as raw material, a powder blend is used comprising at least two different metallic powders with two different melting points, or with different laser absorption, of which at least one is precipitation hardening and where the non-precipitation hardening powder preferably is the one with the lowest melting point.

The basic principles outlined above for the present invention apply both to the use of copperas well as steel-based metallic powder blends. Both these metals can be alloyed so that they become precipitation hardening.

Another basic principle that is essential in this context is the size of the actual powder particles, and according to the present invention the powder particles shall preferably have a particle size that does not exceed 50μm.

Furthermore, according to the present invention the precipitation hardening pulverous ingredients preferably shall be of a quantity that equates to at least 50% by volume of the complete quantity of powder.

It was stated above that the precipitation hardening pulverous ingredients as claimed in the present invention can beneficially be constituted of a maraging steel, and the general analysis for a maraging steel is also disclosed in the text. According to a further development of the present invention the preferably more low-melting, normally non-precipitation hardening ingredient shall consist of a low-melting Cu-, Fe-, Ni- or Co-based alloy containing one or more elements that have greater affinity to oxygen than iron. Examples of such elements are P, B, Si, Mn, Ti and Zr. Interesting alloys in this context are, therefore, among others, the Cu-alloy Cu7P6Sn with 7%P and 6%Sn, the Ni-alloy Ni20Cu2SilB with maximum 0.05%C, 2%Si, 1.1 %B and 0.5%Fe, as well as an existing alloy on the market containing 40%Cu, 20%Mn, 20%Fe, 15%Ni, 2%Si and 1%B.

With regard to that part of the present invention involving the use of copper-based precipitation hardening alloys, we have found that it is possible to use a number of such alloys. Specially suitable are alloys containing Ni 10-20% by weight, Sn 4-12% by weight with the remainder Cu and normal contents of impurities.

Besides precipitation hardening alloys this variant of the invention can also comprise powder of a copper-based alloy with a lower melting point and containing varying contents of Sn, P, Zn and/or Mn.

One copper-based variant of pulverous material in accordance with the present invention that we have found to function specially well comprised 50-90% by weight of a precipitation hardening copper alloy containing Ni 10-20% by weight, Sn 4-12% by weight and the remainder copper plus the usual impurities, while the non-precipitation hardening ingredient with the lower melting point contained P 2-10% by weight, Sn 5-15% by weight with the remainder Cu and the normal contents of impurities.

In addition, we have found that further improvements of the powder blend as claimed in the present invention can be achieved if 5-15% by weight of bronze powder is added to the blend, which has been shown to produce a denser metal piece. Bronze powder contents in the interval 5-30% by weight have also been shown to give positive results.

hi the powder blend that is characteristic for the present invention the precipitation hardening pulverous material will not normally melt when the powder is used within the scope of the above referenced SLS method or other freeform fabrication method. Consequently, it will be necessary for the second, more low-melting pulverous ingredient to wet the precipitation hardening powder particles to enable a satisfactory sintering between the ingredients incorporated. Various sintering additives can be used to facilitate the sintering process. One additive that is good in this context is boric acid that is deposited on the surface of the precipitation hardening powder particles before this pulverous ingredient is blended with the other pulverous ingredients.

For the more low-melting ingredient it generally applies that when it is in molten state it shall have low viscosity and also, as already mentioned, shall be able to wet the more high-melting ingredient. The latter in turn means, purely metallurgically, that the more low-melting ingredient shall incorporate substances that can function as a fluxing agent, i.e. can react with oxides on the surface of the powder particles of the more high-melting ingredient. Fluxing agent elements for steel powder are elements with higher affinity for oxygen than Fe, such as Si, B, Al, Ti, etc, but also P for non-stainless powder.

Narious processes that are known per se for obtaining a denser metal piece, for example in the form of infiltration, and various methods that are also known per se for increasing the surface hardness of the product by nitration or other surface treatment methods such as PND treatment or shot peening can, of course, also be utilised for products fabricated in accordance with the present invention.

Another method of increasing the density of the finished product is based on increasing the quantity of available melt by right from the beginning incorporating a third pulverous ingredient that is more low-melting compared with the high-melting ingredient, which third pulverous ingredient can be present in the proportion of 5-15% by weight for example.

Concerning tests performed of the present invention

To test the fundamental principles presented above for the present invention a first trial series was performed based solely on powder in precipitation hardening alloys. Both powder in precipitation hardening copper alloys and powder in precipitation hardening steel were tested. None of the trials gave the desired results. The surfaces especially were very rough/granular.

The next stage was to test two- and multi-ingredient systems, i.e. powder blends consisting of powder of a precipitation hardening alloy as well as powder of one or more other alloys. These trials indicated that if one selected a powder blend consisting of powder of a precipitation hardening alloy (steel) in combination with powder of one or more alloys with another melting point (in the first instance a lower melting point) than that of the said precipitation hardening alloy, and this (these) alloy(s) also produced a melt that exhibits good wettability in relation to the powder in the precipitation hardening alloy, there were good prospects for achieving the desired result. Development work has been focused on this, and has led to a number of steel- and copper-based powder blends that give a good combination of hardness, surface finish and tolerances in the SLS-fabricated tool. One of these powder blends has given specially good results. This blend is described in detail below.

Detailed description of the present invention

A number of precipitation hardening copper alloys (Cu-Be, Cu-Cr, Cu-Ti, CuNiSn and CuSiNi) have been SLS tested with varying results. The best result so far was achieved with the precipitation hardening copper alloy Cu-15Ni-8Sn which, among other things, is characterised by high precipitation hardenability (compared with beryllium copper), very good dimensional stability (better than beryllium copper), as well as low sensitivity for the atmosphere in the SLS chamber. Powders of this alloy in different size classes and with contents of 50-90% have been combined with different sorts of solder powder with a lower melting point than the Cu-15Ni-8Sn alloy and with good wettability compared with the Cu-15Ni-8Sn alloy. Several solder alloys have been shown to work well, especially those containing P and Mn. Among others, a solder bronze with 7%P, 6%Sn and the remainder Cu has shown itself to work surprisingly well. Using a pulverous blend consisting of 10-30% Cu7P6Sn powder and the remainder Cu-15Ni-8Sn powder (both types of powder with a particle size of maximum 50 μm), it has been possible to laser sinter bodies of various geometrical design to very close tolerances and with good surface finish. By selecting different process parameters it has been possible to reduce porosity down to approximately 10%. Ageing for 2 hours at 400°C has given a hardness of almost 300 Nickers (hardness before ageing was scarcely 200 Nickers). Complex tools have also been fabricated from this pulverous blend with good results in respect of tolerances, surface finish and hardness.

To further improve the internal structure and characteristics of the finished tool the pulverous blend was supplemented with another powder of low-melting metals. A small amount of finegrained bronze powder additive was shown to have a positive effect in reducing porosity.

Some precipitation hardening steels were also SLS tested, with varying results. The use of a nitrogen gas atmosphere and a certain amount of residual oxygen in the SLS chamber has not, however, produced any specially good results with steels with a Ti content nor with steels with a high Cr content. After transition to a completely inert gas atmosphere (e.g. 100% argon) in the SLS chamber a marked improvement in results was achieved. Thus 18%Ni maraging steel in combination with the low-melting 0.03%C-2%Si-l.l%B-20%Cu-0.5%Fe- 76%Ni alloy produced a good result. Even the stainless 17-4PH steel in combination with this low-melting alloy produced a good result.

In addition, tests that produced good results were performed using maraging steel powder in combination with various Ni-based powders (e.g. 0.03%C-2%Si-l.l%B-20%Cu-0.5%Fe- 76%Ni). Especially the combination with 20% Cu7P6Sn powder produced a virtually completely dense material with high hardness (>300 Nickers) after ageing.

Claims

PATENT CLAIMSWe hereby claim and desire to secure by Letters Patent the following.

1. A method utilising a freeform fabrication process for directly fabricating integral products in principally final form with good strength and durability via a CAD-controlled layer-by- layer metallurgical bonding which herein refers to a sintering or coalescence of the particles in a metallic powder blend with each other and with the previous powder layer h e r e i n the raw material used comprises at least one powder of a first precipitation hardening metal alloy and at least one powder of a second metal alloy with a different melting point from the first alloy and whereby the product fabricated is given its final hardness and durability by a subsequent precipitation hardening also designated as ageing.

2. A metallic powder blend for freeform fabrication of bodies directly to their principally final form in accordance with the method claimed in Claim 1 by means of a CAD-controlled layer- by-layer build-up of powder to the desired body involving a metallurgical bonding which herein refers to sintering or coalescence mutually between the powder particles in the powder blend and with the immediately prior powder layer h e r e i n the said metallic powder blend comprises at least one powder of first type of a precipitation hardening metal alloy and at least one powder of a second type of metal alloy with a different melting point from the first alloy.

3. A metallic powder blend as claimed in Claim 2 w h e r e i n besides the precipitation hardening first type of powder the said blend also comprises a second type of powder with a lower melting point than that of the first powder type.

4. A metallic powder blend as claimed in either of Claims 2 or 3 w h e r e i n the first type of precipitation hardening alloy is incorporated in an amount equivalent to at least 50% by volume.

5. A metallic powder blend as claimed in Claims 2 through 4 w h e r e i n the powder types comprised have a particle size that does not exceed 50μm and that the blend comprises a first type of powder of a precipitation hardening alloy and at least a second type of powder with a different melting point from the first alloy.

6. A metallic powder blend as claimed in Claim 2 w h e r e i n the precipitation hardening alloy consists of a precipitation hardening steel alloy.

7. A metallic powder blend as claimed in any of Claims 2 through ό h e r e l n the precipitation hardening alloy consists of a maraging steel of 18Ni type comprising C<0.05% by weight, Ni 16-20% by weight, Mo 3-6% by weight, Co 6-10% by weight, Ti 0.2-1.0% by weight, Al 0.05-0.2% by weight as well as the customary contents of impurities for his type of steel, i.e. low contents of Si, Mn, Cr, etc.

8. A metallic powder blend as claimed in any of Claims 2 through 7 w h e r e i n the low- melting, non-precipitation hardening pulverous ingredient consists of a B- and/or Si- containing iron-, nickel- or cobalt-containing alloy with a lower melting point than that of the precipitation hardening steel.

9. A metallic powder blend as claimed in any of Claims 2 through 5 w h e r e i n the precipitation hardening pulverous ingredient consists of a precipitation hardening copper- based alloy.

10. A metallic powder blend as claimed in Claim 9 w h e r e i n the precipitation hardening alloy consists of a copper-based alloy comprising 10-20% by weight Ni, 4-12% by weight Sn with the remainder being copper and the normal contents of impurities.

11. A metallic powder blend as claimed in Claims 9 and lO w h e r e i n besides the powder of precipitation hardening copper alloy the said metallic powder blend also contains a copper alloy with a lower melting point, i.e. a Sn-, P-, Zn- or Mn-alloyed copper-based alloy.

12. A metallic powder blend as claimed in Claim lO w h e r e i n 50-90% by weight of the said metallic powder blend consists of the precipitation hardening copper alloy which in turn contains 10-20% by weight Ni, 4-12% by weight Sn with the remainder copper and the normal contents of impurities, while the copper alloy with the lower melting point contains 2-10% by weight P, 5-15% by weight Sn with the remainder Cu and the normal contents of impurities.

13. A metallic powder blend as claimed in Claims 2 through 12wherein it also contains an additive of bronze powder.