BACKGROUND

[0001] The present invention relates to alloys for semi-solid and liquid injection molding and die casting machine components and components made from such alloys.

[0002] Generally semi-solid metal injection molding is the process whereby an alloy feedstock is heated, subjected to shearing and injected under high pressure into a mold cavity. Heating brings the feedstock into a state where both solid and liquid phases are present while the application of shearing forces prevents the formation of dendritic structures in the semi-solid alloy. In this state, the alloy may exhibit thixotropic properties.

[0003] The feedstock may be received into the barrel of the semi-solid metal injection molding machinery in one of three forms: liquid, semi-solid or particulate solid. The former two forms require additional equipment and special handling precautions to prevent contamination of the alloy material and therefore increase costs. The latter form, while being more easily handled, results in longer cycle times and significant thermal gradients in the first encountered portions of the barrel and more pronounced thermal shock to that portion of the barrel. [0004] More specifically, semi-solid metal injection molding (SSMIM) involves the feeding of alloy feedstock into the barrel of the semi-solid metal injection molding machinery. In the barrel, the alloy feedstock is heated and subjected to shear, often by rotating a screw or paddles located therein. As a result of heating and shearing, the temperature of the alloy feedstock is raised so as to be above its solidus temperature and below its liquidus temperature. Within this temperature range, the feedstock is transitioned into semi-molten material having co-existing solid and liquid phases. In addition to aiding to heating, shearing further prevents the formation of dendritic structures in the alloy. In this thixotropic state, the semi-solid alloy material is injected, either through reciprocation of the screw or transfer and reciprocation of a plunger to a shot sleeve, into a mold cavity and solidified to form the desired part. [0005] Typically, components for the injection molding machine are formed from conventional carbon-hardened steels. These steels, however, are not very

tough and are not truly weldable. These steels temper back at service temperatures of about 1200 ⁰ F, thus softening. When these steels are formed into components for an SSMIM machine, such as check rings, they split from the radial impact fatigue stresses. Many failures have occurred in other types of components such as screw tips, piston rings, push rings, flanges, barrels and screws due to this marginal toughness. Some of the failures of check rings and push rings appear to be aggravated by the heat affected zone under weld deposits. These steels are also susceptible to embrittlement during "torching" when operators tend to overheat the components by gas torches, causing components such as nozzles to fail by a brittle mode at the flange, as well as by bulging and splitting longitudinally. [0006] Thus, welding of these carbon hardened alloys is prone to variation in the skill of the welder. Careful pre-and post-heating is required to prevent cracking in the heat-affected zone of the steel. Even with good practice, however, the toughness of the heat affected weld zone appears to be quite inferior to the base steel.

[0007] As seen from the above, there exists a need for an improved material for components of an SSIMM machine and component made of such material.

SUMMARY

[0008] In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an alloy for components of semi-solid injection molding machinery. In particular, the alloy is an intermetallic-hardened steel, known as a Maraging steel alloy. The Maraging steel alloy includes Cr, Co, Mo, and about 0.15% or less by weight C. [0009] Since during welding of these alloys, the heat-affected zone is both soft and tough, these maraging steels are very weldable. This heat-affected zone can be returned to the hardness and toughness of the base alloy by simple post-weld aging, thus avoiding the three-stage temper treatment cycle commonly employed for conventional carbon hardened steels. To save heat treating costs, the aging can be accomplished in start up of a machine.

[0010] Further features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with

reference to the drawings and claims that are appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic illustration of one version of a semi-solid metal injection molding machine with which the present invention may be utilized;

DETAILED DESCRIPTION

[0012] Referring now to the drawings, seen in FIG. 1 is an apparatus/machine

10 used for semi-solid metal injection molding (SSMIM). The construction of the machine 10 is, in some respects, similar to that of a plastic injection molding machine.

[0013] The machine 10 includes a feed hopper 11 for the accommodation of a supply of pellets, chips, or powder of a suitable metal alloy at room temperature. For purposes of describing the salient features of the subject invention, magnesium alloys will be referred to as examples of suitable metal alloys that may be used in practicing the invention. Al and Zn are other such alloys.

[0014] A suitable form of feeder 12 is in communication with the bottom of the hopper 11 to receive pellets therefrom by gravity. The feeder 12 includes an auger (not shown) which functions to advance pellets at a uniform rate from the feeder 12. The feeder 12 is in communication with a feed throat 13 of a barrel 14 through a vertical conduit 15 which delivers a quantity of pellets into the barrel 14 at a rate determined by the speed of the feeder auger. An atmosphere of inert gas is maintained in the conduit 15 and barrel 14 during feeding of the pellets so as to prevent oxidation thereof. A suitable inert gas is Argon and its supply is effected in a conventional manner.

[0015] As is conventional in a thermoplastic injection molding machine, barrel

14 accommodates a reciprocable and rotatable screw 16 provided with a helical flight or vane 17. Adjacent the discharge end of the barrel, the screw has a nonreturn valve assembly 18 and terminates in a screw tip 19. The discharge end of barrel 14 is provided with a nozzle 20 having a tip 20a received and aligned by a sprue bushing mounted in a suitable two-part mold 22 having a stationary half 23 fixed to a stationary platen 24. The mold half 23 cooperates with a movable moid

half 25 carried by a movable platen 26. The mold halves define a suitable cavity 27 in communication with the nozzle. Mold 22 may be of any suitable design including a runner spreader 28 in communication with the cavity 27 and through which the semi-solid material may flow to the cavity in the mold. Although not shown in the drawings, suitable and conventional mold heating and/or chilling means may be supplied if required.

[0016] The opposite end of injection molding machine 10 includes a known form of high speed injection apparatus A including an accumulator 29 and a cylinder 30 supported by stationary supports 31 on a suitable support surface S. Downstream from the cylinder 30 a shot or injection ram 32 projects into a thrust bearing and coupler 33 for operational connection in known manner with a drive shaft 34 for the rotary and reciprocable screw 16. Thrust bearing and coupler 33 may separate the shot ram 32 from drive shaft 34 so that shot ram 32 may merely reciprocate and not rotate when desired. Drive shaft 34 extends through a conventional form of rotary drive mechanism 35 which is splined to drive shaft 34 to permit horizontal reciprocation of drive shaft 34 in response to reciprocation of shot ram 32 while the drive shaft 34 rotates. This shaft is in turn coupled with the screw 16 through a drive coupling 36 of known type to transmit rotation to the screw 16 as well as high speed axial movement within barrel 14 in response to operation of high speed injection apparatus A. It will be understood that suitable and conventional hydraulic control circuits will be used in the conventional manner to control the operation of injection molding machine 10.

[0017] Typically, operation of injection molding machine 10 involves rotation of the screw 16 within the barrel 14 to advance and continuously shear the feed stock supplied through feed throat 13 to a material accumulation chamber C between the screw tip 19 and the nozzle. Suitable heating means of a type to be described supply heat to barrel 14 to establish a temperature profile which results in conversion of the feed stock to a slushy or semi-solid state at a temperature that is above its solidus temperature and below its liquidus temperature. In this semi-solid state the material is subjected to shearing action by the screw 16 and such material is continuously advanced toward the discharge end of the barrel to pass the non-return valve 18 in sufficient accumulated volume ultimately to permit high speed forward movement of the screw 16 to accomplish a mold filling injection or shot. High speed

injection apparatus A functions at the appropriate time (in a manner to be explained) to move shot ram 32 forwardly, or toward the discharge end of the barrel 14, which results in forward movement of the thrust bearing 33 and drive shaft 34. Since drive shaft 34 is coupled to the shaft of the screw 16 through coupling 36, extrude screw 16 moves forward quickly to accomplish the mold filling shot. Non-return valve assembly 18 prevents the return or backward movement of the semi-solid metal accumulated in the chamber C during the mold filling shot.

[0018] As opposed to other methods of semi-solid molding, the above described method has the advantage of combining slurry generation and mold filling into a single step. It also minimizes safety hazards which occur when separately melting and casting reactive semi-solid metal alloys. Obviously, and as will be further appreciated, the alloy of the present invention will have utility with machines other than the one of the illustrated variety. By way of illustration and not of limitation, such other variety machines and apparatus include two stage machines and plastic injection molding machines, similar to die casting machines, where slurry generation and injection molding occur in separate portions of the apparatus, and non-horizontally oriented machines. Additionally, it will be understood by those skilled in the art that other mechanisms could be used to advance the material in the barrel (including gravity), that other mechanisms could be used to induce shear (such as rotating paddles, fins or electromagnetic fields) and that other mechanisms could be used to eject the material from the machine (such as a plunger of a shot sleeve). Furthermore, alloys of this invention can be applied to all-liquid injection molding and die casting.

[0019] The barrel 14 of the machine 10 is divided along its length into a series of different heating zones, with the exact number of zones being, to a certain extent, a matter of design choice. Proceeding from the end of the barrel 14 where the feedstock is received, the respective heating zones are increasingly hotter until leveling out in the latter half of the barrel 14. The barrel temperatures may be measured by a thermocouple positioned approximately three-quarters of the way through the barrel (towards the interior of the barrel).

[0020] The feedstock may be designed to exhibit a gradual melting reaction to match the desired temperature profile along the barrel 14. In this manner, processing of the feedstock material is done while imparting vigorous shear to the

semi-solid, avoiding plugs, reducing thermal shock and stress on the barrel and while being able to precisely fix the fraction solids in the subsequently molded part. [0021] Such a feedstock enables faster cycle times while decreasing thermal shock and stress on the machine 10. A preferred feedstock exhibits a mild on- setting of melting or a spreading of the eutectic reaction over a larger temperature range when initially introduced into the barrel. This decreases the thermal shock in the initial portion of the barrel, and, further, upon the on-set of melting and the introduction of the liquid phase in the feedstock, thermal transfer is enhanced and further melting is activated.

[0022] In accordance with the invention, various components of the machinery

10 are made of intermetallic-hardened steels, referred to as Maraging steels formed by martensite aging, rather than conventional carbide-hardened steels. These alloys are hardened by nano-sized intermetallic precipitates within a soft and tough martensite matrix, rather than coarse carbide phases that form a brittle martensite matrix that occurs in conventional steels. The intermetallic precipitates are more resistant to softening than carbides at barrel temperatures of 1100 to 1200° F. [0023] Components formed of Maraging type steels are very weldable in that the heat-affected zone is both soft and tough. This zone can be returned to the hardness and toughness level of the base alloy by simple post-weld aging at, for example, about 900 ⁰ F for about 3 hours, thereby avoiding the long and rigorous quench and three stage temper treatment cycle associated with conventional steels. Furthermore, dimensional changes that occur during aging are minimal, such that final machining can be done in the soft annealed state before hardening and aging may be accomplished in machine start-up. These steels are also designed with sufficient Cr to resist oxidation at the service temperatures of the molding machine 10, while also resisting liquid Mg attack.

[0024] Shown below in Table 1 is a comparison of conventional carbon hardened steels (first four entries), referred to hereinafter as C steels, with Maraging steels (next nine entries):

TABLE 1 Composition (wgt.%)

[0025] In general, Maraging types of steel employ a Co/Mo hardening mechanism. As to hardening precipitates, the T-30 composition, for example, uses carbides of M ₂₃ C ₆ that precipitate at about 900 ⁰ F; but overage, however, at about 120O ⁰ F. The T-31 composition is precipitation hardened by the more stable Laves, R and Chi phases (Fe ₁ Cr ₁ Co ₁ Mo intermetallics) which precipitate at about 1200°F in fine arrays that are most resistant to overaging and softening. [0026] The T-31 composition has soft and ductile martensite matrix which forms near room temperature upon cooling from the austenite matrix that exists during solution treatment at about 1900 ⁰ F. Moreover, using the T-31 composition avoids the delta phase during annealing. In contrast to carbon hardened steels, severe quenching is not be needed after solution treatment of the T-31 composition. That is, air cooling may suffice to transform the alloy to martensite. In some implementations, to obtain complete transformation of austenite to martensite, refrigeration can be used. The transformation defects in the martensite help nucleate nanometer precipitates upon Maraging (martensite aging) between about

900 and 1200 ⁰ F. The soft martensite (having a hardness of about 30 Rc) can be formed, welded and machined before final aging which provides a hardness up to about 67Rc. Dimensional changes that occur during final aging are less than 0.0001 in/in compared to +0.0006 in/in for C steels.

[0027] During subsequent heating at 1000° to 1250 ⁰ F, martensite reverts to austenite in a time dependent mode. Some reverted austenite of about 5 to 15% improves the toughness. This reverted austenite is of nanometer dimensions but still contains the fine precipitates and is still strong while acting as a tough crack stopper. Note, however, that more reversion softens the steel. Thus, to obtain sufficient life spans for the machine components, the Maraging steels are alloyed to obtain the proper amount of reverted austenite. In accordance with the invention, preferred ranges for the composition of the various alloying elements are listed as the fourteenth entry of Table 1 , and broad ranges are listed in the last entry of the table. It is advantageous to stabilize the alloy at 125O ⁰ F before service. It is feasible to rejuvenate used components to extend their life, by re-annealing at 1500 to 1900 ⁰ F followed by aging/stabilizing at 1100 to 1250°F.

[0028] The alloying elements in the Maraging steels provide at least the following benefits: a. Cr imparts oxidation resistance and participates in the hardening intermetallic and carbides. For example, raising Cr from 5 to 15% diminishes oxidation in 300 hrs at 1200 ⁰ F from 3.81 to 0.32 mg/cm2. b. Cobalt prevents embrittling delta formation, maintains martensite transformation above room temperature, speeds the aging reactions, participates in the intermetallic hardening phases, and slows formation of too much austenite at 1200°F. c. Mo, in synergism with Co, participates in the intermetallic hardening phases. Moreover, Mo in synergy with Cr enhances the stability of the carbide phase. d. C provides for hard, brittle martensite and introduces the delta phase. Thus, the delta phase increase as the amount of C increases. e. Ni toughens the martensite phase. Note, however, too much Ni depresses the martensite transformation temperature and the austenite reversion temperature.

[0029] Heat treatment, for example, stabilizing heat treatment or regenerating heat treatment, of molding machine components made of the Maraging steels offers flexibility in obtaining a desired life span of the components. Solution temperature can be from 1500 ⁰ F to about 1900 ⁰ F. The higher temperatures dissolve the coarse precipitates and minimize the delta phase. Aging temperature and time can be designed to provide fine precipitates along with 5 to 15% reverted austenite. A pre- service aging treatment at about 1200 ⁰ F serves to stabilize the age hardening reaction to prevent over-hardening or softening at lower service temperatures. For example, aging at 1200°F in one grade provides components with a hardness of about 40 Rc, which does not change during service times of 200 hrs at 1200 ⁰ F. [0030] Surface treatment also provides certain benefits to Maraging steels.

For example, gas nitriding in NH ₃ increased surface hardness by 20-30%, while boosting fatigue life, rolling contact life and wear resistance. Ion nitriding may improve wear resistance by 100-150%. In contrast, such nitriding treatments may embrittle the marginally tough C steels, so that such treatments may not be useful on these alloys.

[0031] Table Il below illustrates the hardness stability of the T-31 alloy as compared to conventional C steel (H-13):

TABLE Il LOSS OF HARDNESS DURING SERVICE

Thus, H-13 as well as H-1 1 are not strong enough for the rigorous wear, impact and fatigue exposure of certain machine components. Furthermore, they soften and oxidize very quickly at 1200 ⁰ F.

[0032] As for the T-2888 and Volvic 10 alloys, these suffer from low toughness, as measured by Charpy v-notch (CVN) impact energy tests, and softening. Thus, components such as screws 16, barrels 14 or sections thereof, nozzles 20, nozzle flanges, screw tips, screw adaptor, check rings, piston rings and push rings made of Volvic 10 may experience failures at undesirable rates. For example, check rings and piston rings made of Volvic 10 may require replacement

after 40,000 to 50,000 cycles of machine operation. Not only are replacement parts costly, but the down time associated with replacing the parts raises the production costs very significantly.

[0033] Examples of semi-solid injection molding machinery components made of Maraging steels in accordance with the invention have been tested without failing include:

A. Nozzle made of T-30: After 5000 cycles, the nozzle retained its original hardness at 40 Rc, bulged a slight 1 .5% and did not oxidize significantly. This compares to the conventional carbon hardened steel Volvic 10 which softened from the original 45 Rc to 20 to 35 Rc and bulged 1 % in 3000 cycles.

B. Piston rings made of T-30: After 5000 cycles, the rings retained a hardness at 47 Rc and were ductile enough to be removed. In contrast, Volvic 10 rings embrittle in service and then fracture upon removal.

C. Check rings made of T-31 : After 7000 cycles, no distress was observed; hardness retained at 40 Rc; some pitting on seal face observed.

D. Push ring made of T-31 : After 3000 cycles, no distress was observed, hardness retained at 47 Rc.

[0034] Thus, in accordance with the invention, the Maraging and Maraging +C alloys are useful for machine components such as hot runners, hot sprues, nozzles, nozzle retaining flanges, barrel end caps, barrels, barrel liners, screw tips, check rings, piston rings, push rings, screw extensions and screws. [0035] Referring now to Table III, the mechanical properties of the conventional carbon hardened steels (first three entries) are compared with that of some of the Maraging steels (last five entries):

TABLE III MECHANICAL PROPERTIES

where UTS is the ultimate tensile strength, YS is the yield strength, El is the elongation, RA is reduction in area, CVN is the Charpy v-notch impact energy, K| _C is the fracture toughness, Rc is the Rockwell hardness, and R.S. indicates the rupture strength.

[0036] Of particular note is that with a hardness between 48-50 Rc, the room temperature toughness of Maraging steel is the highest at 33 ft-lb (CVN impact energy), with Maraging plus C being lower at 18 ft-lb, and conventional C hardened T2888 and Volvic 10 being even lower at 5 ft-lb.

[0037] As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementations of the principles of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.