Chadee Persad

Effect of Geometry Change on Armature Behavior

Diffusion-controlled erosion of armatures, which is accentuated by the velocity skin effect, gives rise to nonuniform current conduction and consequent nonuniform heating. Earlier computational studies have highlighted the role played by armature geometry in current density and temperature distribution. Recently, we designed modified C-shaped armatures, called saddle-shaped armatures, to study the effect of geometry change on armature erosion and its consequent effect on rail deposits. The trailing edge and the throat of the armature were curved to better align with the magnetic field, thus reducing the nonuniformity of current density distribution. A front tab was also provided to reduce current concentration and heating at the leading edge of the contact area. Low-speed tests, in which armatures were recovered and compared with recovered conventional C-shaped armatures, were conducted. Similar experiments at speeds above 2 km/s were conducted and compared with corresponding C-shaped armature experiments. It was found that in conformation with our hypothesis, the saddle-shaped armatures recovered from low-speed shots showed less erosion both on the contact surface and in the throat area. The high-speed tests showed that the contact voltage for saddle-shaped armatures was significantly lower than that for conventional C-shaped armatures. This implies less energy loss at the rail-armature interface and potentially less lateral force generated during contact arcing

Thermal stress in EML rail-conductor overlays

In an electromagnetic (EM) railgun, a thin, unbonded overlay on the rail conductor develops residual strains as evident from its post-shot curvature. Heating due to the transient EM field and a large thermal gradient due to deposition of a molten layer of armature material contribute to the straining of the overlay. In this paper, the authors solved the one-dimensional coupled magnetic and thermal diffusion equations to estimate the thermal gradient in the overlay. Heat sources included both Joule heating and the enthalpy from a resolidified thin layer of molten aluminum deposited by the armature. The thermal gradient so calculated was used in a one-dimensional stress model to compute residual stresses from which residual curvature was calculated. The calculated curvature compares well with the experimental value. Finally, based on the estimated residual stress, they present an estimate for the remaining life of the overlay.

Railgun Tribology: Characterization and Control of Multishot Wear Debris

Railgun tribology describes the friction and wear sciences of interacting surfaces in relative motion. The novel tribological feature of railguns is the use of high-speed, high-current sliding electrical contacts for the conversion of the electrical energy in a current pulse into projectile kinetic energy. Several wear processes affect the life of components used to construct the bore of a railgun depending upon the severity of the duty cycle. Following multishot testing without refurbishment in solid-armature railguns, wear products have been observed to accumulate on the contact surfaces of rail conductors. Aluminum-alloy armatures produce wear debris that is chemically complex and structurally inhomogeneous. Because electromagnetic propulsion requires that electrical conduction take place across these debris layers, it is important to understand the factors that influence their structure. A critical question for repeatable and predictable railgun performance and for extended rail life is whether easy conduction occurs through these accumulated wear products. It is possible to control the dimensions and properties of these wear debris accumulations on rail contact surfaces using chemical and mechanical aids. Films of rapidly solidified aluminum with layered structures and high levels of gas porosity are the most common debris on copper rails. It is possible to control the structures of these films and to plow them away when they grow thick. Here, results of characterization and control of wear debris are presented and discussed

Railgun Tribology—Chemical Reactions Between Contacts

Many electromagnetic launcher experiments have been conducted with armature-rail pairs made of dissimilar alloys. When the armature is made of an aluminum alloy, the eventual formation of aluminide intermetallics is likely at the sliding contact interfaces with nonaluminum rails. Because most launcher bores are freely accessed by laboratory air, other reactions-oxidation, nitridation, and water vapor-may also occur during the reactive phase of the transfer of aluminum from the armature to the rail surface. The structure and composition of the thin layer that adheres to the rail contact surfaces is important, because it separates the new armature contact in a subsequent launch from the original rail contact surfaces and, thus, affects current transfer and the contact lubrication process. The chemistry and the morphology of the reaction layer on the rails will vary along the length of the launcher. In the armature start-up and low-velocity regions of the rails, the adhering rail deposit shows evidence of liquid-metal-enhanced, high-temperature reactions. Experimental data confirming the presence of these reaction products are presented for the case of the very low-speed and highly energetic propulsion of a copper armature between aluminum alloy rails. A copper aluminide intermetallic-CuAl2-is formed, as predicted from the binary phase diagram. Intermetallics have properties that will degrade the multishot load-carrying capability of rail conductors

Thermal stresses in an actively cooled two-piece rail structure

In this paper, we evaluate the thermal stress arising in rails and supporting structures of an electromagnetic launcher. We examine the effects of using rail overlays and active cooling. These not only reduce the peak temperature, but also affect the overall thermal gradient. The transverse temperature distribution within the rail structure is obtained by a numerical solution of magnetic and thermal diffusion equations. The effects of cooling rate and cooling channel location on the temperature distribution are evaluated. The resultant thermal stress in the rail structure is then computed. The analysis yields two fruitful approaches: use of thin resistive overlays, which minimize thermal stress on the structure by effecting a more even temperature distribution; and use of active cooling at a central location, which even though ineffective at very early times, enables more efficient heat removal in the time between launches.

Characterization of hypervelocity gouge craters in rail conductors

Gouging is a frequently encountered rail damage phenomenon and has been observed in several large-bore guns. In this study the size and number of gouges have been determined as a function of position and velocity. Detailed analysis was performed on rails recovered from single shot experiments. Rail hardness and surface and microstructural characteristics of gouges have been determined and analyzed to relate materials properties to gouge formation. We observed hardening and plastic flow of copper beneath a gouge crater. Transmission electron microscope studies showed twinning in copper. We find that gouge sizes are closely related to armature contact size. The gouge is generally shorter than the contact length and no wider than the contact. Our study shows that rail hardness is affected in the vicinity of a gouge. After gouges are formed on a rail, the hardness is no longer uniform and performance of the rail conductor is affected.

Liquation cracking and its effects in aluminum alloy armatures

Experiments were performed using four different aluminum alloy armatures to study liquation cracking and its effects on armature performance. The four materials tested were Al 1100, Al 6082, Al 7075, and Al 7475. Of these, only the Al 1100 (commercially pure aluminum) armatures were free of gross cracking after exit from the launcher. The Al 6082, Al 7075, and Al 7475 armatures experienced fracture and structural deterioration as observed in high speed photographs of the in-flight bodies and the condition of the recovered armatures. Metallography of the recovered armatures revealed that liquation of the grain boundary phases in Al 7075 and Al 7475 had contributed to the structural failures in these armatures. The results suggest that a composite armature should be designed using different aluminum alloys to separately perform the sliding contact and structural functions.

Post-test characterization of the hardness and microstructure of copper rails from a 9 MJ electromagnetic launcher

Following a test series consisting of sixty-seven experiments, the rails from a 90 mm diameter /spl times/10 m long launcher were removed for metallurgical characterization. The rails were machined from OFHC copper extrusions. The measured Brinell hardness values of the rails ranged from HB 60 to HB 86. The starting material had a hardness of HB 86. Observed values increased from the breech up to a distance of approximately 4 m and tended to decrease and level off beyond 4 m. These hardness values are normal for worked OFHC copper, falling between those for fully annealed (HB 34) and cold rolled (HB 91) conditions. The compressive flow strength of the as-received material was determined to be /spl sigma//sub f/=331 /spl epsi//sup 0.12/ MPa. Extensive cratering and hypervelocity gouging damage was observed except in the regions 1 m from the breech end and between 7.5 and 10 m. The microstructure of the as-received rail material was fully dense and equiaxed and the average grain size was 100 /spl mu/m. >

High energy rate modification of surface layers of conductors

Multimicrosecond, high power density current pulses have been employed in a study of the surface response of railgun rail materials. Many of the surface transformations observed appear to result from self-quenching effects. The pulse heating produces a hot skin while the specimen interior remains cold. The hot skin is in intimate thermal contact with the cold interior. These conditions result in a rapid self-quench. The thermophysical properties of the materials affect the extent of the surface modification and the level of damage that results from pulsed joule heating.

Obtainable microstructures in electrical conductors made of a copper-silver alloy

The goal of this study was to understand the relationships between the obtainable properties of copper-silver conductor alloys and the materials microstructure. Processing routes to obtain property combinations of high yield strength and high electrical conductivity were desired. The processing method involved melting and casting of pure elemental materials in selected mass ratios, electrical discharge machining of rectangular prismatic blocks from the cast billets, and flat rolling of the blocks to a 51% reduction in thickness. Standard electrical conductivity and hardness measurements were made. The microstructures of the cast ingots and of the rolled materials were evaluated. It was observed that manipulation of the processing parameters and microstructure produced material with room-temperature yield strengths of 600 MPa and corresponding electrical conductivities of 85% IACS at a 51% thickness reduction. These property values are sufficient for use of this alloy in either the as-cast state or in the cast-and-rolled state.