EP1558416A1

EP1558416A1 - Tool steels and method of rapid tooling by spray forming

Info

Publication number: EP1558416A1
Application number: EP03750776A
Authority: EP
Inventors: Yunfeng; c/o VTT Industrial Systems YANG
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2002-10-16
Filing date: 2003-10-16
Publication date: 2005-08-03
Also published as: FI20021835A0; WO2004035250A1; CN1705535A; AU2003268999A1

Abstract

The invention relates to a series of tool steels for spray forming die inserts comprising iron, carbon, silicon, manganese, chrome, molybdenum and vanadium. According to the invention the vanadium content is over 1,4 w-%. Further more the invention relates to a method of rapid tooling by spray forming in which a melted tool steel is as above, and it is atomized and sprayed onto a ceramic mould under the atomizer. Further more the ceramic mould is rotated under the atomizer and the rotating axis of the mould is tilted from the spraying direction to a deposition angle.

Description

TOOL STEELS AND METHOD OF JRAPID TOOLING BY SPRAY FORMING

The invention relates to a series of tool steels to be used in spray forming of die inserts and a corresponding method of spray forming die inserts.

Many mass-produced parts ranging from cell phone parts to automotive components are made by using metal moulds or dies through such processes like pressure die-casting, die forging, and injection moulding. Conventionally, the dies and die inserts are made of a block of forged tool steel with specialised machining. Because of high requirements and complexity of geometry, manufacturing of the dies is very time consuming and costly. Spray forming is a rapid solidification process, in which metal melt is atomised by an inert gas into droplets of 10-200 microns in size, flying at subsonic speed onto a deposition substrate. The cooling rates are between 100 to 100,000 degrees per sec- ond. As a result, spray formed materials are free from macro-segregation and have much finer, more homogeneous and more equiaxed grain structures than the conventionally made ones. Spray formed steels have similar microstructure and mechanical property features as powder metallurgy processed steels that are considered to have the highest quality available, while the costs and lead times are remarkably reduced because of single-step operation of converting molten alloy directly into a semi-finished product. Osprey ™ process represents one of the major developments in spray forming technology. It has been currently used to make preforms such as billets, cladding rolls and pipes, and also developed to produce plates and ring type near-net shapes. It has been found out that standard hot work tool steels are extremely difficult to be spray formed without porosity. Process parameters such as metal/gas flow rates, spray height, and the substrate movement have a great influence on the as-sprayed soundness. The optimal processing window is very narrow. If the process parameters result in a little too cold spray, layered porosity is formed. If the temperature of the spray is a little too high, bigger pores caused by liquid shrinkage form.

Fig 1 shows the porosity problem in a spray formed die insert of standard AISI H13 steel. The spray forming parameters have been optimised, but some uncontrollable variation of the parameters has caused pores to form and a transformation from layered porosity to liquid shrinkage pores, sometimes even large cavities of hot defects to occur. The objective of the invention is to eliminate the drawbacks referred to above. One specific objective of the invention is to disclose a technically and economically feasible method to spray form high- quality die inserts for different production methods, especially for demanding processes with high thermal and mechanical stresses on the die such as pressure die-casting and hot die-forging. Further objective of the invention is to disclose a new series of hot work steels, which are better in processing properties than conventional ones to eliminate the porosity in the die inserts. Also one objective of the invention is to disclose a method that leads to fine and uniform mi- crostructure of die inserts. Still further objective of the invention is to improve the wear resistance of hot work steels by increasing the vanadium carbide amount without significantly impairing the toughness because of the fine and uniform spray formed micro- structures .

As for the features characteristic of the in- vention, reference is made to them in the claims. As shown in Fig. 1 the big pores have a smooth and round appearance. It was suspected that gas precipitation might be involved.

Because vanadium is a high temperature car- bide-forming element and VC is formed at a temperature close to the liquidus, it was surprisingly realized that vanadium carbide would reinforce the deposition that is normally in a semi-solid state, thus resisting gas pore formation. So a series of tool steels in ac- cordance with the invention for spray forming die inserts comprises iron (Fe) , carbon (C) , silicon (Si) , manganese (Mn) , chrome (Cr) , molybdenum (Mo) and vanadium (V) . According to the invention the vanadium content is over 1,4 w-%. Test results confirmed the new invention, as can be seen from the figure 2. This steel alloy comprises C 0,35%, Si 0,80%, Mn 0,49%, Cr 4,91%, Mo 1,48% and V 2,71%. This die insert spray formed of above mentioned steel with a higher vanadium content has no visible internal pores at all. Mirror surface is obtained and no micro pores are visible from the middle and tooling surface area adjacent to ceramic mould.

Advantageously the vanadium content is selected from the range 1,4 - 3,5 w-% for each case spe- cifically.

In a further embodiment of the invention the carbon content is at least 0,35 w-%.

Also advantageously the carbon content is selected from the range 0,35 - 0,9 w-% for each case specifically.

In a method of spray forming die inserts a melted tool steel is atomized and sprayed onto a ceramic mould under the atomizer. The steel comprises iron (Fe), carbon (C) , silicon (Si), manganese (Mn) , chrome (Cr) , molybdenum (Mo) and also vanadium (V) , the content of which is at least 1,4 w-%. According to the invention the disc shaped ceramic mould is rotated under the atomizer and the rotating axis of the mould is tilted from the spraying direction to a deposition angle. The deposition angle is between 1-50° depending on the shape chatacters of the mould. In this inventive method it is possible to use an Osprey ™ spray forming machine to make die inserts or tools of sophisticated shapes, the tooling surfaces of which are ready for direct mass-production use or subject to minor finishing by grinding and/or polishing if needed. However it is also possible to use other types of spray-forming machines based on melt atomisation.

Advantageously the atomized melted metal is sprayed onto the ceramic mould as two separate sprays apart from each other.

In a further embodiment said two sprays are situated symmetrically on the opposite sides of the rotating axis of the mould.

In figure 3 a disc shaped ceramic mould 1 is placed under an open tundish 2 with a melt nozzle 3 at the bottom and protruded into a 2-stage free-fall nitrogen atomizer 4, and a nitrogen-sealed spray chamber. In the spray chamber mainly a horizontal disc shaped deposition substrate, i.e. a ceramic mould 1 is rotating. The rotating axis of the mould is tilted from the vertical spraying direction to a deposition angle, which is selected for each case separately. This tilting of the mould is a solution to the columned porosity with vertical walls. However this solu- tion works properly only if the mould has vertical walls only in the outer edges or in the center so that the vertical walls do not face to both directions in the radius as happens in figure 3. This leads unavoidably to porosity 5. This porosity adjacent to the other vertical wall in shadow can be avoided by using a twin atomizer spray forming of figure 4. In figure 4 a disc shaped ceramic mould 1 is placed under an open tundish 2 with two melt nozzles 3 at the bottom and protruded into a 2-stage free-fall nitrogen atomizer 4, and a nitrogen- sealed spray chamber. The nozzles 3 are placed apart from each other so that the rotating axis of the mould is under and between the nozzles. In the spray chamber mainly a horizontal disc shaped deposition substrate, i.e. a ceramic mould 1 is rotating. The rotating axis of the mould is tilted from the vertical spraying di- rection to a deposition angle of about 30°. As becomes clear from figure 4, the porosity problems in the both sides of any vertical walls in the mould are avoided completely.

Considerable advantages are associated with the steels and the method in accordance with the invention as compared to the prior art. The inventive steels contain up to 3,5% vanadium. These higher vanadium contents than conventional hot work steels are an effective way to prevent formation of macro pores in the disc shaped spray deposition. The new steels have not only good as-spray formed soundness, but may also have a simplified heat treatment, and superior wear resistance as compared to conventional hot work steels, leading to longer service life. Small and simple inserts can be made with a single atomizer (single spray) . For larger and/or more complex inserts with high vertical walls facing to both directions in the radius of the substrate mould, a twin atomizer (two separate sprays) is needed for proper soundness.

Remarkable cost benefits derive from converting molten alloy directly into a net-shape die insert. Tooling cost reduction can be quoted from reference to be 30-50% comparing to traditional machining-on-ingot processes. Lead times for die making can be shortened from months to weeks. In most cases, an insert can be made in 5-8 days. It is more rapid and the costs are significantly lower with follow-on tools.

Dimension accuracy should be +/- 0,05 mm in 100-200 mm. Surface finishing satisfy most cases in die forging and pressure die casting.

Lifetime of die inserts is expected to extend because of the metallurgical benefits of the spray forming process as well as the invented steel grades. Potential applications cover nearly all mass- production manufacturing sectors, such as die casting, injection moulding, forging, extrusion, stamping, blow moulding, etc.

The applicant has named the new and inventive method as PSF Process that stands for Precision Spray Forming Process.

In the foregoing, the invention has been described in detail with reference to the accompanying drawings while various embodiments of the invention are possible within the scope of defined by the claims.

Claims

1. A series of tool steels for spray forming die inserts comprising iron (Fe) , carbon (C) , silicon (Si) , manganese (Mn) , chrome (Cr) , molybdenum (Mo) and vanadium (V), ch a r a c t e r i s e d in that the vanadium content is over 1,4 w-%.

2. A series of tool steels according to claim 1, ch a r a ct e r i s ed in that the vanadium content is selected from the range 1,4 - 3,5 w-% for each case specifically.

3. A series of tool steels according to claim l or 2, ch a r a c t e r i s e d in that the carbon content is at least 0,35 w-%.

4. A series of tool steels according to claim 3, ch a r a ct e r i s e d in that the carbon content is selected from the range 0,35 - 0,9 w-% for each case specifically.

5. A method of spray forming die inserts in which a melted tool steel according to any one of claims 1-4 or a melted metal alloy is atomized and sprayed onto a ceramic mould under the atomizer, ch a r a ct e r i s e d in that the ceramic mould is rotated under the atomizer and the rotating axis of the mould is tilted from, the spraying direction to a deposition angle.

6. A method according to claim 5, ch a r a c t e r i s e d in that the deposition angle is between 0-50° and is selected according to the mould shape characters for each case specifically.

7. A method according to claim 5 or 6, c h a r a c t e r i s e d in that the atomized melted metal is sprayed onto the ceramic mould as two separate sprays apart from each other.

8. A method according to claim 7, ch a r - a c t e ri s e d in that said two sprays are symmetrically on the opposite sides of the rotating axis of the mould.