WO2013074028A1

WO2013074028A1 - A gradient weld stud and method of preparation

Info

Publication number: WO2013074028A1
Application number: PCT/SE2012/051250
Authority: WO
Inventors: Erik Karlsson; Mohamed Radwan; Anders BREMMER; Katarina Flodström
Original assignee: Diamorph Ab
Priority date: 2011-11-18
Filing date: 2012-11-14
Publication date: 2013-05-23
Also published as: SE536766C2; EP2780127A1; EP2780127A4; SE1151094A1

Abstract

A weld stud adapted to be welded against a substrate. The weld stud (1;10;21;31;41) comprising: a first (I), second (II) and third (III) portion. The first portion comprises a first material M1, and the second portion comprises a second material M2, weldable to the substrate (2). The third portion comprises at least one of: a material being weldable to the substrate, and a flux material adapted to facilitate the welding of the second material M2 to the substrate, wherein the weld stud comprises a length axis (LA) running through said first, second and third portion, and wherein the third portion comprises a cross section (B-B) perpendicular to the length axis having a smaller area (a2) than the average cross-section area (a1) of the first and second portions perpendicular to the length axis. The first material M1 and second materials M2 are joined with a gradual transition (5), the first material M1 is not weldable to the substrate, and the second material M2 is weldable to the substrate, and the first and second portions comprises a sintered gradual transition region comprising a mix of the first M1 and the second M2 materials.

Description

A GRADIENT WELD STUD AND METHOD OF PREPARATION Technical field

[0001 ] The present invention relates generally to a weldable component with a gradient structure going from one material which is weldable to a substrate, to a second material which is not weldable to the same substrate, and a method of preparation of such as a shape by sintering, preferably by spark plasma sintering (SPS).

Background art

[0002] The use of ceramic and cermet materials as wear protection and wear parts is attractive due to their hardness and thus their resistant to wear. A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. Tungsten carbide (WC), often referred to as cemented tungsten carbide or hard metal, for example, is extensively used in the mining and construction industry as heavy wear components. Some consider the material to be a ceramic and some a cermet material. It is the addition of a metallic binder, generally cobalt, nickel, iron, or their equivalent that generally makes the carbide a cermet and differentiates it from truly brittle materials, that is, the ceramic family of materials. The term

"cemented" is also used to show that the carbide alloy powder includes an amount of metallic binder. During the sintering process, the tungsten carbide particles are captured in the metallic binder and cemented together by forming a metallurgical bond.

[0003] Ceramic materials and cermets are not weldable by nature, hence the use of other joining methods such as brazing. Brazing is a relatively slow manufacturing process compared to welding and has limitations in where it can be used. These limitations have in turn limited the scope of using these materials for wear protection and wear parts.

[0004] There is a clear drawback in that the most desirable materials for wear protection cannot be produced or fastened with the optimal technology for their intended use, in other words they cannot be welded. [0005] Many metals and thermoplastics can be welded, but some are easier to weld than others.

[0006] Arc welding is a fusion welding process that uses electricity to generate the heat needed to melt the base metals. Stud welding is an industrially mature technology, which is a kind of arc welding. By leading current through a bolt, nut or other specially formed parts, the component is welded onto another metal part by the heat generated through electrical resistance. There are three different types of stud welding techniques; Drawn Arc (DA), Short Cycle (SC) and Capacitor

Discharge (CD).

[0007] DA studs are typically loaded with an aluminium flux ball on the weld end, which aids in the welding process. The DA studs are used along with a ceramic ring or inert gas to protect the joint from oxidation during the melting phase of the welding process. SC and CD studs differ from DA studs in that the studs do not require flux nor the oxidation protection in form of a ceramic ring or inert gas during welding, due to shorter welding times than for DA.

[0008] Stud welding is very versatile. Portable stud welding machines are available. Welders can also be automated, with controls for arcing and applying pressure. Typical applications include automobile bodies, electrical panels, shipbuilding and building construction.

[0009] There is a wide range of different weld studs made from steel available that are used as weld-on wear protection. To achieve a high wear resistance of the steel, the weld studs are often tempered. However, due to the heat during the welding process, the properties of the tempered steel might be compromised.

Even the best tempered steel is also much softer than ceramic materials, and therefore has a shorter lifespan in wear applications.

[0010] Previous inventions for joining of different materials in a weld stud are generally based on mechanical anchoring or surface coatings. US patent

5,054,980 describes a composite weldable stud consisting of two different metals joined mechanically. [001 1 ] US patent 6,860,687 describes an aluminum stud with a surface coating of titanium.

[001 2] The German company BETEK produces a weldable stud comprising tungsten carbide, where a solid cemented tungsten carbide core is shrink-fitted to a steel body which in turn fits a stud welding machine. The downside with this product is the more complicated manufacturing process, where the cemented tungsten carbide needs to be sintered, the steel body needs machining and finally the two components are joined by shrink-fitting. Also, if the ceramic core falls out, the wear protection is severely reduced.

Summary of invention

[0013] An object is to provide a weld stud of a functionally graded material (FGM) component prepared by sintering, preferably by spark plasma sintering (SPS), where a weldable material base is combined with an outer surface of a different, non-weldable material. The component is intended for welding to a substrate.

[0014] Another object is to provide a method of preparation of said weld stud.

[0015] A weld stud adapted to be welded against a substrate is provided. The weld stud comprises a first, second and third portion. The first portion comprises a first material M1 , the second portion comprises a second material M2, weldable to the substrate, and the third portion comprises at least one of: a material being weldable to the substrate, and a flux material adapted to facilitate the welding of the second material M2 to the substrate. The weld stud further comprises a length axis running through said first, second and third portion. The third portion comprises a cross section perpendicular to the length axis having a smaller area than the average cross-section area of the first and second portions perpendicular to the length axis. The first material M1 and second material M2 are bonded with a gradual transition, wherein the first material M1 is not weldable to the substrate, and the second material M2 is weldable to the substrate. Furthermore, the first and second portions comprise a sintered transition region comprising a mix of the first M1 and the second M2 materials. [0016] By providing the FGM weld stud, a non-weldable material can be welded onto a substrate which makes it possible to apply materials with superior properties to substrates by means of welding.

[0017] According to one embodiment, the weld stud comprises a rounded (4) or tapered (4a; 4b) portion adapted to initiate contact with the substrate which is crucial for a homogeneous melt of the materials and the best possible result when welding. The rounded or tapered portion may comprise a flux for facilitating and/or improving the weld between the substrate and the weld stud.

[0018] The non-weldable surface can preferable be of a ceramic material or a cermet.

[0019] The non-weldable surface preferably has a good wear resistance. Other desirable properties of this surface can be low weight, high corrosion resistance and an insulating nature.

[0020] The non-weldable surface can have different shapes, such as flat, tapered and spherical.

[0021 ] One suitable wear resistant material of this invention is tungsten carbide (WC), often referred to as cemented tungsten carbide or hard metal.

[0022] Other suitable materials are ceramic oxides, nitrides, borides or other carbides.

[0023] Especially aluminium oxide (AI2O3) and zirconium oxide (ZrO₂) are suitable for this invention.

[0024] The non-weldable material can also be a metal or metal alloy which is not weldable to the substrate.

[0025] The weldable material is preferably a metal or metal alloy, but can also be plastic. Examples of weldable metals, including their alloys are aluminium, nickel, gold, platinum, titanium, tantalum and zirconium. Further, steel and some stainless steel alloys (300 and 400 series) are weldable. [0026] The weldable material is preferably a steel alloy or a stainless steel alloy.

[0027] According to one embodiment, the weld stud is a FGM weld stud made of steel/cemented tungsten carbide (steel / WC-Co FGM).

[0028] According to another embodiment, the weld stud is a FGM weld stud of stainless steel /cemented tungsten carbide (SS / WC-Co FGM).

[0029] According to another embodiment, the weld stud is a FGM weld stud of stainless steel or steel /aluminium oxide (SS / AI2O3 FGM).

[0030] According to another embodiment, the weld stud is a FGM weld stud of stainless steel or steel /zirconium oxide (SS / Zr02 FGM).

[0031 ] In another embodiment, the non-weldable material M1 does not comprise 100% of one material, but also some portion of the weldable material.

[0032] By forming the FGMs into weld studs for stud welding, cermets and ceramic materials can easily be welded in a quick manner using a commercially available technology, which has previously not been possible.

[0033] The shape of the weld stud can be achieved directly through the sintering, or the stud can be machined to proper geometry after the sintering.

[0034] A cylindrical shape with circular cross section is very common for weld studs, and is one shape possible for the present invention.

[0035] The shape of the weld stud is however not limited to the circular cross- section, but the cross-section can also be square, rectangular, hexagonal, octagonal or of other similar shapes.

[0036] A tip or a slightly conical structure of the welding surface is in many cases suitable for the weld studs. This shape can be machined after the sintering or the component can be sintered into this shape. [0037] A hole in the tip of the conical part can be formed for receiving a piece or slug of welding flux which may be aluminium or some other similar material used as flux in welding.

[0038] The wear resistant material of a gradient weld studs is present throughout the wear surface and the gradient region, because of the gradual compositional change between the two materials. This further improves the wear properties and the lifespan of the components, compared to components with a wear resistant surface coating.

[0039] A ceramic material's properties such as hardness are not affected by the heat during a welding process, and the properties will not change after the welding.

[0040] The invention also relates to a method for producing the weld stud. More specifically the invention relates to a method for producing a ceramic or cermet/metal FGM, shaped as a weld stud. The method comprises the following steps:

[0041 ] 1 ) Forming a FGM powder structure

[0042] 2) Sintering of the prepared FGM-structure with the spark plasma sintering technique (SPS).

[0043] 3) Performing necessary finishing of the FGM component by methods such as blasting, cutting, turning, grinding, milling and possible addition of a flux.

Brief description of drawings

[0044] Some possible embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

[0045] Fig. 1 shows a weld stud according to one embodiment having a functionally graded material structure, where portion I comprises a wear resistant part, portion II comprises a weldable part and portion 5 comprises a gradual transition between the two materials with four intermediate layers i-iv,

[0046] Fig. 2 shows a weld stud 1 in an embodiment similar to that of fig. 1 , the difference being that there is a third portion III, consisting of two tapered parts 4ab,

[0047] Fig. 3a, 3b shows a weld stud in an embodiment similar to that of fig. 1 and fig.2, the difference being that the third portion III has a spherical and frustoconical shape, respectively,

[0048] Fig. 4 shows a weld stud according to an embodiment in which the weld stud comprises a gradient portion 5 being an intermediate region comprising a mix of non-weldable and weldable material,

[0049] Fig. 5 is a perspective view of a weld stud according to any of the embodiments of figs. 1 - 4 when welded onto a substrate 2,

[0050] Fig. 6 shows the weld stud in form of a nut having a plurality of weld flux portions,

[0051 ] Fig. 7 shows a scoop for excavators comprising weld studs according to any of the embodiments herein,

[0052] Fig. 8 shows a drill bit for a rock drill comprising weld studs according to any of the embodiments herein,

[0053] Fig. 9 shows a feed roller for example for a forest harvester head comprising weld studs according to any of the embodiments herein.

Detailed description

[0054] A functionally graded material (FGM) is a material design concept which provides a joining solution to incorporate incompatible properties of two dissimilar materials, such as the heat, the wear, and the oxidation resistance of a ceramic or a cermet, such as for example cemented tungsten carbide, with the high

toughness, the high strength, weldability and machinability of a metal, such as steel, by placing graded composite interlayers of the two materials between the pure end layers. A functionally graded material is thus a material bonded with a gradual transition from at least a first to at least a second material.

[0055] FGMs can be prepared through different techniques such as

conventional powder metallurgy processing, vapour deposition and sintering techniques. The spark plasma sintering method (SPS), also referred to as for example field assisted sintering technique (FAST), is a powerful sintering technique which allows very rapid heating under high mechanical pressures. This process, hereafter referred to as SPS, has proved to be very well suited for the production of functionally graded materials. Other sintering techniques could possibly also be used for preparing FGMs, such as for example direct hot- pressing, hot-pressing or hot isostatic pressing.

[0056] A flux is to be understood as a chemical cleaning agent, flowing agent, or purifying agent used in welding for preventing oxides from forming on the surface of the molten metal and/or absorbing impurities. The flux could for example comprise a piece of aluminium, however it is also conceivable that the flux is a flux comprising ammonium chloride, hydrochloric acid, zinc chloride or borax.

[0057] Embodiments will now be described in more detail under reference to the accompanying drawings. All examples herein should be seen as part of the general description and therefore possible to combine in any way in general terms. Again, individual features of the various embodiments and methods may be combined or exchanged unless such combination or exchange is clearly

contradictory to the overall function of the functionally graded material shape or its method of production.

[0058] Fig. 1 shows an example of a metal / ceramic FGM with a graded portion 5 consisting of several composite layers, there is a gradual variation of the microstructure with the compositional change. The matrix is replaced gradually from metal to ceramic, and the microstructure profile varies concurrently from II a pure metal , iii-iv a metal-rich region (the ceramic particles are dispersed in metal matrices), i-ii a ceramic-rich region (the metal matrix diminishes and turns into discrete phases or particles in ceramic matrices), to finally (portion I) a pure ceramic. This gradient in the composition-microstructure-properties along the FGM is the key for its stability and performance.

[0059] Fig. 2 shows a weld stud 1 according to one embodiment. The weld stud 1 is adapted to be welded against a substrate (shown as reference numeral 2 in fig. 5) by means of electric resistance welding. In electric resistance welding heat is generated by the electrical resistance of the material to be welded which makes a portion of the material of the weld stud 1 and a portion of the substrate melt and thus forming the weld between the weld stud 1 and the substrate. The first portion I of the weld stud 1 comprises a first material M1 which is a material which is not possible to weld against the substrate. The material M1 could for example be a ceramic, cermet or polymer material. The material M1 could be a wear resistant material adapted to increase the wear resistance of weld stud or a chemically resistant material adapted to increase the chemical resistance of weld stud 1 . The weld stud 1 further comprises a second portion II comprising a second material M2 being a material weldable against the substrate. The second material M2 could for example comprise steel or stainless steel. The first portion I is joined to the second portion II by means of the weld stud having a functionally graded region

comprising a mix of the materials M1 and M2, such that the materials of portion I and portion II are materially bonded. The weld stud further comprises a third portion III comprising two tapered portions 4a, 4b such that the third portion comprises a cross section perpendicular to the length axis of the weld stud 1 having a smaller area than the average cross-section area of the first and second portions perpendicular to the same length axis. The tapered portions 4a, 4b are needed to initiate the welding process and could furthermore comprise a flux for facilitating the welding of the second material M2 against the substrate.

[0060] Fig. 3a shows a weld stud 1 according to an embodiment similar to the embodiment of fig.1 , with the difference that the third portion III comprises a rounded portion 4 which, just as the tapered portions of fig. 1 , is needed to initiate the welding, and could also comprise a flux for facilitating the welding of the second material M2 against the substrate. [0061 ] Fig. 3b shows a weld stud 1 according to an embodiment similar to the embodiment of fig.1 , with the difference that the third portion III comprises a frustoconical portion 4 which is used to initiate the welding, and could also comprise a flux for facilitating the welding of the second material M2 against the substrate.

[0062] Fig. 4 shows an embodiment of the weld stud 1 very similar to the embodiment shown in fig. 2, the difference being that the weld stud of fig. 4 has a transition region 5 which is a substantial part of the weld stud 1 . The transition region 5 comprising a mix of the materials of the first I and second II portions and creates the material joint between the first and second portion. The weld stud has a length axis LA running through said first I, second II and third III portion. The third portion III comprises a cross section B - B perpendicular to the length axis LA having a smaller area a² than the average cross-section area a¹ of the first and second portions perpendicular to the length axis LA, in figure 4 shown with the cross-section A - A, as the weld stud of fig. 4 is cylindrical and thus has an equal cross-section over the entire length of the weld stud 1 . The sintered graded region comprises a mix of the first M1 and the second M2 materials which could be a mix creating a gradual variation in composition, smoothly or stepwisely, throughout the transition region 5. The material M1 could in one example be tungsten carbide and the material M2 could be steel and the gradient change throughout the third portion could be 20vol% (i.e. 80/20, 60/40, 40/60, 20/80 vol%). The first I and second II portion may comprise the first M1 and second M2 material in its pure form, respectively, or it may comprise a mix of the materials M1 and M2, with the percentage of M2 being higher in the second portion II than in the first portion, and the percentage of M1 being higher in the first portion I than in the second portion II.

[0063] Fig. 5 shows the weld stud 1 according to any of the embodiments herein when welded to a substrate 2 such that a weld 3 is formed between the substrate 2 and the weld stud 1 , fixating the weld stud 1 to the substrate 2.

[0064] Fig. 6 shows an embodiment of the weld stud 10 in which the weld stud 10 has the shape of a nut comprising internal threads 15 enabling the fixation of an object having external threads to the nut 10. The weld nut 10 comprises a first portion I comprising a first material M1 , alone or in combination with at least a second material M2, the material of the first portion I is not weldable to the substrate but chosen for the reason of a particular material property, which may be a mechanical property, such as good wear resistance, or a chemical property, such as good chemical resistance. The weld nut further comprises a second portion II comprising a material weldable against the substrate. The weld nut 10 further comprises a third portion III comprising a plurality of rounded portions needed to initiate the welding process. The rounded portions 14 could be tapered or otherwise shaped such that they have a cross section area being smaller than the average cross-section area of the weld nut for initiating the welding process. The plurality of rounded portions 14 may comprise a flux for facilitating and/or improving the weld between the weld nut 10 and the substrate 2.

[0065] The weld nut is to be seen as an example showing that there is no limitation to the shape in which the weld stud may be produced as long as the basic principle of a first non-weldable material integrated with a weldable material applies.

[0066] Fig. 7 shows a scoop 20 for an excavator comprising a substrate 22 adapted to form the scoop 20. The substrate 22 is for example steel or a steel based alloy such as stainless steel. The scoop 20 furthermore comprises weld studs 21 , according to any of the embodiments herein, welded to the substrate 22 for improving the wear resistance of the scoop 20. In fig. 7 the weld studs 21 are shown welded to the side of the scoop 20, however it is equally conceivable that the weld studs 21 are welded to the front 23 of the scoop, the inside 24 of the scoop 20, or the teeth 25 of the scoop 20. The scoop 20 shown in fig. 7 is to be seen as an example of an application area of the weld studs 21 according to any of the embodiments disclosed under reference to figs. 2 - 6.

[0067] Fig. 8 shows a drill bit 30 for a rock drill. The drill bit 30 comprises a substrate 32 adapted to be rotated for exerting a drilling force on a target material, such as a rock wall or a sediment layer. The drill bit 30 furthermore comprises weld studs 31 a, 31 b, according to any of the embodiments herein, welded to the top surface of the substrate 32 (the weld stud 31 a) and along the periphery or lateral surface of the substrate 32 (the weld stud 31 b). As the weld studs 31 a, 31 b for example comprises a ceramic or cermet material they substantially improve the wear resistance of the drill bit 30 at the same time as they are easy to apply or replace as they can be welded directly on to the substrate 32 by means of resistance welding. The drill bit 30 shown in fig. 8 is to be seen as an example of an application area of the weld studs 31 a, 31 b according to any of the

embodiments disclosed under reference to figs. 1 - 5.

[0068] Fig. 9 shows a roller 40 for feeding, for example for use in a forest harvester head. The roller 40 comprises weld studs 41 according to any of the embodiments disclosed herein. According to the embodiment disclosed in fig. 9 the roller 40 comprises a substrate 42 having a substantially circular periphery adapted to rotate for feeding for example logs when harvesting. The weld studs 41 are welded to the circular periphery of the substrate 42. The roller 40 shown in fig. 9 is to be seen as an example of an application area of the weld studs 41 according to any of the embodiments disclosed under reference to figs. 2 - 6.

[0069] The invention also relates to a method for producing weld studs according to any one of the embodiments herein. More specifically the invention relates to a method for producing a ceramic or cermet / metal FGM, shaped as a weld stud. The method comprises the following steps:

[0070] 1 ) Forming a FGM powder structure, wherein the first material surface comprises up to 100% of the first material M1 , the weldable second surface comprises up to 100% of the second material M2, and the intermediate graded region has several or at least one composite interlayers together creating an intermediate graded composite region, essentially consisting of an intermix of the first M1 , second M2 and possible a third material M3, by loading mixtures of all layers in order, layer by layer, into a sintering tool called die, preferably consisting of graphite and of a desirable shape such as cylindrical or rectangular.

[0071 ] 2) Sintering of the prepared FGM-structure with the spark plasma sintering technique (SPS). [0072] 3) Performing necessary finishing of the FGM component by methods such as blasting, cutting, turning, grinding, milling and possible addition of a flux.

[0073] The starting materials (M1 , M2) may be delivered continuously into a sintering die in which the material is sintered, creating at least one interlayer with gradual variation in composition, smoothly or stepwisely, throughout the FGM shape consisting of different mixtures of the materials. As is well known in the art, sintering additives may further be added to the first and/or the second material M1 , M2 in order to improve its properties. The gradient region may further comprise at least one more material, with an expansion coefficient intermediate to the two outer materials.

[0074] The ingredients of each composite interlayer may be automatically or manually weighed and mixed, by dry mixing or wet mixing, until homogeneity, and if necessary dried and sieved. According to one example, the numbers of graded layers are between two and twenty. However, other numbers of layers are of course also possible. The change in composition profile along the layers can be linear as well as non-linear

[0075] In order to decrease the temperature-rise locally in one of the materials, generally in the metal, M2, an electrically insulating layer of an electrically insulating powder or coating can be inserted in the FGM structure.

[0076] The whole die is according to this example pre-pressed by cold uniaxial pressing prior to sintering.

[0077] In one embodiment a weldable metal substrate is used as a base in the FGM structure prior to sintering, and the powder layers are joined with the weldable substrate during the sintering. In such a case the production costs can be reduced as the amount of powder is reduced.

[0078] Cutting can preferably be performed with techniques such as laser cutting, water jet cutting, cutting wheel, plasma cutting or wire EDM. [0079] In one embodiment the outer surfaces of material M1 and M2 are flat and parallel, shaped during the sintering by the so called pressing punches, which form the sintering tool together with the die, these punches having flat surfaces.

[0080] In another embodiment at least one of the pressing punches has a non flat surface, giving at least one of the FGM component surfaces a non-flat nature, to reduce the amount of finishing needed.

[0081 ] In one embodiment the FGM components are sintered one at a time, in a single sintering tool.

[0082] In another embodiment several FGM components are sintered

simultaneously in a multi-component sintering tool.

[0083] In another embodiment one large FGM component is sintered at a time, which is subsequently cut into smaller components.

[0084] In one embodiment the weldable end of the gradient weld stud and the metal substrate to which it is to be attached are being brought together in a substantially parallel relationship, and thereafter welded.

Examples

[0085] The present invention is further illustrated by the following experimental results, which should not limit the claims in any way. For example, other metals and ceramics can be used. Other sintering techniques than SPS can also be used.

[0086] A steel / WC-Co FGM was designed to comprise four composite interlayers between the pure steel and tungsten carbide layers at the two ends. The composites consisted of steel - cemented carbide mixtures with a 20vol% gradient change (i.e. 80/20, 60/40, 40/60, 20/80 vol%). A die for production of 6 cylindrical components was used. The total six layers for each component were loaded in order, layer by layer, in a graphite die and a BN insulating layer was interposed between the punch and the steel layer. The WC had a Co content of 1 1 % and the grain size was approximately 2 pm. The steel had a D50 size of 10 pm. The die was sintered in a SPS unit at 1 100 °C during 6 minutes and at a pressure of 30 MPa. The heating rate was 50 °C/min. The sintering took place in vacuum. The dimension of the sintered FGMs was 012x22 mm. The steel surface of the FGM was turned into a slightly conical shape, a small hole was made in the steel and aluminium was added as flux. The components were welded onto steel substrates through drawn arc stud welding.

[0087] A FGM component of stainless steel and yttria-stabilized zirconia was sintered with the SPS technology in a single component graphite tool. The dimensions of the sintered component were 020x17 mm. The component was sintered at 1 100 °C during 22 minutes and at a pressure of 75 MPa. The heating rate was 50 °C/min. The sintering took place in vacuum. The stainless steel part of the component was turned into a slightly conical shape and aluminium was added as flux.

[0088] A FGM of stainless steel and alumina was prepared, with zirconia as an additive in the intermediate layer. 21 different powder mixtures were prepared from the materials stainless steel SUS316L (D90 < 22 μητι), Al₂0₃ (100 nm) and/or Zr0₂(3Y) (D50=0.6 pm). The sample was densified with SPS at 1 100 °C for 30 minutes. The sintering took place in vacuum. The SPS pressure was kept at 75 MPa. A heating rate of 100 °C/ min was applied. The FGM was produced as a cylinder with a diameter of 20 mm and a height of 22 mm.

[0089] It will be appreciated that the figures described are for illustration only and are not in any way restricting the scope of the invention. Please note that any embodiment or part of embodiment as well as any method or part of method could be combined in any way. All examples herein should be seen as part of the general description and therefore possible to combine in any way in general terms.

Claims

1 . A weld stud adapted to be welded against a substrate, the weld stud

(1 ;10;21 ;31 ;41 ) comprising: a. a first (I), second (II) and third (III) portion, and wherein b. the first portion comprises a first material M1 , and c. the second portion comprises a second material M2, weldable to the substrate (2), and d. the third portion comprises at least one of: a material being weldable to the substrate, and a flux material adapted to facilitate the welding of the second material M2 to the substrate, wherein e. the weld stud comprises a length axis (LA) running through said first, second and third portion, and wherein the third portion comprises a cross section (B-B) perpendicular to the length axis having a smaller area (a₂) than the average cross-section area (a-i ) of the first and second portions perpendicular to the length axis, characterized in that the first material M1 and second materials M2 are joined with a gradual transition (5) , and wherein the first material M1 is not weldable to the substrate, and the second material M2 is weldable to the substrate, and wherein the first and second portions comprises a sintered gradual transition region comprising a mix of the first M1 and the second M2 materials.

2. The weld stud according to claim 1 , wherein the third portion comprises a rounded (4) or tapered (4a;4b) portion adapted to initiate contact with the substrate when welding.

3. The weld stud according to any of claims 1 and 2, wherein the material M1 is a material selected from: a ceramic material, a cermet material, a metal not weldable to the substrate, and a polymer material.

4. The weld stud according to claim 3, wherein the material M1 is a material selected from: an oxide, a nitride, a carbide or a boride.

5. The weld stud according to any of claims 3 and 4, wherein the material M1 further contains sintering additives.

6. The weld stud according to any of the above claims, wherein the material M1 further contains some of the weldable material M2.

7. The weld stud according to any of the above claims, wherein the weldable material M2 is a metal or metal alloy.

8. The weld stud according to claim 7, wherein the weldable material M2 is a material selected from: aluminium, nickel, gold, platinum, titanium, tantalum and zirconium or their alloys, steel and stainless steel.

9. The weld stud according to any of the above claims, wherein the weldable material M2 is a material selected from steel and stainless steel,

10. The weld stud according to any of the above claims, wherein the weldable material M1 is a material selected from steel and stainless steel, and the non-weldable material M2 comprises at least one of cemented tungsten carbide, aluminum oxide, and zirconium oxide.

1 1 . The weld stud according to any of the above claims, wherein the third portion comprises a flux material comprising aluminum.

12. Method of manufacturing the weld stud according to any of the above claims, the method comprising: a. preparing a powder compound comprising the first material M1 and second material M2 for sintering, and

b. sintering the powder compound for preparing a sintered material bonded with a gradual compositional transition region.

13. The method according to claim 12, wherein the step of sintering of the

powder compound is performed by means of spark plasma sintering (SPS).

14. The method according to any of claim 12 and 13, wherein the method

further comprises at least one of: blasting the sintered material, cutting the sintered material, turning the sintered material, grinding the sintered material, milling the sintered material and adding a flux to the sintered material.

15. The method of manufacturing the weld stud according to any of claims 12 - 14, wherein the method further comprises placing a metal substrate comprising the second material M2 in contact with the powder compound, and wherein the sintering of the powder compound joins the solid piece to the sintered material.

16. A roller for feeding, wherein the roller (40) comprises weld studs (41 )

according to any claims 1 - 1 1 .

17. The roller according to claim 16, wherein the roller comprises a substrate (42) having a substantially circular periphery for feeding, and wherein the weld studs are welded to the circular periphery of the substrate.

18. The roller according to any one of claims 16 and 17, wherein the roller is a feed roller for a harvester head.