Shock-Wave Consolidation of Metallic Powders

Citation

Kasiraj, Prakash (1985) Shock-Wave Consolidation of Metallic Powders. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7cxy-vx62. https://resolver.caltech.edu/CaltechETD:etd-09202002-161800

Abstract

Conventional powder metallurgical techniques have the drawback in that metastable properties of the powder can be lost during the sintering stage. The use of shock waves to bond particles of powder together can, in principle, circumvent this drawback and produce bulk solids which retain metastable properties of the initial powder. However, the effects of the relevant shock and powder parameters on the final properties of the compacts must be understood before this technique can be used optimally.

In the present investigation the influence of shock pressure, shock duration, and surface oxides on the mechanical and metallurgical properties of compacted steel, molybdenum, and nickel-molybdenum alloy powders were studied. In addition, to improve our fundamental understanding of shock waves in metallic powder media, the shock temperature at the junction between two layers of copper and constantan powders was measured via the thermoelectric effect for varying shock pressure and energy.

The measured homogeneous temperatures in the copper-constantan powders varied from 150 C to 940 C as the shock energy was increased from 50 to 360 kJ/kg (shock pressure from 1.3 to 9.4 GPa). These results indicate that almost all of the energy in the shock front is converted into thermal energy. Furthermore, the rise time of the shock front is less than 23 ns which corresponds to a shock front width of less than 37 µm,a dimension comparable to the powder particle diameter.

Rapidly solidified AISI 9310 steel powders were consolidated and the dependence of the microhardness and the ultimate tensile strength of the compacts on the shock energy from 94 to 770 kJ/kg (3.6 to 19.0 GPa) were measured for an initial powder distension of 1.64 and a shock duration of 2-3 µs. Photomicrographs and SEM fractographs were used to study the interparticle bonding in the compacts. Results show that, for shock energies below 200 kJ/kg (4.9 GPa), the compacts have negligble strength. However, above this threshold the strength of the compact rises rapidly until a maximum value of 1.3 GPa is reached at a shock energy of 500 kJ/kg (12.4 GPa). This strength which is larger than that of wrought AISI 9310 remains constant before decreasing at the higher shock energies. In marked contrast, with increasing shock energy, the diamond pyramid hardness increases very gradually from a value of 340 for the initial powder to 500 at a shock energy of 500 kJ/kg. Microhardness also begins to decrease at higher shock energies. The maximum strength obtained correlates well with the strength expected from microhardness measurements.

The AISI 9310 powders were also used to study the effect of shock duration on the compact's strength. Results indicate that, for consolidations with a shock energy of 400 kJ/kg (10 GPa), shock durations greater than 0.4 µs are needed to produce strong compacts. This lower limit on shock duration can be attributed to the condition that the duration of the compressive shock wave must exceed the solidification and strengthening time of the melt produced by the shock wave.

Molybdenum powders with a distension of 1.67 were used to study the effect of surface oxides. Results indicate that, by reducing the surface oxides, the tensile strength of the compact can be increased from nearly zero to 0.76 GPa for a shock energy of 580 kJ/kg (17.8 GPa). The final strength is comparable to that of bar stock of molybdenum.

Powders of glass forming Mark-1064 alloy (Ni_55.8Mo_25.7Cr_9.7B_8.8) with a distension of 2.0 were used to study the amount of melt produced during the shock consolidation process. Such measurements were possible because the melt was quenched rapidly enough to form the amorphous phase which could be delineated from the microcrystalline phase during metallographic examination. The results indicate that shock energies above 190 kJ/kg (3.4 GPa) are required before the occurence of measurable melting. The amount of melt is much less than the upper bound limit and the melt distribution is nonuniform.

Finally, the conceptual understanding of the shock consolidation process is discussed and the criteria for producing well-bonded compacts are enumerated. These criteria can be conveniently expressed in terms of a shock consolidation map which plots a dimensionless shock duration versus a dimensionless shock energy. The existing models for the shock consolidation process are evaluated in light of the recently acquired results.

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