PH12014000383A1 - Duplex treatment of molds for die-casting - Google Patents

Abstract

This invention pertains to duplex treatment of steel alloy molds, whereby the first step is surface hardening of the mold surface by a suitable nitriding process, followed by a second step of Physical Vapor Deposition (PVD) process yielding a hard coating product with a particular design. The invention covers 1) the process of preparing or conditioning the as-received mold surface for duplex treatment, 2) the process of plasma nitriding with parameters that yield a nitride surface zone compatible with the subsequent PVD coating process, and 3) the composite product which comprises of a plasma nitride zone underlying the PVD coating with a certain compositional and microstructural architecture. A significant extension of mold life and performance in the manufacture of aluminum die-cast components is obtained by this invention.

Description

I

SPECIFICATION oy

Title of the Invention: ~

Duplex Treatment Of Molds For Die-Casting oo »

This invention pertains to surface treatment of steel molds fr Ji! -9 2% 21 aluminum die-casting of various industrial parts in general. The / upper and lower molds are paired such that molten aluminum is Cb injected into the cavity when the molds are in closed position. After the melt solidifies, the molds are opened and the solid aluminum ~ casting shaped by the mold is ejected. This die-casting process is - ys = repeated in cycles, e.g., approximately every 30 seconds.

Heretofore, failure of molds in production can result from any or all of the following causes: the surface region is subjected to a) rapid cycles of thermal stress due to heating up to the melt temperature and cooling to allow solidification; b) abrasive wear due to moving contact with the solidified casting that has hard inclusions; c) high temperature soldering or chemical reactions in air such as oxidation; or d) micro-cracking induced by thermal fatigue as the metal expands and contracts with temperature, with subsequent infiltration of open cracks by the molten metal.

Abrasive wear resistance is improved by increasing the hardness of the surface region of the mold by various metallurgical treatments such as gas nitriding or plasma nitriding, The nitrided oo surface zone may also be in a state of compressive stress that effectively suppresses micro-cracking within this zone. Another proven method of increasing surface wear resistance is Physical Vapor :

Deposition (PVD) that imparts a hard, relatively thin coating. A PV = coating of certain composition can bé quite effective against wear and Man i mm TT erik fat : — oo ok Fo] chemical reactions at high temperatures where nitriding effects fog : weaken. Co Latadl + wo - - Therefore, a combination pf nitriding and PVD processes is conceived to yield higher mold performance. However, ’ Jom ; incompatibilities between the two processes are known to occur, such da as the poor adhesion of the PVD coating on the underlying nitrided = surface, which the current invention of duplex treatment, combining - plasma nitriding and particular PVD coating and structure in one process chamber, has overcome. ee Le HF pndaeory

BACKGROUND OF THE PLASMA NITRIDING PROCESS » et ’

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A common metallurgical surface hardening treatment for eT i rT TTT metalworking dies is any method that diffuses atomic carbon, nitrogen or boron into the surface region under controlled conditions of temperature and atmospheric gas concentration. These are called carburizing, nitriding or boronizing, respectively, which result in fine precipitation of metal alloy carbide, nitride or boride phases in the surface region of the order of 100 microns deep that increases hardness and imparts higher wear resistance. Traditionally, such surface hardening suffices to improve the performance of a die or mold. : :

This invention uses the concept of duplex treatment, where the first step such as nitriding is supplemented by PVD coating. It is n ' : necessary that conditions for the mold surface to be suitable for good - adhesion of the PVD coating be fulfilled. ~

Conventional gas nitriding at atmospheric pressure is not a =O suitable first step because a “white layer” by-product of the nitriding - cycle, ie. iron nitride compounds, formed at the outer surface would ~ etn i the adhesion of thé PVD coating, : - A more recent version of gas nitriding is plasma nitriding in = controlled vacuum environment where the gas molecules of nitrogen = are ionized and accelerated by bias voltage to penetrate the surface - region of the die or mold. This is a plasma-assisted process that allows faster kinetics and higher chemical activity such that high temperature reactions can take place at lower temperatures. White layer formation can be essentially eliminated by plasma nitriding.

This process lends itself to adaptation inside the same vacuum chamber where physical vapor deposition is done. Many years of advances in plasma nitriding and PVD technologies have led to design of commercial systems that allow duplex treatment in the same PVD chamber.

BACKGROUND ON EDM SURFACE ARTIFACT ON PVD COATING

ADHESION

Electro-discharge machining (EDM) is often employed to imprint the intricate geometric pattern on the surface cavity of the die used in ] casting. An electrode generates the pattern by electrical sparking to remove material. A melted and re-solidified surface layer, also termed white layer, is left on the surface by EDM. This is brittle and not

. tightly bound to the underlying steel. A PVD coating on such a layer > would be non-functional if the white layer peels off under load. Its O complete removal is therefore a prerequisite to good PVD coating . adhesion. -

BACKGROUND ON MICROBLASTING PREPARATORY TO DUPLEX =

TREATMENT tw

The current invention includes the preparatory surface 1. treatment of the mold in optimized condition for the sequences of - plasma nitriding and PVD (comprising duplex treatment) processes. =

The embodiment refers to an aluminum-die casting mold, H13 steel - with hardness of about 52 Rockwell C (equivalent to Vickers hardness 550) where the cavity pattern has been shaped by mechanical machining, electro-discharge machining and surface polishing.

Under ordinary cleaning procedures of ultrasonic washing with alkaline solutions, which are standard preparatory procedures prior to

PVD coating, it was found that subsequent duplex treatment of plasma nitriding and PVD coating led to many instances of coating peel-off, which would not pass quality control in production.

Specific microblasting parameters were optimized and applied to the mold surfaces to completely remove any remaining “white layer” left by EDM. Microblasting is also a means of shot-peening that creates surface defects that facilitate penetration or diffusion of nitrogen during plasma nitriding. Removal of the EDM white layer consequently eliminated PVD coating peel-off, particularly at sharp edges of the mold pattern.

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BACKGROUND ON__PVD COATING DESIGN FOR DUPLEX =

TREATMENT ~

PVD equipment types are classified according to the method of g ion generation from the metal targets or sources mounted on the = inside walls of the PVD chamber, such as by arc evaporation, = magnetron sputtering, or variants of ion plating methods. on

The metal ions (for instance, Ti ions) are mixed with reactive gas = ions (for instance, nitrogen ions) in the electric plasma from which oo these coating specie ions are extracted and directed energetically to - form the coating (for instance titanium nitride) on a substrate or object to be coated. Substrates are fixtured and rotated such that the exposure times of functional surfaces are controlled under parameters of bias voltage, gas pressures and vacuum chamber temperature; altogether these result in the sequential deposition of individual layers of given composition and thickness. Following a programed coating recipe, the detailed architecture of multilayers for the total PVD coating can be realized.

Those knowledgeable in the art design the PVD layer structures to meet functionality requirements. Each layer contributes to the overall mechanical hardness, toughness and chemical inertness of the composite coating. The basic functionality and synergistic effects derived from these layer combinations, in relation to the service environments, are well studied in the literature particularly in cutting ; tool applications, concepts of which can be extended to die casting mold applications. The following invention focuses on an adherent

PVD coating (coupled with plasma nitriding described in an earlier > section) on steel molds for aluminum die- casting, operating in an i" environment where surface protection is needed against abrasive wear, micro-cracking, and chemical corrosion at high temperatures. o

PVD coating layer architectures can be designed in terms of total - thickness, comprised of individual layers each with given thickness -

J and composition, as programmed in the coating recipes in industrial =

PVD equipment. 5 fot

SUMMARY OF THE INVENTION =

In summary, the increase in mold performance is effected by duplex treatment as described in this invention, the embodiment of which includes the following essential features: 1) A particular microblasting process to prepare the mold surface to enhance the diffusion of nitrogen during the plasma nitriding process as well as providing a surface condition that optimizes good adhesion of the PVD coating; 2) A particular plasma nitriding process that yields a hardened surface region of the mold without formation of a “white layer” that would inhibit good adhesion of the subsequent PVD coating. Such nitrided zone or layer is known to inhibit thermal : cracks during repeated cooling and heating cycles (termed heat checking or thermal fatigue) and therefore mitigates micro-cracking; 3) Particular coating architecture of the PVD coating layers is designed featuring an interfacial innermost layer in contact with the ; plasma nitrided surface, a series of nanolayers and an outermost layer. These coating layers have specific functionalities for good > coating adhesion with the nitrided steel base, providing high hardness oO and toughness and high temperature chemical inertness of the o coating to protect the mold surfaces under the operating conditions of - aluminum die casting. -

PREFERRED EMBODIMENT OF THE PVD COATING PRODUCT -

INVENTION oN

The following describes the product invention of duplex = treatment of the mold surface, from the outermost coating towards to oO the plasma nitrided surface zone of the base steel and their respective - functionalities. Although this invention has broader application to die-casting molds in general, the description is particularized to molds used to manufacture aluminum alloy hard disk drives.

Good Coating Adhesion

It is generally recognized that the innermost interlayer next to the tool surface serves as an interface bond and provides strain compatibility between the coating and the underlying tool, both important for adhesion of the coating. This layer may have composition of metallic Ti or Cr, singly or in combination with TiN or

CrN, respectively. The interfacial layer may range 0.1 to 0.5 micron : in thickness.

High Coating Microhardness and Toughness

The stacking of many thin layers (termed nanolayers, typical thickness range 10-50 nm), with built-in compressive stress due to ion bombardment during deposition, serves to inhibit crack

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Co propagation from the top surface towards the inner layers, thus > increasing the coating micro-toughness. The overall hardness of the oO coating imparts to the coating its resistance to abrasive wear at given - temperature. These nanolayers comprise the main body of the coating i. structure, and typically are in the range of 2 to 4 microns thickness. -

Depending on the choice of targets used in the coating recipe - in this N fa invention the targets used were Cr, Ti, AlTi (with 66 atomic percent -

Al), TiSi (with 20 atomic percent Si)- these nanolayers may be = comprised of combinations of CrN, TiN, AITiN, or TiSiN.

Chemical Inertness

The composition of each layer determines the coating chemical stability against oxidation and other chemical reactions at given temperature. It is generally known that chemical stability at elevated temperature increases in the sequence TiN < TiAIN <AITiN < TiSiN.

The outermost layer is the first line of defense against chemical reactions such as oxidation, which may be 1-3 microns thick.

EMBODIMENT OF MICROBLASTING PROCESS INVENTION

Microblasting with alumina powder, 120 mesh, at 5 bar for about 5 minutes on the patterned cavity area ~ 100mm x 70mm, generated a dull, surface that is not “appealing” to the eye because it is not very shiny; nevertheless coating peel-off is no longer observed.

When finer powder size such as 320 mesh was used as microblasting powder, the original polished surface was largely preserved, however

PVD coating peel-off could be frequently observed at sharp edges of ; :

the pattern. The more aggressive action of relatively larger = microblasting particles results in complete removal of any vestiges of ~

EDM white layer that causes poor coating adhesion. When longer » times using 120 mesh alumina were used, some dimensional ~ tolerances were degraded (e.g., rounding of sharp edges) due to too po much material removal. Therefore, it is important to control the : microblasting parameters for optimum PVD coating adhesion. =

It is also known that shot peening effects resulting from = microblasting activate the surface for atomic diffusion, therefore the nitrided diffusion zone extends deeper into the interior of the mold.

This diffusion zone is characterized by higher hardness near the surface and internal compressive stress that mitigates against cracking from the interior region.

EMBODIMENT OF THE PLASMA NITRIDING PROCESS INVENTION

It is also known that a hard coating is more effective in wear applications when it is supported by a harder base provided by the nitrided layer or zone (in comparison to a base steel without surface ] hardening).

Typical plasma nitriding parameters used in the same PVD coating chamber are: temperature 480 - 500 C, total pressure of 0.080 mbar, with hydrogen/nitrogen partial pressure ratio of 180/140, bias voltage of 900 V, total nitriding time of 2 hours. This produces a plasma nitrided surface zone of 30 - 40 microns with gradient microhardness. This diffusion zone has a Vickers = microhardness HV 0.050 of ~1100 at the top surface, gradually - diminishing to the interior microhardness HV 0.050 of ~550. o > .

Bich

Claims (3)

CLAIM: ®

1. Applicant claims the particular microblasting process to i» prepare the mold surface to enhance the diffusion of nitrogen during ~ the plasma nitriding process by (Controlling the microblasfing 1-0 B > parameters for optimum PVD coating adhesion. =o

2. | ~ Applicant also claims the particular plasma nitriding / process that yields a hardened surface region of the mold by - optimizing and applying specific microblasting parameters to prevent © formation of a “white layer” that would MBB good adhesion of tHe - subsequent PVD coating.

3) Applicant also claims the particular coating architecture of the PVD coating layers featuring an interfacial innermost layer in contact with the plasma nitrided surface, a series of nanolayers and an outermost layer, providing high hardness and toughness and high temperature chemical inertness of the coating to protect the mold ] surfaces under the operating conditions of aluminum die casting.