GB2563333A

GB2563333A - Manufacture of metal articles

Info

Publication number: GB2563333A
Application number: GB1806744.7A
Authority: GB
Inventors: Guttamindapalli Aswathanarayanaswamy Ravi; John Sutcliffe Christopher
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 2017-04-27
Filing date: 2018-04-25
Publication date: 2018-12-12
Also published as: GB201806744D0; GB201706715D0

Abstract

Additive manufacturing of a titanium alloy and at least one grain refinement element, preferably Yttrium or Boron, e.g. 0.01 wt % to 0.1 wt %, e.g. in an alpha-beta, near-beta, or beta alloy, e.g. Ti-6Al-4V, commercially pure titanium, a Ti5553 alloy or a biocompatible alloy such as Ti-6Al-7Nb. No more than 0.5 wt % Iron and 0.2 wt% oxygen may be present. The grain refinement element is designed to act to limit the size of the grains, decrease the anisotropy of the grains, and so reduce stress-induced fractures 202.

Description

MANUFACTURE OF METAL ARTICLES

Field of the Invention

The invention relates to the manufacture of metal articles, more specifically the manufacture of metal articles by additive manufacturing techniques. In particular, the invention relates to the manufacture of metal articles by an additive manufacturing technique that may involve the selective melting or sintering of a metal powder or metal wire. Examples of such techniques may include selective laser melting (SLM), selective laser sintering (SLS), wire plasma arc deposition, and techniques that use an electron beam rather than a laser or plasma arc.

Background

Additive manufacturing methods such as selective laser melting (SLM) are rapid prototyping (RP) and/or rapid manufacturing (RM) technologies which may be used for the production of metallic solid and porous articles. Conveniently, the articles may have suitable properties to be put straight in to use. For instance, SLM may be used to produce one-off articles such as parts or components which are bespoke to their intended application. Similarly, SLM may be used to produce large or small batches of articles such as parts or components for a specific application.

Additive manufacturing processes such as SLM build articles in a layer-by-layer fashion. Typically in SLM, this requires thin (e.g. from 20 pm to 100 pm) uniform layers of fine metal powders to be deposited on a moving substrate. The powder particles are then fused together by selectively laser scanning them, usually according to a model’s 3D CAD data. SLM relies on converting a powder into a melt pool, from which material solidifies to form a new solid component. The solid weld bead must also fuse to the underlying and surrounding solid if a dense, strong component is to be produced.

An advantage of SLM, particularly in comparison with powder sintering used in some other RP/RM processes, is complete metal powder melting which may lead to higher densities and better mechanical properties. Further, this may reduce or even eliminate the need for binders and/or for post-processing.

In addition, additive manufacturing techniques such as SLM or SLS typically may be more cost effective and/or time effective for making articles having more complex geometries when compared with conventional manufacturing techniques, due to the absence of any tooling. There may also be a significant reduction in design constraints. The production of fully functional parts directly from metal powders that can be used in place of parts that would normally be machined or cast is one reason for the widening application of additive manufacturing techniques such as SLM or SLS, e.g. in the medical, dental, aerospace and electronics sectors.

In alternative additive manufacturing techniques such as wire plasma arc deposition (also known as wire arc additive manufacturing (WAAM)), a metal wire is used as the material source rather than a metal powder. A plasma arc is used to melt the wire, and components are built up layer by layer. Such techniques can provide high deposition rates with low material costs, and have been used to produce medium to large-sized components for aerospace applications.

The production of articles using additive manufacturing techniques such as SLM, SLS, or WAAM often requires the use of powders of metals or metal wires. Titanium alloys such as Ti6A14V have been used as the metal powder or metal wire. Ti6A14V in particular is widely used in aerospace and medical applications because it offers a combination of high strength, low weight, and good weldability. Due to the good welding characteristics of Ti6A14V, a powder of Ti6A14V can be melted to a high density (> 99.5 % theoretical), making it ideal for additive manufacturing techniques.

Additive manufacturing techniques are capable of producing large and complex parts with thick and thin sections and also complex features on their surfaces like through holes, squares and rectangles. Some of these features may have sharp corners (90° angles). These sharp corners can act as stress raisers, which may lead to fractures forming in the part.

Summary of the Invention

In accordance with a first aspect there is provided a method of manufacture of an article comprising additive manufacturing using a material comprising, or consisting essentially of, a titanium alloy and at least one grain refinement element. For example, the material may be provided as a powder or a wire.

Advantageously, the grain refinement element(s) may act to limit the size of the grains of the titanium alloy in the manufactured article. In particular, the grain refinement element(s) may act to decrease the anisotropy between the dimensions of the grains compared with carrying out a similar method with the titanium alloy in the absence of the grain refinement element(s). For example, the grain refinement element(s) may act to equalise substantially the length of a given grain in a build direction with the length of the given grain perpendicular to the build direction. Consequently, the given grain may be almost or substantially isotropic. By decreasing the anisotropy (i.e. increasing the isotropy) of the grains, the occurrence of stress-induced fractures occurring, in use, in the manufactured articles may be reduced.

The material may comprise up to 1 wt% of the grain refinement element(s). The material may comprise up to or at least 0.5 wt% of the grain refinement element(s). The material may comprise up to or at least 0.1 wt% of the grain refinement element(s). The material may comprise at least 0.01 wt% of the grain refinement element(s).

In some embodiments, the grain refinement element(s) may include boron and/or yttrium.

For example, the material may comprise up to 0.5 wt% of boron. The material may comprise up to or at least 0.1 wt% of boron. The material may comprise at least 0.01 wt% of boron. For instance, the material may comprise approximately 0.02 wt%, 0.05 wt% or 0.1 wt% boron.

Alternatively or additionally, the material may comprise up to 0.5 wt% of yttrium. The material may comprise up to or at least 0.1 wt% of yttrium. The material may comprise at least 0.01 wt% of yttrium. For instance, the material may comprise approximately 0.02 wt%, 0.05 wt%, or 0.1 wt% yttrium.

The grain refinement element(s) may be added to the material in elemental form.

For instance, elemental boron and/or elemental yttrium may be added to the material. Typically, the grain refinement element(s) may include elemental boron and/or yttrium.

In some embodiments, the titanium alloy may be an alpha-beta alloy. Alpha-beta alloys contain a mixture of titanium in the alpha phase and titanium in the beta phase. Such alloys may contain an alpha stabiliser and/or a beta stabiliser to stabilise titanium in the alpha and beta phase respectively.

In some embodiments, the titanium alloy may contain from 5 wt% to 7 wt% aluminium. The titanium alloy may contain from 3 wt% to 5 wt% vanadium.

Typically, the titanium alloy may contain around 6 wt% aluminium and/or around 4 wt% vanadium.

The titanium alloy may contain no more than 0.5 wt% iron, typically no more than 0.25 wt% iron. The titanium alloy may contain no more than 0.2 wt% oxygen. The balance of the titanium alloy may be made up of titanium and unavoidable impurities.

In some embodiments, the titanium alloy may comprise, or consist essentially of, a Ti6A14V alloy to which the grain refinement element(s) has/have been added. Ti6A14V alloys are known for their combination of high strength, low weight and good weldability.

In an embodiment, the material may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% boron: with the balance made up of titanium and unavoidable impurities.

In an embodiment, the material may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% yttrium; with the balance made up of titanium and unavoidable impurities.

In an embodiment, the material may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; from 0.01 wt% to 0.5 wt% yttrium; and from 0.01 wt% to 0.5 wt% boron; with the balance made up of titanium and unavoidable impurities.

In some embodiments, the titanium alloy may comprise the grain refinement element(s).

Some embodiments may further comprise a preliminary step of producing the material. The material may be produced for example by atomisation. Advantageously, atomisation typically may produce substantially spherical particles.

For example, elemental boron and/or yttrium may be atomised with the metals used to produce the titanium alloy.

Alternatively, the grain refinement element may be added to the material using mechanical blending or shear blending, after production of the titanium alloy. For example, a powder comprising the grain refinement element in its elemental form may be mechanically mixed with the titanium alloy powder. Alternatively a powder comprising the grain refinement element in its elemental form may be coated onto the titanium alloy powder using shear blending.

The powder may have an average particle size, e.g. average diameter, of less than 1 pm or at least 1 pm, e.g. at least 5 pm or at least 10 pm, preferably at least 20 pm. The powder may have an average particle size, e.g. average diameter, of up to 100 pm, preferably up to 80 pm or up to 50 pm. For instance, the powder may have an average particle size, e.g. average diameter, of 45 pm.

The particles of the powder may have a size distribution with a median diameter of between 30 pm and 50 pm.

The particles of the powder may have a distribution for particle size versus volume fraction (as a percentage), wherein d50 is between 30pm-50 pm, dlO is between 10 pm-30 pm and/or d90 is between 40pm-60 pm. The distribution may be determined using a laser diffraction particle analyser.

In some embodiments, the additive manufacturing may comprise selective melting and/or selective sintering of the powder comprising a titanium alloy. An electron beam or a laser may be used to selectively melt and/or sinter the powder. Typically, the selective melting and/or sintering may be carried out under an inert environment. The inert environment under which the selective melting and/or sintering is carried out may be argon-based or nitrogen-based. Preferably, the inert environment may contain no more than 0.2 vol% oxygen. A laser or electron beam power of 2kW or less, lkW or less, 500 W or less, 400 W or less, 300 W or less, 200 W or less, 150 W or less, or 100 W or less, may be used. Preferably, the laser or electron beam power may be 50 W or more. Typically, the laser or electron beam power may be 50 W, 100 W, 200 W, 300 W, 400 W, 500 W, lkW, or 2kW.

Preferably, the laser or electron beam may have a beam spot diameter of 100 pm or less. For instance, the beam spot diameter may be 80 pm or less, or 50 pm or less. The beam spot diameter may be 5 pm or more, e.g. 10 pm or more.

The laser or electron beam may follow one or more of: a meander pattern, a chequerboard pattern or a stripe pattern. A laser or electron beam scanning speed of no more than 400 mm/s, preferably no more than 200 mm/s, may be used. Preferably, the laser or electron beam scanning speed may be 100 mm/s or more.

Optionally, a layer thickness of up to 0.5 mm and/or 30 pm or more may be used. Typically for SLM, a layer thickness of up to 100 pm may be used. The layer thickness may be for example 10 pm or more. For instance, the layer thickness may be 10 pm or more or 20 pm or more. The layer thickness may be 50 pm. Multiple layers may be processed at once, for example two layers, three layers, four layers, or five layers.

Some embodiments may further comprise the step of providing a prefabricated part, e.g. a metallic part typically comprising a titanium alloy, and selectively melting and/or sintering the powder on at least a portion of a surface of the prefabricated part. The prefabricated part may constitute a part of the manufactured article. The prefabricated part may have been manufactured by any suitable process, e.g. by casting, extrusion, cutting and/or machining or by an additive manufacturing process. The prefabricated part may have the same composition as the powder or the prefabricated part may have a different composition from the powder.

According to a second aspect there is provided an article manufactured according to the method of any embodiment of the first aspect.

According to a third aspect there is provided powder for use in a method of manufacture of an article comprising additive manufacturing using the powder, the powder comprising, or consisting essentially of, a titanium alloy and at least one grain refinement element.

The powder may comprise up to 1 wt% of the grain refinement element(s). The powder may comprise up to or at least 0.5 wt% of the grain refinement element(s). The powder may comprise up to or at least 0.1 wt% of the grain refinement element(s). The powder may comprise at least 0.01 wt% of the grain refinement element(s).

For example, the powder may comprise up to 0.5 wt% of boron. The powder may comprise up to or at least 0.1 wt% of boron. The powder may comprise at least 0.01 wt% of boron. For instance, the powder may comprise approximately 0.02 wt%, 0.05 wt% or 0.1 wt% boron.

Alternatively or additionally, the powder may comprise up to 0.5 wt% of yttrium. The powder may comprise up to or at least 0.1 wt% of yttrium. The powder may comprise at least 0.01 wt% of yttrium. For instance, the powder may comprise approximately 0.02 wt%, 0.05 wt%, or 0.1 wt% yttrium.

For instance, elemental boron and/or elemental yttrium may be added to the titanium alloy. Typically, the grain refinement element(s) may include elemental boron and/or yttrium.

In an embodiment, the powder may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% boron: with the balance made up of titanium and unavoidable impurities.

In an embodiment, the powder may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% yttrium; with the balance made up of titanium and unavoidable impurities.

In an embodiment, the powder may comprise or consist essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; from 0.01 wt% to 0.5 wt% yttrium; and from 0.01 wt% to 0.5 wt% boron: with the balance made up of titanium and unavoidable impurities.

The powder may be produced for example by atomisation. Advantageously, atomisation typically may produce substantially spherical particles.

In some embodiments, the grain refinement element(s) may be added to the titanium alloy whilst producing the titanium alloy. For example, elemental boron and/or yttrium may be atomised with the metals used to produce the titanium alloy.

Detailed Description

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure 1 shows cracks in a typical article produced by additive manufacturing; figure 2 shows cracks in an alternative typical article produced by additive manufacturing; and figure 3 shows an SEM image of an article produced by additive manufacturing.

Figures 1 and 2 show typical articles produced by additive manufacturing. Additive manufacturing can be used to produce complex articles, for example articles comprising both thick and thin sections, and also articles with complex features such as holes. Some features may have sharp corners (with angles of approximately 90 degrees or less). Sharp corners can induce stress in the article, which may lead to fracturing of the article. In particular, sharp corners may result in part failure during the laser melting stage of SLM manufacturing.

Figures 1 and 2 respectively show articles 100, 200 comprising sharp-angled features. Both articles 100, 200 were produced by SLM of a Ti6A14V powder.

Figure 1 shows an article 100 with circular 101, triangular 102, and square 103 shaped holes running through the article. Without wishing to be bound by theory, the sharp corners of the triangular 102 and square 103 shaped holes induce stress, which led to the cracks 104 in the article 100.

Similarly, figure 2 shows an article 200 with sharp corners 201, likely leading to the creation of cracks 202.

The fractures and cracks in articles produced by additive manufacturing such as articles 100 and 200 occur despite the high strength of titanium alloys such as Ti4A14V. Cracked or fractured articles most likely cannot be used for their intended purpose, leading to waste of materials, time, and expense. It is therefore desirable to reduce the occurrence of fractures and cracks in articles produced by additive manufacturing.

In order to understand how fractures might occur, a scanning electron microscope (SEM) was used to analyse the microstructure of a Ti6A14V article manufactured by additive manufacturing. A resulting SEM image is shown in figure 3, showing the microstructure along the build direction, i.e. the direction in which the article is built up by the layer-on-layer process of additive manufacturing.

Grains of Ti6A14V are visible in figure 3. It is apparent that there is a mismatch in the typical dimensions of the grains. The grains are longer in the build direction by a factor of 5 to 10 compared with their width in the direction perpendicular to the build direction. The dimensions of some typical grains are shown on the SEM image. For example, a grain may have a length of around 400 pm in the build direction, but a width of only around 14 pm.

The anisotropy of the dimensions of the grains may cause anisotropic mechanical properties between the build direction and the direction perpendicular to the build direction. Typically, articles may be weaker in the direction perpendicular to the build direction - corresponding to the direction of the cracks 104, 202 in figures 1 and 2.

Therefore, making the grain size more isotropic, by substantially equalising the length and width of typical grains of titanium alloy in an article, may limit the occurrence of fractures in the article. A proposed solution to increasing the isotropy of the grain size dimensions is to add at least one grain refinement element to the powder of titanium alloy, e.g. Ti6A14V. The grain refinement element may, for example, be elemental boron or elemental yttrium, although other elements may be added to achieve the effect. The or each grain refinement element may be an element which increases the isotropy of the typical grain dimensions in the article manufactured by additive manufacture. For example, the or each grain refinement element may be an element which decreases the typical grain length in the build direction, or which increases the typical grain width in a direction perpendicular to the build direction. In some examples, the grain refinement element may comprise a mixture of both elemental boron and elemental yttrium.

The grain refinement element(s) may be added to the titanium alloy during preparation of the titanium alloy powder, for example during atomisation of the titanium alloy. Alternatively, a powder of the or each grain refinement element may be added to a powder of the pre-alloyed titanium alloy using a mechanical blending technique. For example the grain refinement element(s) may be coated onto the titanium alloy powder using shear blending.

Typically, only small quantities of grain refinement element(s) may be required. For example, the amount of grain refinement element in the mixed powder may be less than or equal to 0.5% by weight of the mixed powder, for instance 0.1 wt%, 0.05 wt%, or 0.02 wt%.

The titanium alloy to which the grain refinement element(s) is/are added may typically be a Ti6A14V titanium alloy, but other titanium alloys may be used, for example alloys containing a mixture of titanium in the alpha phase and in the beta phase (alpha-beta alloys), alloys containing a majority of titanium in the beta phase (nearbeta alloys), or alloys containing titanium in the beta phase but not in the alpha phase (beta alloys), or alloys containing a majority of titanium in the alpha phase (nearalpha alloys), or alloys containing titanium in the alpha phase but not in the beta phase (alpha alloys). Boron may particularly be used as the grain refinement element, or one of the grain refinement elements, in alloys containing some titanium in the beta-phase. Yttrium may particularly be used as the grain refinement element, or one of the grain refinement elements, in alloys containing some titanium in the beta-phase.

For example the titanium alloy may be commercially pure titanium (CP-Titanium), a beta-titanium alloy such as Ti5553 (Ti-5Al-5V-5Mo-3Cr), or a bio-compatible titanium alloy such as Ti-6Al-7Nb.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims.

Claims

1. A method of manufacture of an article comprising additive manufacturing of a material comprising, or consisting essentially of, a titanium alloy and at least one grain refinement element.

2. The method of claim 1, wherein the at least one grain refinement element includes boron.

3. The method of claim 2, wherein the material comprises 0.5 wt% or less of boron.

4. The method of claim 3, wherein the material comprises from 0.01 wt% to 0.1 wt% boron.

5. The method of any one of claims 1 to 4, wherein the at least one grain refinement element includes yttrium.

6. The method of claim 5, wherein the material comprises 0.5 wt% or less yttrium.

7. The method of claim 6, wherein the material comprises from 0.01 wt% to 0.1 wt% yttrium.

8. The method of claim 7, wherein the material comprises 0.02 wt%, 0.05 wt%, or 0.1 wt% yttrium.

9. The method of any preceding claim, wherein the titanium alloy is an alpha-beta, near-beta, or beta alloy of titanium.

10. The method of any preceding claim, wherein the titanium alloy contains from 5 wt% to 7 wt% aluminium.

11. The method of any preceding claim, wherein the titanium alloy contains from 3 wt% to 5 wt% vanadium.

12. The method of any preceding claim, wherein the titanium alloy contains no more than 0.5 wt% iron.

13. The method of any preceding claim, wherein the titanium alloy comprises, or consists essentially of, a Ti6A14V alloy, commercially pure titanium, a Ti5553 alloy or a biocompatible alloy such as Ti-6Al-7Nb.

14. The method of claim 1, wherein the material comprises or consists essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% boron: with the balance made up of titanium and unavoidable impurities.

15. The method of claim 1, wherein the material comprises or consists essentially of: from 5 wt% to 7wt% aluminium; from 3 wt% to 5 wt% vanadium; no more than 0.5 wt% iron; no more than 0.2 wt% oxygen; and from 0.01 wt% to 0.5 wt% yttrium; with the balance made up of titanium and unavoidable impurities.