The origin of the yield strength in thin sections of high pressure die castings

Yang, Kun (2012). The origin of the yield strength in thin sections of high pressure die castings PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Yang, Kun
Thesis Title The origin of the yield strength in thin sections of high pressure die castings
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2012
Thesis type PhD Thesis
Supervisor Carlos H. Cáceres
Mark A. Easton
Total pages 200
Total colour pages 79
Total black and white pages 121
Language eng
Subjects 0912 Materials Engineering
Formatted abstract
In cold chamber high pressure die casting (HPDC) process, partial solidification occurs in the unheated shot sleeve during which externally solidified grains (ESGs) are generated. Since the ESGs are larger and have lower solute content, they are softer than the grains solidified in the die cavity. A strong bimodal grain microstructure is thus observed in almost all the HPDC Mg alloys. A thin layer following the casting periphery, or skin, is normally reported. The skin has higher integrity and hardness due to the supersaturation of primary α-(Mg), increased volume fraction of intermetallics and finer grain size. The centre region of the specimens, or core, is much softer due to the concentration of ESGs. The overall behaviour during yielding is thus a weighted average of those of the skin and core regions.

Aside from grain size, solid solution, and dispersion hardening effects, the increased strength of thinner castings of the concentrated Mg alloy is thought to be accounted for by the formation of a percolating β-phase intermetallic structure which forms a 3D skeleton throughout the casting. Both the degree of interconnection and the scale of this skeleton may increase the material’s strength. Therefore a quantitative understanding of the contributions of different possible mechanisms to the overall strength is essential to fully understand the origin of the strength of HPDC Mg castings.

To unveil the microstructural details behind these issues, seven binary Mg-Al alloys with Al contents from 0.5~12 mass % and thicknesses of 3 mm were cast using a 250 T cold chamber HPDC press and subject to tensile testing, microstructural examination (SEM) and microhardness mapping. These alloys were chosen for microstructural features from only primary α-(Mg) for the very dilute alloys, to concentrated alloys with partly or completely interconnected intermetallic networks. Likewise, a number of Mg-RE alloys with predetermined amounts of either intermetallics or eutectic, as well as with different eutectic morphologies, lamellar or rod-like, were also studied. The Scheil-Gulliver equation was used to calculate the solute partitions for the alloys.

Tensile testing was carried out on cast-to-shape tensile specimens. SEM (BSE, SEI and EBSD) was used for the characterization of the grain microstructure, especially the distribution of the ESGs and the scale and volume fraction of the β-phase intermetallics. The microhardness was mapped over the entire cross section of the un-deformed specimens to identify the skin and core regions of the castings. The Kocks-Mecking method of analysis was applied to investigate the yielding behaviour.

The following conclusions were drawn:

• The skin effect in Mg-Al alloys The skin is patchy, uneven and asymmetric; it is generally harder than the core; a distinct skin and core regions forms when Al concentration reaches 4.37 mass % or above, due to the formation of ESGs and a reduced volume fraction of intermetallics at the core region.

The skin imposes a radially tensile constraint on the core during deformation, which delays the onset of full plasticity of the core region until the skin itself starts to yield. About 30 % of the cross section of the 11.6 mass % Al alloy still remains elastic when the core becomes fully plastic. That fraction decreases to less than 10 % for the more dilute alloys.

• The skin formation in Mg-RE based alloys Whether a continuous and uniform skin layer forms over the cross section depends on the distribution of ESGs, which varies with solute concentration and nature. The Mg-RE alloys tend to have larger solid fraction for given temperature drop during solidification. This translates into a larger volume fraction of ESGs formed in the shot sleeve and a densely packed pre-solidified skin layer compared with that of the Mg-Al alloys. A relatively more viscous semisolid forms in the Mg-RE alloys, preventing the migration of ESGs towards the edge of the casting. The resulting grain microstructure at the skin is therefore more uniform than in the Mg-Al alloys, and this is reflected by a better defined and more continuous skin layer. By the same token, the core and skin regions are better differentiated in the Mg-RE alloys.

• The modelling of strength of Mg-Al alloys Calculations using the collected microstructural data and standard models for the strength of polycrystalline alloys containing dispersed particles and solid in solution led to the following conclusions:

Grain boundary strengthening accounts for the majority of the yield strength. Solid solution and dispersion hardening make smaller contributions but account for the differences in strength between alloys of a given family.

The calculated strength closely matched the experimental yield strength as well as the hardness data for the skin and core regions for alloys with Al contents below 5 mass %; a shortfall of 30~50 MPa was observed for the concentrated alloys. It is suggested that the interconnected intermetallics network adds an extra strengthening effect to the dispersion hardening component, accounting for the observed shortfall.
Keyword Mg alloys
High pressure die casting
Skin effect
Section thickness effect
Microhardness maps
Yielding behaviour
Elasto-plastic transition
Additional Notes Pages in colour: 44-45,50-51,54-55,58-59,61,65-67,73-75,78,85-90,95-96,98,107-115,117-118,121-123,132-139,141-144,152-153,159,170-177,179-191,193-196

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Created: Fri, 27 Jul 2012, 12:55:42 EST by Miss Kun Yang on behalf of Scholarly Communication and Digitisation Service