EP1307311A1

EP1307311A1 - Method for the production of precise components by means of laser sintering

Info

Publication number: EP1307311A1
Application number: EP01956414A
Authority: EP
Inventors: Abdolreza Simchi; Frank Petzoldt; Haiko Pohl; Holger LÖFFLER
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2000-08-07
Filing date: 2001-07-27
Publication date: 2003-05-07
Also published as: AU2001278408A1; DE10039143C1; WO2002011928A1

Abstract

The invention relates to a method for the production of precise components by means of laser sintering of a powdered material, comprising a mixture of at least two powdered elements. The invention is characterised in that the powder mixture comprises the major component iron powder and further powdered alloying elements, present in elemental, pre-alloyed or partially pre-alloyed form and that during the laser sintering process a powder alloy arises from the powder elements. The following powdered alloying elements, either alone or in any combination are added to the iron powder: carbon, silicon, copper, tin, nickel, molybdenum, manganese, chromium, cobalt, tungsten, vanadium, titanium, phosphorus or boron. The produced components are subjected to the following treatment steps, individually or in any combination; homogenisation, stress-relief tempering, heat treatment, removal of internal faults and improvement of the surface quality.

Description

Process for the production of precise components using laser sintering

description

The invention relates to a method for producing precise components according to the preamble of the main claim.

Such a method is known from EP 0 782 487. A component is then produced using the laser sintering method by sintering metal powder mixtures with three components. The most important aim of the invention is to increase the melting temperature of the finished component.

The problem with the production of metallic components from conventional powder mixtures is that the porosity of the components produced is relatively high. If the components are made with a steel powder, a density of approx. 90% of the theoretical density can be achieved. A residual porosity of approx. 30% remains in the laser-sintered bronze parts. When increasing the density of the finished components e.g. infiltration reduces the operating temperature to approx. 180 ° C. Further disadvantages of conventional powder mixtures are the achievable mechanical properties of the component. The thermal stresses that arise in the component during the laser sintering process are retained as residual stresses and lead to the formation of cracks at certain points, e.g. B. strong cross-section transitions.

The invention has for its object to produce metallic components in the laser sintering process inexpensively with very good mechanical properties and in high quality.

This object is achieved by a method with the features of claim 1. The subordinate claims represent advantageous developments. Thereafter, the powder mixture with which components are to be produced in the laser sintering process consists of the main component iron and further powder components, which can be in elementary, pre-alloyed or partially pre-alloyed form. In the course of the laser sintering process, these powder alloy elements form a powder alloy from which the component is made within milliseconds. The powder mixture / alloy is heated to a temperature range close to / above the melting point, sintered and converted into a metastable state by subsequent very rapid cooling. This metastable condition is essential for carrying out a subsequent post-treatment, which decisively improves the properties of both the material and the components made from it. The aftertreatment consists of the following steps, which can be carried out individually or in combination: compression, homogenization, stress relieving, heat treatment, reduction of internal defects, improvement of the surface quality.

Depending on the requirements of the finished component or the manufacturing process, the following additional powder elements are added individually or in any combination to the main component iron of the powder mixture: carbon C, silicon Si, copper Cu, tin Sn, - nickel Ni, molybdenum Mo, manganese Mn, chromium Cr , Cobalt Co, tungsten W, vanadium V, titanium Ti, phosphorus P, boron B.

These powder components can be added individually or in any combination, depending on the requirements for the properties of the prefabricated component or the manufacturing process, in the following quantities: carbon C: 0.01-2% by mass, silicon Si: up to 1% by mass , Copper Cu: up to 10% by mass, tin Sn: up to 2% by mass, nickel Ni: up to 10% by mass, molybdenum Mo: up to β% by mass, manganese Mn: up to 2% by mass or 10-13% by mass, chromium Cr: up to 5% by mass or 12-18% by mass, cobalt Co: up to 2% by mass, tungsten W up to 5% .-%, vanadium V: up to 1% by mass, titanium Ti: up to 0.5% by mass, phosphorus P: up to 1% by mass, boron B: up to 1% by mass.

The invention provides that the individual powder components are in elementary, alloyed or partially alloyed form. These can be powder particles that are alloyed with the main component iron. In this case, they are present as, for example, ferroboron, ferrochromium, ferrophosphorus, or iron silicide. It can also be added other elements in powder alloyed _¬ ter or alloyed form such as Kuofer- phosphide, which, however, are not listed individually here. It is also provided that the powder mixture formed from the above-mentioned powder components is pre-alloyed in a separate process step.

According to an advantageous embodiment of the invention, the powder mixture consists of water or gas atomized powders, carbonyl powders, ground powders or a combination of these.

It is provided that the powder particles of the powder mixture have a size of <50 μm, preferably between 20-30 μm.

In an advantageous embodiment of the invention it is also provided that the powder particle size can be between 50 and max. 100 μm. This particle size is particularly advantageous if the components are to be manufactured quickly, i.e. if the powder layers in the laser sintering process have a layer thickness of ax. 100 μm, at which layer thickness the process can be rotated relatively quickly.

It has been found that a particle distribution of 30% <20 μm and a residual amount of particles between the size. see 20 and βOμm leads to particularly good process results, since high bulk density with good flowability is achieved.

According to the advantageous embodiment according to claim 9, it is provided that the main constituent of the powder mixture, the iron powder has a proportion between 5 and 20% of particles of size <10 μm and the remaining amount of the powder particles has a size between 50 and 60 μm.

Through the optimized choice of exposure parameters, the density of the components after laser sintering can be adjusted so that either short construction times with a lower component density or high property requirements (high densities with longer construction times) are taken into account. In the case of very high requirements with regard to temperature and wear, the residual porosity can now be eliminated by targeted post-treatment (sintering, infiltration, HIP). The temperature range remains unchanged or is improved. At the same time, the internal stresses can be removed by the heat treatment / sintering and the component quality be increased. When the components are manufactured using the laser sintering process, the minimum density is determined by the necessary part strength. It must be high enough that the part remains fixed on the construction platform and that post-treatment is possible. The upper limit of the density is determined by the material and the ge _¬ desired properties. In addition, economic concerns are taken into account. For example, it has already been shown that by suitable selection of the powder characteristics and the chemical composition of the material system, laser- _sintered parts with densities of 92% of the theoretical density could be produced. It is a niedriglegier _¬ th steel, in which the construction rate of 5.4 cm / hr. Higher Baura _¬ rates can be set if economic concerns play a greater role than the component properties. To save costs, so a part Gerer laser sintered density can be manufactured with higher construction rate At low _¬. The notwen _¬ ended density can be obtained by a post-treatment.

The individual process steps of the aftertreatment are explained in more detail below:

^V fiction: Laser-sintered components can be made directly to retired union under _¬ densities. However, internal pores have a negative influence on the mechanical properties _¬ properties of a component. Therefore, reducing the residual porosity is one way of expanding the field of application of laser sintered parts. This goal can be achieved by post-treatment with different steps. The steps include a ⁾ conventional sintering, b) infiltration, or c ₎ isostatic compression. During sintering various Me _¬ mechanisms can be activated, for example. B. solid sintering and / or liquid phase sintering. Pressure sintering is also applicable. Laser sintered structures made of steel can be particularly well compacted tern by a combination of solid state and Flüssigphasensin _¬. Laser-sintered steel powder mixtures ^w iron to a high inhomogeneity. Strong Mii_ _ö - an _d Makrose- gregationen occur during exposure to _d em laser _¬ s ^t rahl as a result of shock heating and -abkühlung. Since _d hrough some alloying elements such. B. copper, not completely dissolved in the steel structure. Continue to emerge nied _¬ r ⁱ gschmelzende phases such. B. the eutectics of Fe-C, Fe-B Fe-P. ^W now ird a laser sintered component made of steel in a αazu suitable furnace with a typical heating rate of 10 K / min heated, a liquid phase can be between fixed laser-

(Epoxy resins) or metal infiltration with copper, bronze or hard solders can be used to close pores. However, such methods can only close open pores. Mechanical methods, e.g. B. include hot isostatic pressing can also be applied to closed pores.

Homogenization: Directly laser-sintered components can have an inhomogeneous structure. For example, even in stem Sy _¬ iron-carbon ferrite, pearlite and martensite and tempered the martensite are the same. The existence of alloying elements such as e.g. B. molybdenum and boron, result in the formation of intermediate structures. The microhardness thus varies between values from 150 to 900 HV and the inhomogeneity increases if elemental and / or partially pre-alloyed powder is used. The formation and segregation of hard phases such as B. carbides, borides and nitrides reduce the mechanical properties, especially the toughness. Homogenization, which can be carried out together with sintering to increase the density, improves the component properties of laser-sintered structures. The homogenization of alloy elements can also be achieved at high temperatures by the formation of a liquid phase. The system Fe-0.8 C-0.3 B or Fe-1.2 C-0.4 P with copper, molybdenum and nickel (up to 2%) can be produced at approx. 1150 "C by the formation of diffusing eutectic, liquid phases from iron-carbon-boron or iron-carbon-phosphorus result in a densification and at the same time very rapid homogenization.The structure then consists of fine, homogeneous bainite with a low proportion of solidified eutectic phase.

Stress relieving: heating and cooling of the powder particles in the laser sintering process take place in a very short time window (<50 ms). This leads to the accumulation of thermal stresses in certain areas of the component. If the material also undergoes a phase change, other internal stresses are superimposed on the thermal stresses. For example, for a laser-sintered steel with 1% C, a ^β minute hold at a temperature of 200 ° C can increase the bending strength by 20%. Stress-relief annealing improves the component properties to a considerable _extent . This method can connected with the steps a) and b) who _¬. Heat treatment: This post-treatment consists of homogenization and stress relief annealing.

Dismantling inner. Defects: Thermal stresses _that arise during the construction process remain as residual stresses in the component after the laser sintering process. Accumulated internal stresses often cause cracks to form perpendicular to the direction of construction. These stresses have particularly in hochbe _¬ anspruchten parts such. B. tools are eliminated. A heat treatment of the aforementioned steel at a temperature Tem _¬ of 1220 ° C (20 min) caused an improvement in bending strength by 40% without changing the density.

Improvement of the surface quality: The surface quality can be positively influenced by heat treatment. Different methods have been used primarily, such as. B. blasting with different abrasives or Infiltra _¬ tion. However, if an optimized sintering cycle applied to the density increase, thereby at the same time the surfaces _¬ be improved quality. The formation of a diffusive liquid phase near the surface plays an important role. This leads to a closing of the pores and at the same time to a leveling of the surface structures. This feature is advantageous for all complex components (mold inserts for plastic injection molding) in which other processes do not lead to the desired result.

The technical applications of the invention are to produce metal prototypes (rapid prototyping) of individual parts (Direct Parts) or tools (eg Formein _¬ rates for the plastic injection molding or die-cast metal - Rapid Tooling) Direct sinter me the generative process Metal laser _^. Due to the very good mechanical properties, such parts. be used in mold and tool construction as well as in machine, plant and vehicle construction.

He ^'invention proper procedure is based on the following examples described in more detail:

Example 1:

Conventional powders are mixed in the desired alloy additions composition _¬ ^t mi each other, the powder intrinsic properties ^• _¬ be adjusted so that they gen the require _¬ correspond to the precast or method. It it is essential that good flow behavior is achieved with a high bulk density. The role of the additives consists in the setting of certain mechanical, physical and chemical properties of the finished component. Furthermore, the role of the additives can be to increase the absorbency of the iron powder from laser beams, to lower the melting point of the powder system, to use low-melting elements / alloys, to reduce the surface tension and viscosity, and to reduce deoxidation to improve the sintering activity in order to achieve high densities , For example, carbon as a fine elemental graphite (powder size 1 - 2 μm) increases the absorption capacity of iron / steel powder and reduces the melting point of the powder mixture through eutectic reaction and deoxidation. Copper or bronze powder with a powder size of less than 45 µm acts as a low-melting element or compound and improves the sintering activity. Phosphorus and boron reduce the surface tension and the viscosity of the melt, which arises during the laser sintering process, in order to achieve a good surface quality by avoiding the formation of spheres. The role of the other powder alloy elements can lie both in the setting of the desired mechanical properties and in the reaction with other elements for increased melt formation (Fe-C-Mo). The powder elements. Carbon, molybdenum, chromium, manganese, nickel bring about the high mechanical properties of the finished component. Phosphorus, boron, copper and tin have a high sintering activity. By choosing suitable laser sintering parameters, the density can be varied between 70 and 95% of the theoretical density. A further increase in density to almost 100% is achieved by sintering in the solid-liquid transition temperature range.

With direct laser sintering of the powder mixture described, densities of 70-95% of the theoretical density are achieved. The maximum density depends on the exposure strategy and the chemical composition, the type of alloy and the properties (powder form, particle distribution, powder size) of the powder mixture used: e.g. with the laser sintering parameters, 215 W cw C0 ₂ lasers with the construction speed of

5.4 cm ^J / h a density of 92 ± 1% of the theoretical density for powder consisting of (in mass%): 0.7 - 1 c, 2 - 4 Cu, up to

1.5 Mo, up to 2 Ni, up to 0.4 Sn, 0.15 B can be achieved. Example 2:

A powder mixture consisting of iron, 0.8 M .-% C, 0.3 M .-% B is with the laser sintering parameters 215 W CO ₂ laser, 100 mm / s laser scanning speed, 0.3 mm laser track width with a layer height of 100 microns. Laser sintered to a density of 80 - 85% of the theoretical density. The component hardness after laser sintering is approx. 200 HV30. By resintering in the temperature range 1200 - 1260 ° C (sintering time 10 min - 1 h) the density is increased to 98% of the theoretical density. The hardness remains constant at 200 HV30. Example 3:

A powder mixture consisting of iron, 0.7 - 1% by mass C, 2 - 4% by mass Cu, 1.5% by mass Mo, 0.15% by mass B is made with the laser sintering parameters 215 W C0 ₂ lasers, 100 mm / s laser scanning speed, 0.3 mm laser track width with a layer height of 50 μm to a density of 92 +/- 1% of the theoretical density. The component hardness after laser sintering is approx. 370 ^' HV30. Resintering in the temperature range 1200 - 1260 ° C (sintering time 10 min - 1 h) increases the density to 98% of the theoretical density. The hardness is reduced to 320 HV30 by slow furnace cooling.

Example 4:

A powder mixture consisting of iron, 1 - 1.2 M .-% C, 2 - 4 M .-% Cu, 0.4 M .-% P is with the laser sintering parameters 215 W CO ₂ laser, 100 mm / s laser scanning speed , 0.3 mm laser track width with a, compared to the first example, reduced layer height of 50 μm to a density of 90 +/- 1% of the theoretical density. The component hardness after laser sintering is approx. 450 HV3 ^Q. Resintering in the temperature range 1200 - 1260 ° C (sintering time 10 min - 1 h) increases the density to 98% of the theoretical density. The hardness drops to 370 HV30 due to a relatively slow furnace cooling and thus guarantees very good toughness properties.

Example 5:

An iron powder mixture with 0.8 M .-% carbon results after the laser sintering roughness of R _z 150 _{microns un (_} R _a 29 microns. If the carbon content is increased to 1.6 M .-%, improve _t he roughness on R _Σ 60 μm and R _a 19 μm very good mechanical properties after laser sintering (see powder mixture from example 3) have roughness values of R _z 75 μm and R _a 11 μm. The surface quality can be further improved by sintering following laser sintering. They decrease the roughness values of the powder mixture iron, 0.8 mass% C and 0.3 mass% B after sintering at 1260 - 1280 ° C, 30 minutes - 1 hour sintering time in a pre-vacuum, to R _z 19.6 μm and R _a 2.6 μm.

Claims

claims

I. Process for the production of precise components by laser sintering of a powder material consisting of a mixture of at least two powder elements, characterized in that the powder mixture is formed by the main constituent iron powder and further powder alloy elements, which are present in elementary, pre-alloyed or partially pre-alloyed form, and in that In the course of the laser sintering process, a powder alloy is formed from these powder elements, and the manufactured components are subjected to the following post-treatment steps, which are used individually or in any combination: homogenization, stress relief annealing, heat treatment, reduction of internal defects and improvement of the surface quality.

2. The method according to claim 1, characterized in that the following, in elemental, alloyed or pre-alloyed form, powder elements are added to the iron powder, either individually or in any combination: carbon, silicon, copper, tin, nickel, molybdenum, manganese, chromium, Cobalt, tungsten, vanadium, titanium, phosphorus, boron.

3. The method according to claim 2, characterized in that the powder elements are added individually or in any combination in the following amounts: carbon: 0.01-2% by mass, silicon: up to 1% by mass, copper: up to 10% by mass, tin: up to 2% by mass, nickel: up to 10% by mass, molybdenum: up to 6% by mass, manganese: up to 2% by mass or 10 - 13 mass%, chromium: up to 5 mass% or 12 - 18 mass%, cobalt: up to 2 mass%, tungsten up to 5 mass%, vanadium: up to 1 mass -%, titanium: up to 0.5% by mass, phosphorus: up to 1% by mass, boron: up to 1% by mass. , Method according to one of the preceding claims, characterized in that the powder elements are present in alloyed or pre-alloyed form as ferrochrome, ferroboron, ferrophosphorus, copper phosphide or iron silicide.

5. The method according to any one of the preceding claims, characterized in that the powder mixture consists of gas-atomized powders, carbonyl powders, painted powders or a combination thereof.

6. The method according to any one of the preceding claims, characterized in that the powder mixture consists of an amount of powder particles with a size smaller than 50 microns, preferably between 20-30 microns.

7. The method according to any one of the preceding claims 1 to 5, characterized in that the powder mixture consists of particles with a size of 50 - max.lOOμm.

8. The method according to any one of the preceding claims, characterized in that the powder mixture consists of 30% of particles that are smaller than 20 microns and that the remaining amount consists of particles with a size between 20 and 60 microns.

9. The method according to any one of the preceding claims, characterized in that the main component of the powder mixture, the iron powder has between 5 and 20% of the particles smaller than 10 microns and that the remaining amount consists of particles of size 50 - 60 microns.

10. The method according to any one of the preceding claims, characterized in that the parameters of the laser sintering process such as laser energy, laser speed, track width and exposure are set depending on the desired properties of the finished part.

11. The method according to any one of the preceding claims, characterized in that the compression step is carried out as conventional sintering, infiltration or isostatic compression.

12. The method according to claim 11, characterized in that the sintering is carried out as solid-state sintering or as pressure sintering.

13. The method according to any one of the preceding claims, characterized in that stress relieving is carried out in connection with the step of compression and homogenization.

14. The method according to any one of the preceding claims, characterized in that the step of heat treatment consists of the steps of homogenization and stress relieving.

15. The method according to any one of the preceding claims, characterized in that the step of improving the surface quality by blasting with different blasting media is carried out by infiltration or by an optimized sintering cycle.