High power fuse assembly

Abstract: A fuse portion (8) of low melting point alloy for high power applications, is provided with a columnar grain structure extending in a direction (I) of flow of electrical current through fuse. The columnar grain structure enhances creep resistance of the fuse portion when subject to tensile forces acting in the direction of current flow. The columnar grain structure is produced by directional solidification of the fuse alloy. A laser is provided as heat source.

Description: This invention relates to a fuse and a method of making a fuse, in particular a fuse adapted for high currents, for example to fuse a powerline of an automobile battery. A high power fuse for an automobile battery is described in European application EP A 699 565. One of the fuses described in the latter application comprises a bar of fusible alloy supported between support bars of a material with a higher temperature melting point than the fuse alloy. The support material is for example a copper-alloy such as brass (CnZn30), and the fuse portion of a tin-alloy, for example a lead-tin alloy. It is suggested that the fuse portion is mechanically and electrically connected to the support bars by reflow (melting and subsequence solidification of the fuse material between the support bars). In order to reduce mechanical solicitation of the fuse portion, the adjacent support bars are securely attached to an insulating support member. The provision of a low melting point fuse portion as described in EP 699 565, has been found to be particularly effective in providing the desired blowing characteristics, where large currents in the order of between 100-600 Amperes need to be supported for specified times. Fuses formed of high temperature fusing material, for example integrally formed with adjacent support bars, often do not provide reliable blowing characteristics. The latter are subject to a number of problems such as sagging of the fuse, high temperatures of fusion, or ejection of molten tin beads that are placed on the fuse link and that serve to diffuse into the high temperature material and change its resistivity. In the latter case, the tin bead melts sometime before the fuse portion blows, and in certain circumstances such as inertial shock, the molten bead may fly off the fuse before it blows thereby adversely affecting the fuse blowing characteristics. Such problems are overcome by providing the fuse in a low temperature material with respect to the support portions. One of the problems of the low temperature fuse link portion however resides in its reduced mechanical strength. The varying thermal expansion and contraction of the fuse with respect to its insulative support generates cyclic stresses in the fuse. The thermal cycles arise from varying electrical currents through the fuse, and varying external temperature changes. US 4,387,073 is related to gold based contact materials suitable for low energy slip rings and contacts in general, that are fabricated by directional solidification and exhibit increased strength, hardness, wear resistance and undegraded electrical conductivity. Grains with a columnar texture are arranged substantially in the direction of the flow of the electrical current in eutectic structures with a matrix metal, e.g. gold and a second phase rich. The inventor has realised that the thermal or mechanical stresses, in particular tensile stresses, applied on the low-melting-point fuse portion will cause cracks to grow thereby reducing the mechanical resistance of the fuse link, and in addition reducing the electrical conductivity of the fuse link. This adversely affects the fuse blowing characteristics over time. In particular, the formation of cracks that grow transversely to the direction of the electrical current have the most adverse effect on electrical conductivity, such cracks being most affected by tensile stresses applied in the direction of the electrical current, and which are formed by relative contraction and expansion of the fuse with respect to its support. The inventor has further realised that because grain boundaries and other irregularities such as dislocations in the material structure favourise the growth of cracks, the provision of small grains, and in particular grains with boundaries extending transversely to the electrical current flow direction of the fuse will adversely affect the reliability of the fuse link over time. The invention according to the claims takes into account these considerations in view of providing a high power fuse that has reliable characteristics over an extended lifetime. In particular, a fuse is disclosed comprising a fuse link portion extending between support bars, the fuse link portion having a lower melting point than the support bars, wherein the fuse link portion has grains with a columnar texture arranged in the direction of flow of the electrical current through the fuse link. The columnar grains are formed by directional solidification of the fuse portion. The directional solidification may be achieved by providing a heat source that can be focused on the fuse, and moved with respect to the fuse in the direction of electrical current flow such that a temperature gradient from a first end of the fuse link to the other end in the direction of electrical current flow is generated. Germination of grains will occur at an interface between the support portion and molten fuse link portion, such grains growing in a columnar manner in the direction of electrical current flow as the heat source moves away. The heat source may advantageously be provided by a laser beam or beams that are moved with respect to the fuse across from one support portion to the other support portion in the direction of the electrical current flow. By controlling the speed of the heat source, a certain temperature gradient at the solid/liquid interface of the fuse link alloy can be ensured. If the temperature gradient is maintained greater then the solidification curve of the liquidus solution formed in front of the liquid/solid interface , formation of grains ahead of the interface are avoided. In other words by ensuring a sufficient temperature gradient, stable columnar growth of the grains is provided, with the formation of few grains. The reduced grain boundaries extending transverse to the electrical flow direction improves the mechanical resistance and in particular creep properties with respect to tensile forces acting in the electrical flow direction. The temperature gradient during directional solidification may be improved by providing a controlled cooling source at the germination end of the fuse link. For example, the support portion at the germination end of the fuse link may be clamped against a conductive support with controlled cooling. The fusible alloy is advantageously a tin-lead alloy PbSn of 60/40 composition that is readily commercially available. Other low melting point alloys may also be considered however, for example tin-silver AgSn 97.5/2.5. The fuse link portion alloy may be provided on a continuous strip that is cut to length and inserted in the slot between opposed support bars of the fuse, the support bars of the fuse being held in position with respect to each other by integral bridging portions whereby the support portions and integral bridging portions are stamped from a single piece of metal. The fuse link portion material may then be molten by a heat source, and subsequently cooled with the method of directional solidification described above. Pre-heating of the fuse link strip may be effected by a heat source separate from the focused heat source used for directional solidification, such that manufacturing cycle times can be reduced. After the directional solidification process, the bridging portions of the fuse link are severed and the fuse link subsequently mounted to the insulative support. In order ensure a controlled cross-section of the fuse link portion after melting and solidification, the strip of alloy assembled to the support bars is advantageously provided with a length greater then the width between support bars, any surplus material after solidification being severed. Further advantageous aspects of this invention are set forth in the claims, or will be apparent from the following description and drawings. Embodiments of this invention will now be described by way of example with reference to the figures in which; figure 1 is an isometric view of a fuse link crimped to a conducting cable, the fuse link shown without a support member; figure 2 is an isometric view of fuse link assembled to an insulative support member; figure 3 is an isometric view of another embodiment, without support member, comprising a plurality of fuse portions ; figure 4 is an isometric view of a complete assembly comprising the fuse link of figure 3; figure 5 is a view similar to figure 3 where the fuse link is only partially manufactured prior to insertion and brazing of the fuse portions; figure 6 is a top plan view of a fuse support base; figure 7 is a view similar to figure 6 with a strip of fuse link alloy assembled to the support base in a subsequent manufacturing step; figure 8 is a view similar to figure 7 schematically showing a laser heat source in a subsequent manufacturing step; figure 9 is a view similar to figure 8 where bridging portions and excess fuse link portions have been severed in a subsequent manufacturing step; figure 10 shows a subsequent manufacturing step from figure 9 where the fuse support base is crimped to an insulating support member; figure 11 is a top view of a portion of a fuse in which solidification has not been controlled, grains of the fusing portion being schematically shown and enlarged for clarity; figure 12 is a view similar to figure 11 but where the solidification of the fusing portion is controlled according to this invention; figure 13 is a characteristic phase diagram of tin-lead alloy; figure 14 shows a graph of the liquidus curve at the solid/liquid interface of PbSn 60/40; figure 15 is a schematic cross-sectional side view of the fuse section showing the laser beam in three different positions A, B', B and cooling source on a first side of the fuse assembly; figure 16 is a partial top view of the fuse section showing a single laser beam focused by lenses in an oval shape, and further showing the heat flow out of the fuse portion; figure 17 is view similar to figure 16 but showing a dual beam laser; figure 18 is a detailed cross-sectional view of part of the fuse section illustrating the progression of the solid/liquid interface. Referring to figures 1 and 2, a fuse link assembly 2 comprises a conductive base 4, a fuse link section 6 having a fuse portion 8, conductor connection sections 10, 12, and mounting portions 14 in the form of deformable tabs for crimping to a support 16 (see figure 2) of the assembly. The support 16 of insulative material is further provided with a cover 18 mounted over the fuse link section 6. One of the connection sections 10 is a battery terminal section for connection to a battery terminal of an automobile, and the other connection section 12 is in the form of a crimping barrel for crimping to a cable for supplying power to the automobile. The connection sections 10, 12 can be provided with many different shapes and forms for connecting to external conductors, other than automobile batteries and cables. The conductor base 4 in this embodiment is stamped and formed from sheet metal, whereby a variety of different shapes and exit angles for the connection sections 10, 12 can be formed integrally with the base 4. For example as shown in figure 3, another embodiment of a fuse link assembly is shown where the cable connection section 12' extends substantially orthogonally and integrally from the conductor base 4', and the fuse link assembly 2' is provided with two different fuse link sections 6', 6''. A first fuse link section 6' provides the major portion of electrical power from the battery terminal section 10' to the power cable section 12', and a second fuse link section 6'' enables plugging connection via the tab 11' to a further external conductor 13' (see figure 4) that may serve other functions of the automobile such as the safety functions which should remain independent of the main power supply. Rather than supplying the electrical power to the automobile via the power cable 12', a plurality of smaller power cables could also be provided, via a plurality of fuse link sections 6 connected to the base 4. As shown in figure 4, the fuse link assembly 2' also comprises an insulative support 16 onto which the conductive base is crimped by means of deformable tabs 14', the assembly further comprising a cover 18' covering the fuse link sections 6', 6''. The insulative support 16' may further comprise a housing portion 17' for pluggably receiving the connector 19' for mating with the secondary tab 11'. The insulative support or housing 16, 16' thus provides a mechanical support function for the fuse link sections 6, 6', 6'' such that the external forces acting on the cables 13, 13' or connection sections 10, 10' are not transmitted through the fuse link sections but taken up the principally by the supports 16, 16'. This construction enables the fuse link portion 8 to be integrally formed within the assembly and interconnecting terminal sections 10, 12, 10', 12'. The fuse link section 6 comprises support bars 20 either side of the fuse portion 8 which are interconnected to the base via interconnecting portions 22 of a width which may be different from that of the support bars, and in particular with a restricted width W1 which is less than the width W2 of the support bars 20. The widths W1 and W2 are calibrated in order to control the conductive heat flow from heat generated in the fuse section 6 in order tune the fuse blowing characteristics. The tuning takes into account the thickness of the base material 4, the properties of the fuse portion 8 and the length (L) in the direction (I) of flow of electrical current through the fuse. The fuse portion 8 is of a lower melting point conductor then the adjacent support bars 20, a preferred alloy being tin-lead with an approximately eutectic composition. A widely available tin-lead alloy is tin-lead with 60% tin by weight and 40% lead by weight which is close to the eutectic composition of 61.9% tin by weight (see figure 13). The support bars may be of a copper alloy for example CuZn30 which has a melting point far greater than the melting point of 183°C of eutectic SnPb. Other low melting point fuse portion alloys could also be considered, for example SnAg 96.5/3.5 which has a melting point of 221°C. Yet other low melting point alloys may be considered for the fuse portion, depending on the blowing characteristics of the fuse. The preferred alloys are SnPb 60/40 and SnAg 96.5/3.5 in view of their widespread commercial availability, the former being the most cost effective. Both of these alloys are polyphase with compact crystalline structures, having advantages in improving the creep resistance over materials that are single phase or that have less compact crystalline structures. The fuse section 6 may be subject to various loads and temperatures during its lifetime. The loads may on the one hand be mechanical originating from force applied on the connection sections 10, 12. Although the support 16, 16' will absorb a major part of such forces, residual forces may nevertheless act on the fuse section 6. Other loads arise from thermal expansion and contraction of the fuse assembly and particularly of the fuse base 4 with respect to the support base 16, due to different thermal expansion coefficients of the different materials. Thermal variations result from electrical power losses in the fuse assembly or conductors connected thereto, and external temperature changes. Automobile manufacturers specify operating ranges for external temperatures between -40°C and 70°C. Load and temperature on the fuse portion will affect the rate of growth of cracks (cavities), the worst situation being high temperature and high tensile forces acting in the direction (I) of electrical current flow through the fuse. The fuse portion of lower melting point has a mechanical resistance and a resistance to creep that is smaller than that of the supporting base material 4, and therefor any creep or rupture will occur in the fuse portion 8 or at the interface of the fuse portion with the support bars 20. The formation and growth of cracks in the fuse portion 8 or at the interface with the support portions 20 reduces the fuse blowing characteristics by changing the effective cross-sectional passage for electricity flowing through the fuse. The formation and growth of cracks also reduces the mechanical resistance which could possibly lead to rupture of the fuse. It is desirable, for example in automotive applications, to ensure reliable fuse blowing characteristics over the lifetime of the fuse (which for example should exceed ten years), taking into account the mechanical and thermal loads to which the fuse may be subject in the environment of an automobile engine compartment or elsewhere. The fuse portion according to this invention ensures reliable blowing characteristics over the required lifetime of the fuse by provision of a columnar grain structure extending substantially parallel to the direction of current flow (I), as will be described in greater detail further on. Referring to figures 6-10, manufacturing steps of the fuse assembly will now be described. Referring first to figure 6, the conductive base is stamped and formed from sheet metal integrally comprising the conductor connection sections, 10, 12, the support bars 20, and interconnection portions 22 of the fuse sections 6. The support bars 20 have opposed first and second sides 24, 26 respectively that form a gap of length (L) (see figure 1) for receiving the fuse portion 8 therebetween. The connection portions 10, 12 of the fuse base are held together by bridging portions 28 that flank the fuse section 6 and are spaced therefrom with gaps 30. The stamped and formed base 4 shown in figure 6 is then tin plated to improve bonding with the fuse portion alloy that is brazed to the support bars 20. The plating material could be different depending on the alloy used for the fuse portion. In the next manufacturing step shown in figure 7, a bar 7 of solid fuse material such a as SnPb 60/40 is pressed in an interference fit between the first and second sides 24, 26 of the support bars 20. The bar 7 may for example be provided in the form of a wire supplied on a continuous reel and cut to length for assembly to the base 4. The bar 7 is provided with excess length 32 extending beyond lateral sides 34 of the support bars 20, and may even traverse the bridging portions 28 as shown in figure 7. The excess material 32 serves to prevent constriction of the fuse portion 8 during the brazing process. The extensions that traverse the bridging portions 28 also serve as a support for the extensions during the brazing process of the fuse portion. In the next step, the fuse portion between the support bars 20 is melted by provision of a heat source. In this embodiment the heat source is provided by a laser. The molten fuse portion remains between the support bar sides 24, 26 by capillary action, whereby a certain sagging of the fuse portion is to be expected. The extensions 32 of the fuse bar 7 assist in supporting the molten fuse portion and also prevent constriction of the fuse portion between the support bars. Constriction of the fuse portion at the end 34 (i.e. a concave surface of the fuse portion forming between the support bars 20 at the ends 34) reduces reliable manufacturing of fuses with specified electrical current passages. Prior to melting of the fuse bar, there may be a pre-heating stage to bring the temperature of the supports 20 and fuse bar 7 close to the melting point of the fuse bar material. This pre-heating could occur during masked time of the laser melting and subsequent solidification process in order to increase manufacturing cycle times. The pre-heating could be effected by different heating means such as with a hot plate or by induction or other conventional means of heating that are advantageously low cost in relation to laser technology. As best illustrated in figure 8, after pre-heating a laser beam is focused on the fuse bar 7, the oblong path 36 depicting the laser beam reception on the fuse. Laser beam in this context should be considered as one or more laser beams that can be composed in order to form an oval or oblong shape such that the fuse portion 8 can be heated substantially evenly across its width W2. The shape of the laser beam may also be modified by varying the focus of the beam with lenses - the power intensity and spread of the beam may be tuned as required. In the present embodiment, an oblong beam (see figures 16, 17) may be provided by provision of two laser beams 36' adjacent each other in the direction of the support bar 7 and slightly unfocused to increase their area of impact on the fuse, or by a single beam 36 formed into an oval shape by lenses. The laser beam is positioned over the fuse bar in an initial position (A) as shown in figure 15, over the fuse portion 8 for melting the fusible material, and subsequently the laser beam is moved relative to the fuse in a direction (D) of solidification to a second position (B) away from the fuse portion 8 over the base 4. The direction of solidification (D) is parallel to the direction (I) of current flow through the fuse. By proper control of the directional solidification, the fuse portion can be formed with a columnar grain structure as will be explained in more detail hereinbelow. Solidification of the fuse portion starts at the first side 24 and progresses to the second side 26 of the support bars 20. The base section 4 on the first side is provided with thermal regulation means 37 (see figure 15) that maintains the temperature of the base portion 4 on the first side 5, at a certain distance from the fuse portion, at a substantially constant temperature. This is achieved for example by clamping the first side 5 of the base 4 against a block of conductive material (e.g. aluminium) that is maintained at constant temperature by forced convection 40, for example with water circulating through the aluminium block. The cooling of the second side 3 of the fuse base 4 may occur through natural convection 41, as illustrated in figure 15. After directional solidification, as shown in figure 9, the bridging extensions 28 and the fuse bar extensions 32 are then severed to form the fuse section shown in figure 10, and subsequently the base 4 can be mounted to the support 16 by crimping of the mounting tabs 14 thereto. In a subsequent step, the cover is positioned over fuse section 6 and for example thermally bonded to the housing 16 to provide a safe enclosure for the fuse section. The assembly 2 can then be crimped to the power cable 13 and mounted to the battery terminal or other conducting member. A similar manufacturing procedure may be adapted for designs with a plurality of fuse sections, for example as shown in figure 3, where the fuse portions 8', 8'' are directionally solidified, the bridging extensions 28' subsequently severed, and the fuse base 4' mounted to a support. With the aid of figures 11-16 the directional solidification process will now be explained in more detail. The inventor has realised that in order to ensure a high power fuse with reliable characteristics over an extended lifetime, such as ten years or more in an automotive environment taking into account the mechanical and thermal solicitation of the low-melting-point fuse portion, the formation and growth of cracks in the fuse portion should be avoided. It is advantageous to improve the creep resistance of the fuse portion, particularly with respect to forces acting in the direction (I) of current flow. This is achieved by providing the fuse portion with a columnar grain structure. The columnar grain structure is formed by directional solidification. The columnar grain structure extending in the current flow direction (I) reduces the number of grain boundaries, dislocations, cavities and impurities that extend transversely to the current flow direction (I). Grain boundaries, and dislocations that extend transversely to the current flow direction, which is also the direction of the main tensile forces on the fuse portion, will tend to form and grow cracks transverse to the current flow direction thereby reducing the electrical conductivity and mechanical resistance of the fuse. Cracks may even lead to rupture of the fuse portion, but in any event the variation of electrical resistance of the fuse portion is undesirable at it modifies the fuse blowing characteristics. In figure 11, the grain formation of a fuse portion that is melted and left to solidify without control is schematically represented. As the fuse portion is left to cool with natural convection and conduction, there are four main cooling front F1-F4: from opposed sides 24, 26 of the fuse portion and opposed lateral sides 34 respectively. Close to the support bar interfaces 24, 26, small grains are formed due to the rapid germination and cooling at this interface in view of the large heat flow by conduction into the adjacent support bars 20. The germination and formation of many small grains can be explained as follows: The equation for the free enthalpy ΔG for the formation of a germ in the case of homogenous germination is: Δ G = V (- L Δ T T f )+ S γ Where: V: Volume of a germ (m 3 ) - S: Area of a germ (m 2 ) γ: Liquid/solid surface energy (J/m 2 ) L: Latent heat of solidification (J/m 2 ) Δ T : Supercooling (°C) Tf: Fusion temperature (°C) For a germ of radius r: Δ G =- 4 3 Π r 3 L Δ T T f +4Πr 2 γ For a germ of radius r to remain stable and be able to grow, the free enthalpy ΔG must decrease i.e. ∂ΔG / ∂r≤ 0. The minimum radius r* of the first stable germ will be: r*=constant/ΔT The quantity of germs which can be formed at the germination temperature T g = T f-Δ T is: n=N 0 e ( -Δ G HT g ) with N 0 : Number of initial atoms and K: Boltzmann's constant. From the results of the calculation of r* and n, it can be deduced that the lower the supercooling, the fewer stable germs will be formed and the larger they will be. Conversely, in order to avoid the germination of many grains, the supercooling temperature must be very close to the melting (fusion) temperature. The inventor has further recognised that the formation of many grains is disadvantageous due to the increased number of grain boundaries extending transversely to the current flow direction. It would be advantageous to not only provide columnar grain texture, but the germination of few but larger grains, which can be achieved by controlling (reducing the rate of) initial solidification at the first interface 24. The creation of few large germs will thus improve creep resistance in the transverse direction of the fuse portion, and improve the bonding resistance of the fuse portion to the support bar 20. It can be further seen from the non-controlled solidification of the fuse portion in figure 11, that a welding zone 38 occurs in the center of the fuse, comprising many small grains that once again form a zone with unfavourable creep resistance. The cooling fronts F3, F4 of the lateral sides 34 produces a zone 39 with grains directed transverse to the current flow direction (I), which are unfavourable with respect to creep resistance for loads acting in the current flow direction (I). The weld line 38 can be avoided by directional solidification, starting at the first side 24 and terminating at the second side 26 of the support bars 20. The columnar grain structure achieved by directional solidification is schematically represented in figure 12. As best seen in figure 15, initially the laser beam is positioned in a first position (A) striking the fuse portion 8 which is melted thereby. Heat 40, 41 flows out of the fuse portion 8 through the first and second sections 5,3, the first section 5 provided with the thermal regulators 37, which are cooling elements, maintaining the second section 5 at a certain distance from the fuse portion 8 at a substantially constant temperature. This enables effective control of the temperature at the first side 24 of the support bar where the first grains are solidified. The base 4 is positioned against an insulator 43, either side of the fuse portion for better control of the heat flow out of the fuse portion. The power of the laser energy is controlled by providing a pulsed laser beam whereby the frequency can be varied in order to inject more or less heat into the fuse. The power is tuned to melt the fuse portion and maintain the fuse portion during the fusing period close to the melting point of the alloy. A high temperature of the molten material reduces the capillary forces which leads to greater sagging of the molten fuse portion by gravitation. In addition, a temperature close to melting point enables greater control of the initial germination of grains at the first interface 24. Initial germination is effected by moving the laser beam from position (A) in the solidification direction (D) whereby the heat energy 40 removed from the first side 5 provides a cooling front at the interface 24. The slower the movement of the beam from the initial position (A) in the solidification direction (D), the lower the supercooling. This leads to the germination of fewer grains at the interface 24 with a larger size as explained above. In this particular embodiment, because the composition of PbSn is hypo-eutectic, the initial germination will be composed of Pb19 pro-eutectic grains (see the phase diagram in figure 13 for a liquid of composition C 0 as it reaches the fusion point). After initial germination, in order to produce a columnar (or lamellar) grain growth, without the formation of new grains ahead of the solid/liquid interface, it is necessary to maintain the liquid/solid interface substantially stable or plane. This is explained as follows. In front of the solid interface, there is a rejection of solute that modifies the composition of the liquid ahead of the solid interface. This change in composition has the effect of constitutional (or structural) supercooling. In other words the liquid of modified composition has a fusion temperature that increases with respect to the lowest fusion temperature defined by the eutectic fusion point (see phase diagram of figure 13). The increase in the fusion temperature of the liquid composition ahead of the solid/liquid interface means that germs may form (solidify) ahead of the interface if the liquid has a "low" (i.e. supercooled) temperature. This is an unstable situation which needs to be avoided if large columnar grains are to be formed. In figure 14, the liquidus curve represents the fusion temperature of the liquid ahead of the solid interface, as a function of distance from the interface versus composition of the liquid which varies from eutectic composition at the interface C e to the composition C 0 of the alloy. In order prevent supercooling ΔT due to a small temperature gradient T L1 , the temperature gradient in the liquid must be greater than or equal to T L2 . It can be shown that to satisfy this condition the following relationship is required: G L R ≥ - m ( C E - C 0 ) D L R: Interface rate of displacement G L : Thermal gradient in the liquid m: slope of the liquidus curve at X=0 D L : Diffusion coefficient of solute in liquid D L and m are properties of the alloy. For PbSn: D L =6.7x10 -6 cm 2 sec -1 ; m= -2.326 °C For a fuse portion of 0,2 cm length (L), and a desired rate of solidification of R=4x10 -2 cm sec -1 (i.e. 5 seconds for solidification the fuse portion), the temperature gradient G L is equal to 263°Ccm -1 , which is equivalent to a temperature gradient of 52°C between sides 24, 26 of the fuse. The higher the thermal gradient, the higher the solidification rate may be thereby reducing the duration of the solidification cycle. However, since the effect of the increase in the temperature gradient is to increase the mean temperature of the molten alloy, the viscosity of this alloy will decrease and its volume will increase. As the liquid alloy is not supported and only remains in place through capillary action during the soldering or brazing operation, the temperature of the molten alloy is controlled to a temperature just sufficient to reliably satisfy the minimum gradient for stable plane-front solidification. In order to control the temperature gradient, the speed of displacement of the laser beam in the direction (D) and the power output of the laser is controlled. The laser power may also be controlled to take account of the differing emmissivity factors ε of different metals, for example the PbSn alloy has an emmissivity factor of ε=0,22 which differs from that of the tin-plated brass base section with ε=0,13. The fuse portion 8 thus absorbs more of the laser energy than the supports 20 in the present embodiment. By providing, for example a 500 Watt Y.A.G laser in pulsed mode, the frequency can be varied between 1 and 50'000 hertz in order to control the power striking the fuse. The requirements of a low temperature gradient for initial grain germination at the interface 24 and high temperature gradient for subsequent columnar grain growth are to some extent conflicting. Additionally, for the purposes of reducing manufacturing cycles a reasonable rate of displacement of the heat source needs to be provided, for example 0.5 - 1.0 mm per second. An advantageous compromise can be found in providing a slower rate of displacement of the laser in the initial solidification phase, and a faster rate of displacement after the period of initial germination, if less but larger grains are desired. As shown in figures 16 and 17, single or multiple beams 36, 36' can be provided. As shown in figures 16, 17, after the solidification process the fuse bar extensions 32 remain connected to the fuse portion 8 via constriction zones 33 that ensure maintenance of the cross-sectional area of the fuse portion 8. The columnar grains formed by the above mentioned process will have a lamellar structure of juxtaposed Pb and Sn layers with a composition close to C 0 (i.e. 60/40) similar to the lamellar structure of a eutectic composition subject to stable plane-front solidification. Referring now to figure 18, the fuse portion 8 is shown in cross-sectional during the directional solidification process. During solidification, the solid/liquid interface, represented by the line 50 is at an oblique angle with respect to the support sides 24, 26, resulting from the temperature gradient between the opposed sides 26, 24, in combination with the temperature gradient between the top side 52 which receives the laser beam 36, and the opposed bottom side 54 of the fuse which does not receive a heat source. The oblique progression of the solid/liquid interface 50 is particularly advantageous during the final stages of the solidification process where the interface meets the second side 26 as indicated by the dotted line 50'. The volume of liquid alloy is greater than solid alloy, which would result in the formation of asperities due to the relative contraction during solidification if the solid/liquid interface were to meet the second side 26 parallel thereto. Due to the oblique angle of the solid/liquid interface 50' as it meets the side 26, a pocket of liquid alloy 58 feeds the solid interface with material such that the interface 50' progresses upwardly along the side 26 without creation of asperities. Only when a very small pocket of liquid remains at the top corner 60 between the side 26 and top 52, may asperities form due to the contraction in volume. The top corner is however in a zone of low stress due to the convex top surface 62 of the fuse portion 8 that resulted from the gravitational forces on the molten alloy. The relatively poor bonding and grain structure in the upper corner 60 is therefore advantageously out of the zone of tensile stress lines 61 acting on the fuse portion and therefore does not adversely affect the mechanical strength or creep resistance of the fuse. It is therefore advantageous to provide the heat source on the top surface 52, or provide a cooling element 37 only on the bottom surface 54 at the first side, or provide both the heat source and cooling elements on opposed sides as shown in figure 18 such that a solid/liquid interface 50 progresses at an oblique angle with respect to the fuse sides 26, 24. Alternatively, it could also be considered to provide the second side 26 of the fuse portion at an oblique angle with respect to the vertical direction (V) and heat top and bottom sides 52, 54 with the solid/liquid interface 50 progressing roughly vertically. It is particularly advantageous to terminate solidification at the top corner 60 with respect to the gravitational forces (acting in direction V)as this corner is subject to low stresses in view of the concave shape 62 of the fuse portion top surface, thereby depressing the major force lines 61 traversing the fuse portion. In summary, the directional solidification of the fuse portion , which can be easily controlled with the use of a pulsed laser heat source, increases creep resistance in the direction transverse to electrical current flow by: eliminating the central weld line; reducing the formation of small grains at the support interfaces 24, 26; and reducing of the numbers of grains and in particular grain boundaries transverse to the electrical current flow. The latter is achieved by controlled solidification of initial germs at the first side 24, and maintenance of a temperature gradient at the solid/liquid interface by control of the heat source power and displacement speed of the heat source in the solidification direction (D). It may be noted as shown in figures 8, 16 and 17 that the end point of the laser beam 36, 36' is well into the second section 3 of the base 4 to ensure that the temperature gradient at the solid interface is maintained until the grains meet the second interface 26. In order to control the manufacturing process, temperature sensors 46, 48 (figure 15) may be positioned against the fuse base. The temperature variation during the directional solidification process can thus be controlled and the resulting curves compared with characteristic curves in order ensure that the correct temperature gradients over time have been provided. A solidification front at on angle with respect to the support interface, for example by thermal regulation across the thickness of the fuse, improves bonding and creep resistance of the fuse portion to the support.

Claims: A fuse assembly comprising a fuse link portion (8, 8', 8'') extending between support bars (20), the fuse link portion having a lower melting point than the support bars, characterised in that the fuse link portion has grains with a columnar texture arranged substantially in the direction (I) of flow of the electrical current through the fuse link. The fuse assembly of claim 1 characterised in that the columnar grains are formed by controlled directional solidification. The fuse assembly of claim 1 or 2 characterised in that the fuse link portion (8, 8', 8'') is comprised of a tin-alloy. The fuse assembly of claim 3 characterised in that the tin-alloy is a tin-lead-alloy of substantially 60/40 composition by weight. The fuse assembly of anyone of the preceding claims characterised in that the fuse link portion (8, 8', 8'') has a width (W2) substantially the same as the width (W2) of the adjacent support bars (20). The fuse assembly of anyone of the preceding claims characterised in that a plurality of fuse link portions (8', 8'') interconnect a conductive base (4') formed from an integral sheet of metal. The fuse assembly of anyone of the preceding claims characterised in that the assembly comprises a conductive base (4, 4') including the support bars (20), the support bars being stamped and formed from sheet metal, and having a battery terminal section (10) and a conductor connection section (12), the conductor connection section (12) comprising a crimp barrel for crimping to a power cable (13). A method of producing a fuse assembly comprising the following steps: providing a conductive base with support bars (20) having opposed first and second sides (24, 26) forming a gap therebetween; providing molten fuse alloy between the first and second sides (24, 26); directionally solidifying the fuse alloy by moving relatively a heat source (36) across the fuse alloy portion (8) in a direction of solidification (D) substantially parallel to a direction (I) of electrical current flow through the fuse. The method of claim 8 characterised in that during the directional solidification from the first side (24) to the second side (26) of the fuse portion 8, the base (4) at a first side (5) is thermally regulated by cooling means (37). The method of anyone of the two preceding claims characterised in that the heat source is provided by a laser. The method of claim 10 characterised in that the laser is pulsed, the power output of the laser being controlled by varying the pulse frequency. The method of any one of the preceding claims 8-11 characterised in that the base (4) is provided with bridging portions (28) supporting the support bars (20) with respect to each other during manufacturing, the bridging portions (28) being severed after directional solidification of the fuse portion (8). The method of any one of the preceding claims 8-12 characterised in that the fuse portion is provided in the form of a bar or wire (7) provided with surplus extensions (32) that extend beyond lateral ends (34) of the support bars (20), the excess length being severed after the directional solidification of the fuse portion. The method of anyone of the preceding claims 8-13 characterised in that during the solidification process, the fuse is thermally regulated (36, 37) to provide a solid/liquid interface (50, 50') disposed at an angle with respect to the second side (26). The method of claim 14 characterised in that a thermal gradient between a top side (52) and bottom side (54) of the fuse is provided, whereby top and bottom are defined with respect to gravitational force. The method of claim 15 characterised in that the gradient is generated by providing the heat source (36) on the top side (52).

Inventors: BEDNAREK ALAIN, BEDNAREK, ALAIN
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1999
Main classification: H01R11
IPC codes: H01R11, H01H85