A method for producing a mould for high pressure die casting with reduced risk for gross cracking

Abstract

The invention is directed to a method for reducing or eliminating the danger for gross cracking in a hot work tool for high pressure die casting practically eliminates the danger of crack initiation from the inside of the cooling channel by manufacturing a near net shape (NNS) mould for HPDC, wherein the conformal cooling channels are made of stainless steel tubes having a smooth surface. The hot work tool comprises at least one non-liner and / or one conformal cooling channel. The NNS mould is produced by Hot Isostatic Pressing (HIP).

Description

[0001] A METHOD FOR PRODUCING A MOULD FOR HIGH PRESSURE DIE CASTING WITH REDUCED RISK FOR GROSS CRACKING

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for reducing or eliminating the danger for gross cracking in a hot work tool for high pressure die casting.

[0004] BACKGROUND OF THE INVENTION

[0005] High Pressure Die Casting (HPDC) is a metal casting process in which molten metal is forced under high pressure into a closed steel over short cycle times. The mould cavity is created using hot work tool steels, which have been machined into shape. HPDC offers an economical way of producing large quantities of complex, high tolerance thin walled parts comprising sharp corners and edges. HPDC is used for non-ferrous metals, in particular aluminium alloys.

[0006] The moulds are subjected to high temperatures and mechanical forces during the casting process, because of the intense thermal cycling generated by the cooling system.

[0007] Long production series are common and place tough demands on tools and materials.

[0008] Principally, the lifetime of the mould depends on the properties of the mould material, the method of manufacture and the thermal operation.

[0009] Cooling channels are usually drilled in different mould positions in order to obtain a high cooling rate. However, the thermal cycling may lead to deteriorating of the mould surface by heat checking and ultimately gross cracking causing total mould failure, when the pressurized cooling water is forced into the mould cavity.

[0010] In recent years there has been an increased interest to use Additive Manufacturing (AM) for producing near net shaped moulds with conformal cooling channels, which lead to increased productivity by reducing the mould temperature and increasing the cooling rate. The conformal cooling channel produced by AM differs to a great extent from a drilled linear channel. In particular, the distance from the working surface of the mould to the channel is much smaller than for the conventionally linear channel drilled in the mould.

[0011] Although the conformal channels reduces the mould surface temperature, there can be a local increase in stress values by the increased cooling intensity, which may result in heat checking and gross cracking also in AM-moulds having conformal cooling channels. In order to minimize these problems great efforts have been made to optimize the printing parameters, the mould design and the compositions and properties of the powder used making the moulds. However, the problem with gross cracking of HPDC-moulds produced from metal powder has not yet been satisfactory solved.

[0012] DISCLOSURE OF THE INVENTION

[0013] The present invention was made to mitigate the problems inherent in the prior art.

[0014] Accordingly, it is a general object of the present invention to provide a manufacturing method for near net shaped HPDC-moulds having conformal cooling channels, wherein the risk for gross cracking is reduced or eliminated.

[0015] In order to accomplish the above general object of the present invention, the applicant has researched in detail the reasons for gross cracking in HPDC-moulds produced by AM. It was found, that cyclic tensile stresses at the surface of the cooling channel can result in crack initiation in the surface of cooling channel, because of the fact that the inner surface is not smooth but have has a high average roughness as an inherent result of the production method. In addition, corrosion of the surfaces in the cooling channel is supposed to result in a further reduced fatigue life of the mould and, ultimately, in gross cracking.

[0016] The above general object is solved by the method set out in claim 1.

[0017] A further object is to provide a manufacturing method, which is more energy efficient than conventional manufacturing of HPDC-moulds from cast hot work tool steels.

[0018] The invention is defined in the claims. DETAILED DESCRIPTION

[0019] The material used for making moulds is a hot work tool steel. The term ‘hot-work tools’ is applied to a great number of different kinds of tools for the working or forming of metals at comparatively high temperatures, for example tools for die casting, hot-pressing and moulds for forming plastics as well as various other kinds of tools intended for the use in work at high temperatures. Conventional hot-work tool steels are developed for strength and hardness during prolonged exposure to elevated temperatures. For hot working applications it has been common to use different kinds of hot working tool steels, in particular 5 % Cr steels like Hl 1 and Hl 3. Uddeholm DIEV AR® is a premium hot work tool of this type.

[0020] It is also known to use maraging steels for hot work applications. Maraging steel metal powder has been increasingly used in the Additive Manufacturing (AM), since the low carbon content helps prevent cracking during cooling. Maraging steels are not hardened by carbon but by the precipitation of intermetallic phases in a highly alloyed matrix of low carbon martensite. The inventive alloy may be in the form of a pre-alloyed powder produced by melt atomizing, wherein the powder has a composition as set out above. Although maraging steel combines ultrahigh strength and ductility, a drawback of maraging steels is that they contain high amounts of expensive alloy elements. A further drawback in many maraging steels is the partial reversion from martensite to austenite that takes place during aging treatment. This type of austenite is referred to as reverted austenite and is to be distinguished from retained austenite, which can also be present in maraging steels after hardening and aging. The microstructural changes that take place during heat treatment result in transformational stresses and distortion, because austenite and martensite have different densities. The transformation of austenite into martensite causes a volume increase and the transformation of austenite into martensite results in a shrinkage of the tool steel. Accordingly, these unwanted transformations may lead to harmful dimensional changes, which is a difficult problem in high precision moulds, tools and dies.

[0021] Preferred maraging steels to be used in the present invention should be extraordinary stable against the formation of reverted austenite during heating. The martensite-to-austenite transformation temperature (Acl) should preferably be higher than 680 °C, which is a typical temperature for Al-die casting. The Acl can easily be determined in a dilatometer and is taken as the temperature at which the thermal expansion first deviates from linearity. It is preferred that the Act temperature is at least 690 °C, preferably > 700 °C, more preferably > 710 °C and most preferably > 720 °C. It is also preferred that the maraging steel comprises intermetallic precipitates, wherein the at least 50 vol. % of the precipitates is of the type (Fe,Ni,Co)?Mo6, i.e. the intermetallic p-phase.

[0022] Common for the hot work tool steels of the present invention is that they are alloyed with chromium in a content of 2 - 14 %. Hence, this is the definition of a hot work steel in this application.

[0023] All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims and / or in the description. This applies to all elements and all ranges.

[0024] All percentages for the chemical composition of the steels are given in weight % (wt. %) throughout the description. Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. The arithmetic precision of the numerical values can be increased by one or two digits for all values given in the present application. Hence, a value reported as e.g. 0.1 % can also be expressed as 0.10 % or 0.100 %.

[0025] The inventive method for reducing or eliminating the danger for gross cracking in a hot work tool for high pressure die casting practically eliminates the danger of crack initiation from the inside of the cooling channel by manufacturing a near net shape (NNS) mould for HPDC, wherein the conformal cooling channels are made of stainless steel tubes having a smooth surface. The hot work tool comprises at least one non-liner and / or one conformal cooling channel. The NNS mould is produced by Hot Isostatic Pressing (HIP). The inventive method comprises the steps of:

[0026] i) providing a steel container having a shape similar to the shape of the hot work tool,

[0027] ii) processing at least one stainless steel tube to a predetermined shape corresponding to the cooling channel, wherein the at least one stainless steel tube has an average roughness, Ra, of < 3 pm, iii) incorporating the at least one stainless steel tube into the container in a predetermined position, wherein the openings of the at least one stainless steel tube extends to the outside of the container,

[0028] iv) filling the gap between the at least one stainless steel tube and the container with a powder of a hot work tool steel comprising 2 - 14 % Cr,

[0029] v) outgassing and sealing the container,

[0030] vi) subjecting the sealed container to hot isostatic pressing,

[0031] vii) removing the container,

[0032] viii) optionally subjecting the hot work tool to machining,

[0033] ix) optionally subjecting the hot work tool to heat treatment.

[0034] In the following brief details of the i) to ix) will be given.

[0035] The first step in the method is directed to the fabrication of a NNS steel container, which is produced with known methods and may consist of mild steel.

[0036] The second step is the provision of a stainless steel tube having a smooth surface in order to avoid crack initiation. The average roughness, Ra, should be < 3 pm on at least one of the outer and inner surfaces, preferably on both surfaces. The average roughness, Ra, may be set to < 2 pm, < 1 pm, < 0.5 pm or < 0.3 pm. The average roughness, Ra, is commonly used in the art and is defined in ISO 4287:1997.

[0037] The stainless steel is selected to be compatible with the powder of the hot work tool steel, which comprises 2 - 14 % Cr. Although not preferred, it is conceivable to use a non-stainless steel for the tubes, this may be the case if the quality of the cooling water give rise to a low propensity of rusting. In the next steps iii) and iv), the at least one stainless steel tube is incorporated into the container in a predetermined position. The openings of the stainless steel tube(s) extends to the outside of the container in order to withstand the pressure in the HIP. Then the gap between the at least one stainless steel tube and the container is filled with a powder of a hot work tool steel comprising 2 - 14 % Cr.

[0038] Steps v) to viii) are well known in the art of HIP and need no further explanation.

[0039] The last step of heat treating the hot work tool is optional. Although, it is common to heat treat hot work tool steels, there may be a possibility to dispense with a separate heat treatment step after the HIP. This may for instance be the case for certain precipitation hardening steels.

[0040] The powder used is in the form of a pre-alloyed powder produced by melt atomizing.

[0041] The pre-alloyed powder may be produced by gas atomizing, wherein at least 80 % of the powder particles have a size in the range of 5 to 500 pm and wherein the powder fulfils at least one of the following requirements:

[0042] Mean sphericity, SPHT > 0.85

[0043] Mean aspect ratio, b / 1 > 0.85

[0044] wherein SPHT = 47tA / P2, where A is the measured area covered by a particle projection and P is the measured perimeter / circumference of a particle projection and the sphericity (SPHT) is measured by a Camsizer in accordance with ISO 9276-6, and wherein b is the shortest width of the particle projection and 1 is the longest diameter.

[0045] Although the claimed invention is directed to a method for reducing or eliminating the danger for gross cracking in a hot work tool for high pressure die casting, it is conceivable that the method steps i) to ix) could be used to produce moulds for the processing of plastics as well as complicated extrusion dies. EXAMPLE 1

[0046] In this example the premium hot work tool steel Uddeholm DIEV AR® was used to produce a mould with conformal cooling channels for HPDC using the steps defined in claim 1. The surface roughness, Ra, of the stainless steel tube (316L) was < 0.5 pm. The mould was used for HPDC of an Al-alloy having a melt temperature of 685 °C. Minor heat checking cracks were found in the working surface after 200000 shots, but there was no signs of gross cracking.

[0047] EXAMPLE 2

[0048] In this example a maraging steel having an Acl temperature of 730 °C and an Ac3 temperature of 925 °C was used to produce a mould with conformal cooling channels for HPDC using the steps defined in claim 1. The nominal composition included 0.03 %C, 5.3 %Cr, and 5.5 %Mo. The steel was found to comprise the intermetallic p-phase directly after HIP and, therefore, needed no extra heat treatment. The mould was used for HPDC of an Al-alloy having a melt temperature of 685 °C. Again no signs of gross cracking was found after 200000 shots.

[0049] INDUSTRIAL APPLICABILITY

[0050] The claimed method can be successfully applied to moulds for HPDC for mitigating the problems with gross cracking.

Claims

CLAIMS1. A method for reducing or eliminating the danger for gross cracking in a hot work tool for high pressure die casting, the hot work tool being produced by hot isostatic pressing of a hot work tool steel powder, wherein the hot work tool comprises at least one non-liner and / or one conformal cooling channel, the method comprises the steps of:i) providing a steel container having a shape similar to the shape of the hot work tool,ii) processing at least one stainless steel tube to a predetermined shape corresponding to the cooling channel, wherein the at least one stainless steel tube has an average roughness, Ra, of < 3 pm,iii) incorporating the at least one stainless steel tube into the container in a predetermined position, wherein the openings of the at least one stainless steel tube extends to the outside of the container,iv) filling the gap between the at least one stainless steel tube and the container with a powder of a hot work tool steel comprising 2 - 14 % Cr,v) outgassing and sealing the container,vi) subjecting the sealed container to hot isostatic pressing,vii) removing the container,viii) optionally subjecting the hot work tool to machining,ix) optionally subjecting the hot work tool to heat treatment.

2. The method of claim 1, wherein the at least one stainless steel tube has an average surface roughness, Ra, of < 2 pm, preferably < 1 pm, more preferably < 0.5 pm.