Title of Publication Edited By

TMS (The Minerals, Metals & Materials Society), Year

MELT CONDITIONED HIGH PRESSURE DIE CASTING (MC-HPDC) OF Mg-ALLOYS

S. Tzamtzis, H. Zhang, G. Liu, Y. Wang, N. Hari Babu, Z. Fan

Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University, Uxbridge, UB8 3PH, UK

Keywords: Melt conditioning, Die casting, Microstructure, Mechanical properties

Abstract

The high pressure die casting (HPDC) process is characterized by

low cost and high efficiency. However, HPDC Mg-alloy

components have non-uniform microstructure, chemical

segregation, and substantial amount of casting defects, such as

porosity and hot tearing. Recently, we have developed a new

shape casting process named as melt conditioned high pressure

die casting (MC-HPDC) where liquid metal is conditioned under

intensive forced convection and intensive shearing provided by

the MCAST (melt conditioning by advanced shear technology)

unit, and then transferred to a conventional HPDC machine for

shape casting. Melt conditioning can be done at temperatures both

above and bellow the liquidus of the alloy. Compared to

conventional HPDC, the MC-HPDC process offers cast

components with fine and uniform microstructure, much reduced

cast defects and substantially improved mechanical properties. In

this paper we present the microstructures and mechanical

properties of Mg-alloys processed by MC-HPDC under different

conditions and discuss the solidification behavior of conditioned

melt.

Introduction

Magnesium alloys, as the lightest of all structural metallic

materials, are finding increased applications in the automotive

industry for vehicle weight reduction. Since the early 1990s there

has been a 15 % average annual growth rate of die cast Mg-alloy

components in the automotive industry [1]. It is predicted that the

usage of light alloys in cars will continue to rise at an even faster

pace in the next decade. The most commonly used fabrication

process is high pressure diecasting (HPDC). The current cast

components, however, usually exhibit a coarse and non-uniform

microstructure, contain various casting defects and severe

chemical segregation, and thus offer poor mechanical

performance. For wrought products for more demanding

applications, extensive thermo-mechanical processing must be

applied to the continuously cast feedstock to achieve

microstructural refinement and compositional uniformity. Such

thermo-mechanical processing is usually low in materials yield,

intensive in energy consumption, capital investment and

manpower, and hence, inevitably high in cost. A grand challenge,

addressed by the MCAST (melt conditioning by advanced shear

technology) process [2-5], has been to develop solidification

processing technologies which can ensure a fine and uniform as-

cast microstructure free of macro-segregation and cast defects, so

that the cast products can be either directly used in the as cast

state, or only require minimal thermo-mechanical processing. In

this paper we present the application of the melt conditioned high

pressure diecasting (MC-HPDC) [6] process on Mg-alloys.

Figure 1. A schematic illustration of the MC-HPDC process.

Melt Conditioned High Pressure Die Casting (MC-HPDC)

Process

In the MC-HPDC process, the intensive shearing is directly

imposed on the alloy melt in either liquid or semisolid state, prior

to die filling. The implementation of the shearing is carried out by

the MCAST unit, which can be directly attached to a standard

HPDC machine, as seen in Figure 1. The function of the MCAST

unit is to impose the high shearing dispersive mixing action of the

twin screws to the melt, so that the melt is treated in such a way

that either its uniformity in chemistry and temperature is

improved, or the melt is converted to semisolid slurry containing

spherical solid particles. The MCAST process can work either in a

batch manner or in a continuous manner depending on the specific

requirement of the subsequent casting processes. Inside the barrel

of the MCAST unit, there is a pair of specially designed screws,

co-rotating, fully intermeshing and self-wiping, creating an

environment of high shear rate and high intensity of turbulence for

the alloy melt. The sheared melt or slurry is then cast by the

conventional HPDC process and is expected to offer unique

solidification behavior, and an improved fluidity and die-filling

during the subsequent HPDC process.

During the MC-HPDC process, the alloy melt is conditioned

inside the MCAST unit at a pre-set temperature for the barrel and

screws, and mechanically sheared by the pair of the screws before

being transferred to the HPDC machine. When the temperature is

higher than the liquidus, the shearing of the melt is in a totally

liquid state; whilst semisolid slurry is created from the melt if the

temperature is preset to be lower than the liquidus. The volume

fraction of the solid in the semisolid slurry and the superheat for

the liquid shearing can be readily controlled by setting the

MCAST unit at desirable temperatures. The characteristics of the

process for manufacturing Mg and Al alloys have been

investigated in the semisolid shearing domain [2-5, 7-11].

Table I. Chemical compositions of Mg-alloys used in this work.

Chemical Composition (wt. %) Alloy

Al Zn Mn Sr Liquidus Temperature Processing Temperature

AZ61 6.52 0.15 0.18 - 610 ºC 608 ºC

AZ91D 8.8 0.67 0.22 - 598 ºC 593-650 ºC

AM50 4.75 0.05 0.41 - 620 ºC 609-620 ºC

AM70 7.19 0.05 0.36 - 610 ºC 598-610 ºC

AM90 9.04 0.04 0.38 - 600 ºC 589-600 ºC

AJ62 6.3 - 0.36 2.5 612 ºC 612 ºC

In this paper we demonstrate that the MC-HPDC process is an

effective industrial process to manufacture a wide variety of Mg-

alloys with significantly improved mechanical properties

compared to HPDC.

Experimental

The various Mg-alloys (see Table I for compositions) were melted

in a steel crucible under a protective atmosphere of N2 containing

0.4 vol. % SF6, and the melt was then subjected to the intensive

shearing prior to casting. Detailed description of the shearing

mechanism of the MCAST process can be found elsewhere [2-5].

The rotation speed of the twin screws of the melt conditioner was

controlled to be between 500 and 800 rpm. The shearing

temperature was controlled to be either below or above the alloy

liquidus, covering both the semisolid processing and the liquid

shearing domains. A standard 280-ton cold chamber HPDC

machine was used to produce the tensile test samples.

The mechanical properties of the Mg-alloys alloys were measured

by using an Instron tensile test machine (Model 5569), with a

constant strain rate of 6.7×10-4 s-1. The metallographic specimens

for optical microscopy (OM) were prepared by grinding with SiC

abrasive paper and polishing with an Al2O3 suspension solution,

followed by etching in a solution of 5 vol. % concentrated HNO3

and 95 vol. % ethanol. A Zeiss Axio Vision optical imaging

system was used for the OM observation and the quantitative

measurements of microstructural features. In order to obtain

further microstructural information, color etching was used with

the colored orientation contrast being created under polarized light

of the Zeiss optical microscope. Scanning electron microscopy

(SEM) was carried out with an FEG Zeiss Supera 35 microscope,

equipped with an energy dispersive spectroscopy (EDS) facility

and operated at an accelerating voltage of 5~15 kV.

Results

Microstructure of HPDC Mg-alloys

Figure 2 is the general view of the solidification microstructure of

the AZ91D alloy prepared by HPDC. Large and well developed

dendrites are clearly visible, as well as the segregation of the

primary α-Mg phase, resulting in the non-uniform microstructure.

Macro-pores were often observed in the centre of the sample and

the overall porosity of the HPDC processed samples was as high

as 2-8 vol. %. Figure 3 presents the microstructure of the HPDC

processed AZ91D alloy at higher magnification. Two types of

dendrites are found, distinguishable by their dendrite arm spacing.

During the HPDC process, the liquid stays for a few seconds in

the shot sleeve (pouring) and then is injected into the die with the

solidification being accomplished inside the die cavity.

Figure 2. Optical micrograph showing the general view of the

microstructure across the section of a 6mm diameter sample of the

AZ91D alloy prepared by the HPDC process.

Figure 3. Polarized optical micrograph showing the detailed

solidification microstructure of AZ91D alloy prepared by the

HPDC process.

Figure 4. SEM micrograph showing the morphology of shrinkage

porosity in the AZ91D alloy prepared by the HPDC process.

500 µm

200 µm

8 µm

The well developed dendrites in the HPDC processed AZ91D

alloy, as seen in Figure 3, were formed in the shot sleeve of the

HPDC unit. SEM observation showed that the melt between these

dendrites solidified at the final stage with much finer dendrite arm

spacing. Apart from the macro-pores seen in Figure 2, SEM

analysis revealed that the shrinkage porosity was frequently

associated with the melt solidified at the inter-dendritic regions

for the HPDC AZ91D alloy, as shown in Figure 4. Cracks were

often found in connection with the melt between the dendrites

solidified at last.

Figure 5. Optical micrograph showing the general view of the

microstructure across the section of a 6mm diameter sample of the

AZ91D alloy prepared by MC-HPDC process.

Figure 6. Polarized optical micrograph showing the detailed

solidification microstructure of AZ91D alloy prepared by the MC-

HPDC process.

Figure 7. SEM micrograph showing the morphology of shrinkage

porosity in the AZ91D alloy prepared by the MC-HPDC process.

Microstructure of MC-HPDC Mg-alloys

The AZ91D alloy prepared by the MC-HPDC process exhibited a

significant improvement in both microstructural uniformity and

refinement, as shown in Figure 5. The primary α-Mg phase was

uniformly distributed across the whole sample, whilst no macro-

porosity was observed. The dendritic structure seen in Figure 3 for

the AZ91D alloy produced by HPDC is eliminated in the MC-

HPDC processed alloy, as Figure 6 shows. Furthermore, in the

MC-HPDC alloy no dendrite was found in the microstructure as

the result of the secondary solidification inside the die. A mixture

of fine and equiaxed α-Mg grains and eutectic β-Mg17Al12

intermetallic was observed (Figure 7). The eutectic intermetallic

was found to decorate the grain boundaries of the secondary α-

Mg grains, but not necessarily in a continuous way. The alloy

prepared by the MC-HPDC has a much reduced porosity level, as

shown in Figure 7, with the micro-porosity being a few

micrometers in size and usually isolated. The porosity is

associated with the eutectic β-Mg17Al12 intermetallic phase

located between the secondary α-Mg grains. Porosity was

negligible, only as much as 0.2-0.5 vol. % [6] due to the

microstructural refinement and compositional uniformity as a

result of the unique solidification behavior of the sheared melt.

By lowering the temperature of the MCAST unit below the alloy

liquidus, the conditioning process is in the semisolid processing

domain, where a semisolid slurry is created from the melt. Figure

8 shows the microstructure of AZ91D alloy produced by

semisolid processing. Whilst in the semisolid domain, the larger

primary phase globules (α1-Mg) seen in Figure 8 were formed

inside the MCAST unit during shearing. It is interesting that the

α2-Mg phases which formed inside the shot sleeve of the HPDC

machine obtained from both semisolid and liquid processing

exhibited a similar size and morphology.

The MC-HPDC process was used to process various Mg-alloys

and was found to be flexible to Mg-alloys with varying

compositions, which can be either cast or wrought alloys. Figure 9

shows the typical microstructures of AZ61, AM50, AM90 and

AJ62 alloy produced by the MC-HPDC process. The resulting

microstructures of these alloys have similar features to what has

been described in this paper for AZ91D alloy. There is a uniform

distribution of the primary α-Mg phase, a refined structure, with

eliminated porosity and other casting defects.

Figure 8. Optical micrographs showing the typical morphology of

the primary α-Mg phases formed in the AZ91D alloy prepared at

593°, corresponding to semisolid processing. The larger globular

particles were formed inside the MCAST unit.

500 µm

200 µm

4 µm 200 µm

Figure 9. Microstructures of various Mg-alloys produced with the

MC-HPDC process at different conditions (a) AZ61 alloy (608

ºC); (b) AM50 alloy (613 ºC); (c) AM90 alloy (589 ºC) and

(d)AJ62 alloy (612 ºC).

Mechanical Properties

The tensile mechanical properties of the AZ91D alloy prepared by

HPDC and MC-HPDC are shown in Figure 10. Absolute increase

of 20-70 MPa in ultimate tensile strength (UTS) and more than

100 % increase in elongation have been achieved by the MC-

HPDC process. Figure 10 also shows that the variation of the

processing temperature in the HPDC process exerted a more

significant effect on the mechanical properties than in the MC-

HPDC samples. For the HPDC process, the UTS and elongation

increased with the processing temperature up to 650 °C, beyond

this temperature they tended to decrease. In contrast, the effect of

the processing temperature on the mechanical properties was

suppressed by the melt shearing effect. Over the temperature

range of 593 °C to 640 °C, the UTS and elongation of the MC-

HPDC AZ91D alloy did not vary much. In addition, the smaller

error bars in the curves of Figure 10, indicate an improved

consistency of the tensile properties for the MC-HPDC alloy.

Improved mechanical properties can be achieved by melt

processing temperature close to the alloy liquidus, either in

semisolid or liquid processing domain.

Figure 10. Variation of (a) ultimate tensile strength (UTS) and (b)

elongation as a function of the processing temperature for the

AZ91D alloy prepared by the conventional HPDC and the MC-

HPDC processes [6].

160

180

200

220

240

260

280

590 600 610 620 630 640 650 660

Processing Temperature ( oC )

UT

S (

MP

a )

MC-HPDC

HPDC

0

2

4

6

8

590 600 610 620 630 640 650 660

Processing temperature ( oC )

Elo

ng

atio

n ( %

)

MC-HPDC

HPDC

Heat Treatability of MC-HPDC Mg-alloys

The significantly reduced porosity levels make it possible for the

MC-HPDC AZ91D alloy to be subjected to heat treatment without

blistering problems [7, 8]. The mechanical properties of MC-

HPDC samples for the AZ91D under different heat treatment

conditions are tabulated in Table II. It can be seen that the

conventional heat treatment of AZ91D alloy has limited benefits

in the mechanical properties. An optimized heat treatment

schedule, denoted as Tx, was established for the MC-HPDC

AZ91D alloy [8]. As a single treatment process, the Tx caused

partial dissolution of the metastable β-Mg17Al12 phase and broke

up the β-phase network, resulting in a microstructure with a fine

grain size and uniform phase distribution which led to an

improved combination of strength and ductility for AZ91D alloy,

as shown in Table II.

The mechanical properties of all the Mg-alloy series produced by

the MC-HPDC process are summarized in Table III. For AZ and

AM series, an increase in Al content increases the yield strength,

decreases ductility, while the UTS remains fairly constant.

However, the changes caused by the increase in the Al content are

fairly moderate and the minimum 7.4 % elongation obtained is

adequately acceptable for general engineering applications. For

AJ series, the Sr content plays an important role in determining

the mechanical properties. When Sr concentration is low, the

binary Mg17Al12 phase will form. This phase is thought to

facilitate grain boundary migration and therefore decreases the

creep resistance. Thus the Sr content needs to be high enough to

avoid the formation of the Mg-Al binary compound. On the other

hand, the elongation of the AJ alloys is sensitive to the Sr content.

However, the elongation of the MC-HPDC processed AJ62 alloy

with 2.5 wt. % Sr is as high as 9.1 %. Therefore, MC-HPDC

process can be used to process AZ-series alloys with lower Al

content and AJ-series alloys with higher Sr content to improve

ductility, and to process AM-series alloys with increased Al

content for improved processability.

Table II. Mechanical properties of MC-HPDC AZ91D alloy heat

treated under different conditions [7, 8].

Conditions YS

(MPa)

UTS

(MPa)

Elongation

(%)

MC-HPDC As-cast 145 ± 8 248 ± 10 7.4 ± 0.9

MC-HPDC + T4

413 °C, 5 h 91 ± 7 230 ± 9 11.2 ± 1.0

MC-HPDC + T5

216 °C, 5 h 133 ± 9 236 ± 12 6.5 ± 1.1

MC-HPDC + T6

413 °C, 5 h + 216 °C, 5.5 h 134 ± 9 255 ± 9 6.7 ± 1.1

MC-HPDC + Tx

365 °C, 2 h 132 ± 8 249 ± 11 9.1 ± 1.0

Table III. Mechanical properties of AZ, AM and AJ series Mg-

alloys produced by the MC-HPDC process [4, 10].

Mechanical Properties Alloy

UTS (MPa) YS (MPa) Elongation (%)

AZ61 253 126 12.3

AZ91D 248 145 7.4

AM50 249 122 20.5

AM70 251 133 13.3

AM90 253 150 10.2

AJ62 244 130 9.1

Table IV. Tensile properties of MC-HPDC AZ91D using different

volume percentage of scraps [11].

Sample conditions

Yield

Stress

(MPa)

UTS

(MPa)

Elongation

(%)

Primary alloy 140±4 250±7 6.9±0.7 MC-HPDC

As-cast 100 % Scrap 140±3 249±7 6.4±0.6

Primary alloy 97±4 264±11 11.6±1.6 MC-HPDC

+ T4 100 % Scrap 96±2 261±9 11.3±1.3

Primary alloy 143±2 242±9 5.3±0.5 MC-HPDC

+ T5 100 % Scrap 145±1 243±7 4.5±0.5

Primary alloy 134±6 259±9 5.6±0.8 MC-HPDC

+ T6 100 % Scrap 134±6 261±13 4.8±0.9

Mg Scrap Recycling by MCAST

The rapid growth of Mg-alloys in automotive applications means

a similar rapid increase in Mg-alloy scrap from both

manufacturing (new scrap) and end-of-life vehicles (old scrap).

Therefore, recycling Mg-alloy scrap is becoming an important

technical and economical challenge. The MCAST process was

assessed as a physical approach for recycling AZ91D alloy scrap.

Die cast scrap consisting of biscuits (70 wt. %), runners (20 wt.

%), overflows (9 wt. %), and dross (1 wt. %) was processed by

the MC-HPDC process [11]. The experimental results showed that

the MCAST process can be used to recycle high grade Mg-alloy

scrap to produce high integrity Mg-alloy castings. The MC-HPDC

recycled Mg-alloy has fine and uniform microstructure and

extremely low porosity with the chemical compositions being well

within the ASTM specifications for the same alloy. No large

oxide films or particle clusters were found in the MCAST

recycled Mg-alloy. Table IV gives the comparison of the tensile

properties of the MC-HPDC alloy using different volume

fractions of the scrap. It was found that the MCAST recycled

AZ91D alloy exhibited superior tensile mechanical properties,

particularly elongation, over the primary AZ91D alloy processed

by the conventional HPDC process.

Discussion

Effect of Intensive Shearing on the Solidification Behavior

Liquid alloy melts usually contain oxide particle clusters and

oxide films which are non-uniformly dispersed. Additionally, in a

conventional cold chamber HPDC process, formation of the

primary dendrites in the shot sleeve has been found to be

inevitable for both Mg and Al alloys, and their presence, together

with the presence of oxide films and agglomerates has been

shown to be detrimental due to their negative effect on the melt

fluidity and subsequent mould filling and their association with

the occurrence of the cast defects [12-14].

However, it has been demonstrated [15] that intensive melt

shearing is capable of converting these oxide films and

agglomerates into well dispersed fine particles with a narrow size

distribution. Furthermore, the intensive melt shearing creates

uniform temperature and chemical composition fields throughout

the entire volume of the melt. This will lead to what has been

referred to as ‘effective nucleation’ [2], where all nuclei created

will have a chance to survive and eventually contribute to the

microstructural refinement. The combination of the dispersion of

the oxide particles and the enhanced nucleation promoted by

intensive melt shearing will result in improved melt fluidity which

provides better die-filling. Additionally, the sheared melt is

uniform in its composition and temperature and has well dispersed

oxide particles with fine size and a narrow size distribution, which

all promote the heterogeneous nucleation of the primary phase,

resulting in finer, more uniform primary α-Mg phase [15].

Influence of MCAST on Mechanical Properties

The improvement in the tensile mechanical properties by the MC-

HPDC process has been observed in both semisolid and liquid

processing domains, as shown in Figure 10. For the HPDC

AZ91D alloy, the influence of the processing temperature on the

mechanical properties is greater. The increase in the mechanical

properties with increasing temperature from 605 °C to 650 °C

may be attributed to the improved fluidity of the melt, which

therefore improves the castability and leads to the increased

mechanical properties. However, too much superheat for the melt,

especially for the magnesium alloys, causes severe oxidation

during melting and transferring of the melt, and also reduces the

viscosity, which promotes gas entrapment during die filling. This

certainly does not assist with the improvement of the

microstructure and mechanical properties any longer. During the

secondary solidification, the dendrites grow until they touch each

other, pushing the remaining melt into the inter-dendritic regions.

The remaining melt can be readily isolated by the dendrites and

the large reservoirs of the melt solidify locally without adequate

melt compensation for the shrinkage, resulting in the shrinkage

porosity. The Mg-alloys prepared by the MC-HPDC process,

however, have significantly reduced porosity levels and smaller

size of the porosity, which are much less detrimental to the

mechanical properties. Also, what is equally important from an

industrial application point of view, the alloys can be processed

and cast in lower temperatures with the MC-HPDC process

compared to conventional HPDC.

Usually heat treatment is not applicable to HPDC Mg-alloys due

to the problem of blistering, so that further enhancement of

mechanical properties in this way is not possible. However, the

alloys prepared by the MC-HPDC process can be subjected to

heat treatment without the blistering problem due to the negligibly

low porosity level. Detailed investigations have been carried out

on the heat treatment of the semisolid processed AZ91D and AJ62

alloys, and it has been shown that a further enhancement in the

mechanical properties can be realized via customized heat

treatment schedules developed for the AZ91D alloy [7, 8] and the

AJ62 alloy [10] prepared by the MC-HPDC process.

Summary

The novel process of melt conditioning by advanced shear

technology (MCAST), has been developed for conditioning liquid

metal through intensive melt shearing provided by a twin screw

mechanism. It has been demonstrated that the MC-HPDC process

can provide cast products with fine and uniform microstructure,

uniform chemical composition and much reduced or eliminated

cast defects, providing superior mechanical properties over the

traditional HPDC process. The MC-HPDC process is also shown

to be a promising technology for Mg-alloy scrap recycling.

Finally, the various Mg-alloys can be processed and cast in lower

temperatures with the MC-HPDC process compared to

conventional HPDC.

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