with reference to fig1 , rectangular casting mould assembly 10 has mould walls 11 forming part of a water jacket 12 from which a stream of cooling water 13 is dispensed . the feed portion of the mould is divided by a divider wall 14 into two feed chambers . a molten metal delivery trough 30 and delivery nozzle 15 equipped with an adjustable throttle 32 feeds a first alloy into one feed chamber and a second metal delivery trough 24 equipped with a side channel , delivery nozzle 16 and adjustable throttle 31 feeds a second alloy into a second feed chamber . the adjustable throttles 31 , 32 are adjusted either manually or responsive to some control signal to adjust the flow of metal into the respective feed chambers . a vertically movable bottom block unit 17 supports the embryonic composite ingot being formed and fits into the outlet end of the mould prior to starting a cast and thereafter is lowered to allow the ingot to form . as more clearly shown with reference to fig2 , in the first feed chamber , the body of molten metal 18 gradually cools so as to form a self - supporting surface 27 adjacent the lower end of the divider wall and then forms a zone 19 that is between liquid and solid and is often referred as a mushy zone . below this mushy or semi - solid zone is a solid metal alloy 20 . into the second feed chamber is fed a second alloy liquid flow 21 having a lower liquidus temperature than the first alloy 18 . this metal also forms a mushy zone 22 and eventually a solid portion 23 . the self - supporting surface 27 typically undergoes a slight contraction as the metal detaches from the divider wall 14 then a slight expansion as the splaying forces caused , for example , by the metallostatic head of the metal 18 coming to bear . the self - supporting surface has sufficient strength to restrain such forces even though the temperature of the surface may be above the solidus temperature of the metal 18 . an oxide layer on the surface can contribute to this balance of forces . the temperature of the divider wall 14 is maintained at a predetermined target temperature by means of a temperature control fluid passing through a closed channel 33 having an inlet 36 and outlet 37 for delivery and removal of temperature control fluid that extracts heat from the divider wall so as to create a chilled interface which serves to control the temperature of the self supporting surface 27 below the lower end of the divider wall 35 . the upper surface 34 of the metal 21 in the second chamber is then maintained at a position below the lower edge 35 of the divider wall 14 and at the same time the temperature of the self supporting surface 27 is maintained such that the surface 34 of the metal 21 contacts this self supporting surface 27 at a point where the temperature of the surface 27 lies between the solidus and liquidus temperature of the metal 18 . typically the surface 34 is controlled at a point slightly between the lower edge 35 of the divider wall 14 , generally within about 2 to 20 mm from the lower edge . the interface layer thus formed between the two alloy streams at this point forms a very strong metallurgical bond between the two layers without excessive mixing of the alloys . the coolant flow ( and temperature ) required to establish the temperature of the self - supporting surface 27 of metal 18 within the desired range is generally determined empirically by use of small thermocouples that are embedded in the surface 27 of the metal ingot as it forms and once established for a given composition and casting temperature for metal 18 ( casting temperature being the temperature at which the metal 18 is delivered to the inlet end of the feed chamber ) forms part of the casting practice for such an alloy . it has been found in particular that at a fixed coolant flow through the channel 33 , the temperature of the coolant exiting the divider wall coolant channel measured at the outlet 37 correlates well with the temperature of the self supporting surface of the metal at predetermined locations below the bottom edge of the divider wall , and hence provides for a simple and effective means of controlling this critical temperature by providing a temperature measuring device such as a thermocouple or thermistor 40 in the outlet of the coolant channel . fig3 is essentially the same mould as in fig1 , but in this case a pair of divider walls 14 and 14 a are used dividing the mouth of the mould into three feed chambers . there is a central chamber for the first metal alloy and a pair of outer feed chambers for a second metal alloy . the outer feed chambers may be adapted for a second and third metal alloy , in which case the lower ends of the divider walls 14 and 14 a may be positioned differently and the temperature control may differ for the two divider walls depending on the particular requirements for casting and creating strongly bonded interfaces between the first and second alloys and between the first and third alloys . as shown in fig4 , it is also possible to reverse the alloys so that the first alloy streams are fed into the outer feed chambers and a second alloy stream is fed into the central feed chamber . fig5 shows several more complex chamber arrangements in plan view . in each of these arrangements there is an outer wall 11 shown for the mould and the inner divider walls 14 separating the individual chambers . each divider wall 14 between adjacent chambers must be positioned and thermally controlled such that the conditions for casting described herein are maintained . this means that the divider walls may extend downwards from the inlet of the mould and terminate at different positions and may be controlled at different temperatures and the metal levels in each chamber may be controlled at different levels in accordance with the requirements of the casting practice . it is advantageous to make the divider wall 14 flexible or capable of having a variable curvature in the plane of the mould as shown in fig6 and 7 . the curvature is normally changed between the start - up position 14 and steady state position 14 ′ so as to maintain a constant interface throughout the cast . this is achieved by means of an arm 25 attached at one end to the top of the divider wall 14 and driven in a horizontal direction by a linear actuator 26 . if necessary the actuator is protected by a heat shield 42 . the thermal properties of alloys vary considerably and the amount and degree of variation in the curvature is predetermined based on the alloys selected for the various layers in the ingot . generally these are determined empirically as part of a casting practice for a particular product . as shown in fig8 the divider wall 14 may also be tapered 43 in the vertical direction on the side of the metal 18 . this taper may vary along the length of the divider wall 14 to further control the shape of the interface between adjacent alloy layer . the taper may also be used on the outer wall 11 of the mould . this taper or shape can be established using principals , for example , as described in u . s . pat . no . 6 , 260 , 602 ( wagstaff ) and will again depend on the alloys selected for the adjacent layers . the divider wall 14 is manufactured from metal ( steel or aluminum for example ) and may in part be manufactured from graphite , for example by using a graphite insert 46 on the tapered surface . oil delivery channels 48 and grooves 47 may also be used to provide lubricants or parting substances . of course inserts and oil delivery configurations may be used on the outer walls in manner known in the art . a particular preferred embodiment of divider wall is shown in fig9 . the divider wall 14 extends substantially parallel to the mould sidewall 11 along one or both long ( rolling ) faces of a rectangular cross section ingot . near the ends of the long sides of the mould , the divider wall 14 has 90 ° curves 45 and is terminated at locations 50 on the long side wall 11 , rather than extending fully to the short side walls . the clad ingot cast with such a divider wall can be rolled to better maintain the shape of the cladding over the width of the sheet than occurs in more conventional roll - cladding processes . the taper described in fig8 may also be applied to this design , where for example , a high degree of taper may be used at curved surface 45 and a medium degree of taper on straight section 44 . fig1 shows a method of controlling the metal level in a casting mould which can be used in any casting mould , whether or not for casting layered ingots , but is particularly useful for controlling the metal level in confined spaces as may be encountered in some metal chambers in moulds for casting multiple layer ingots . a gas supply 51 ( typically a cylinder of inert gas ) is attached to a flow controller 52 that delivers a small flow of gas to a gas delivery tube with an open and 53 that is positioned at a reference location 54 within the mould . the inside diameter of the gas delivery tube at its exit is typically between 3 to 5 mm . the reference location is selected so as to be below the top surface of the metal 55 during a casting operation , and this reference location may vary depending on the requirements of the casting practice . a pressure transducer 56 is attached to the gas delivery tube at a point between the flow controller and the open end so as to measure the backpressure of gas in the tube . this pressure transducer 56 in turn produces a signal that can be compared to a reference signal to control the flow of metal entering the chamber by means known to those skilled in the art . for example an adjustable refractory stopper 57 in a refractory tube 58 fed in turn from a metal delivery trough 59 may be used . in use , the gas flow is adjusted to a low level just sufficient to maintain the end of the gas delivery tube open . a piece of refractory fibre inserted in the open end of the gas delivery tube is used to dampen the pressure fluctuations caused by bubble formation . the measured pressure then determines the degree of immersion of the open end of the gas delivery tube below the surface of the metal in the chamber and hence the level of the metal surface with respect to the reference location and the flow rate of metal into the chamber is therefore controlled to maintain the metal surface at a predetermined position with respect to the reference location . the flow controlled and pressure transducer are devices that are commonly available devices . it is particularly preferred however that the flow controller be capable of reliable flow control in the range of 5 to 10 cc / minute of gas flow . a pressure transducer able to measure pressures to about 0 . 1 psi ( 0 . 689 kpa ) provides a good measure of metal level control ( to within 1 mm ) in the present invention and the combination provides for good control even in view of slight fluctuations in the pressure causes by the slow bubbling through the open end of the gas delivery tube . fig1 shows a perspective view of a portion of the top of the mould of the present invention . a feed system for one of the metal chambers is shown , particularly suitable for feeding metal into a narrow feed chamber as may be used to produce a clad surface on an ingot . in this feed system , a channel 60 is provided adjacent the feed chamber having several small down spouts 61 connected to it which end below the surface of the metal . distribution bags 62 made from refractory fabric by means known in the art are installed around the outlet of each down spout 61 to improve the uniformity of metal distribution and temperature . the channel in turn is fed from a trough 68 in which a single down spout 69 extends into the metal in the channel and in which is inserted a flow control stopper ( not shown ) of conventional design . the channel is positioned and leveled so that metal flows uniformly to all locations . fig1 shows a further preferred arrangement of divider walls 14 for casting a rectangular cross - section ingot clad on two faces . the divider walls have a straight section 44 substantially parallel to the mould sidewall 11 along one or both long ( rolling ) faces of a rectangular cross section ingot . however , in this case each divider wall has curved end portions 49 which intersect the shorter end wall of the mould at locations 41 . this is again useful in maintaining the shape of the cladding over the width of the sheet than occurs in more conventional roll - cladding processes . whilst illustrated for cladding on two faces , it can equally well be used for cladding on a single face of the ingot . fig1 is a microphotograph at 15 . times . magnification showing the interface 80 between an al — mn alloy 81 ( x - 904 containing 0 . 74 % by weight mn , 0 . 55 % by weight mg , 0 . 3 % by weight cu , 0 . 17 % by weight , 0 . 07 % by weight si and the balance al and inevitable impurities ) and an al — si alloy 82 ( aa4147 containing 12 % by weight si , 0 . 19 % by weight mg and the balance al and inevitable impurities ) cast under the conditions of the present invention . the al — mn alloy had a solidus temperature of 1190 ° f . ( 643 ° c .) and a liquidus temperature of 1215 ° f . ( 657 ° c .). the al — si alloy had a solidus temperature of 1070 ° f . ( 576 ° c .) and a liquidus temperature of 1080 ° f . ( 582 ° c .). the al — si alloy was fed into the casting mould such that the upper surface of the metal was maintained so that it contacted the al — mn alloy at a location where a self - supporting surface has been established on the al — mn alloy , but its temperature was between the solidus and liquidus temperatures of the al — mn alloy . a clear interface is present on the sample indicating no general mixing of alloys , but in addition , particles of intermetallic compounds containing mn 85 are visible in an approximately 200 μm band within the al — si alloy 82 adjacent the interface 80 between the al — mn and al — si alloys . the intermetallic compounds are mainly mnal , and alpha - almn . fig1 is a microphotograph at 200 × magnification showing the interface 80 of the same alloy combination as in fig1 where the self - surface temperature was not allowed to fall below the solidus temperature of the al — mn alloy prior to the al — si alloy contacting it . a plume or exudate 88 is observed extending from the interface 80 into the al — si alloy 82 from the al — mn alloy 81 and the plume or exudate has a intermetallic composition containing mn that is similar to the particles in fig1 . the plumes or exudates typically extend up to 100 μm into the neighbouring metal . the resulting bond between the alloys is a strong metallurgical bond . particles of intermetallic compounds containing mn 85 are also visible in this microphotograph and have a size typically up to 20 μm . fig1 is a microphotograph ( at 300 × magnification ) showing the interface between an al — mn alloy ( aa3003 ) and an al — si alloy ( aa4147 ) but where the al — mn self - supporting surface was cooled more than about 5 ° c . below the solidus temperature of the al — mn alloy , at which point the upper surface of the al — si alloy contacted the self - supporting surface of the al — mn alloy . the bond line 90 between the alloys is clearly visible indicating that a poor metallurgical bond was thereby formed . there is also an absence of exudates or dispersed intermetallic compositions of the first alloy in the second alloy . a variety of alloy combinations were cast in accordance with the process of the present invention . the conditions were adjusted so that the first alloy surface temperature was between its solidus and liquidus temperature at the the upper surface of the second alloy . in all cases , the alloys were cast into ingots 690 mm . times . 1590 mm and 3 metres long and then processed by conventional preheating , hot rolling and cold rolling . the alloy combinations cast are given in table 1 below . using convention terminology , the “ core ” is the thicker supporting layer in a two alloy composite and the “ cladding ” is the surface functional layer . in the table , the first alloy is the alloy cast first and the second alloy is the alloy brought into contact with the self - supporting surface of the first alloy . in each of these examples , the cladding was the first alloy to solidify and the core alloy was applied to the cladding alloy at a point where a self - supporting surface had formed , but where the surface temperature was still within the l - s range given above . this may be compared to the example above for brazing sheet where the cladding alloy had a lower melting range than the core alloy , in which case the cladding alloy ( the “ second alloy ”) was applied to the self supporting surface of the core alloy ( the “ first alloy ”). micrographs were taken of the interface between the cladding and the core in the above four casts . the micrographs were taken at 50 . times . magnification . in each image the “ cladding ” layer appears to the left and the “ core ” layer to the right . fig1 shows the interface of cast # 051804 between cladding alloy 0303 and core alloy 3104 . the interface is clear from the change in grain structure in passing from the cladding material to the relatively more alloyed core layer fig1 shows the interface of cast # 030826 between cladding alloy 1200 and core alloy 2124 . the interface between the layers is shown by the dotted line 94 in the figure . in this figure , the presence of alloy components of the 2124 alloy are present in the grain boundaries of the 1200 alloy within a short distance of the interface . these appear as spaced “ fingers ” of material in the figure , one of which is illustrated by the numeral 95 . it can be seen that the 2124 alloy components extend for a distance of about 50 μm , which typically corresponds to a single grain of the 1200 alloy under these conditions . fig1 shows the interface of cast # 031013 between cladding alloy 0505 and core alloy 6082 and fig1 shows the interface of cast # 030827 between cladding alloy 1050 and core alloy 6111 . in each of these figures the presence of alloy components of the core alloy are gain visible in the grain boundaries of the cladding alloy immediately adjacent the interface . composite ingots of alloy aa6111 clad with ma3003 and also ingots of aa611 clad with aa3104 , produced by the process of the invention , were processed through a rolling mill ( oswego ) to produce sheet of 3 . 46 mm in thickness . this sheet was then directly cold rolled to a final gauge of 1 mm on a laboratory mill ( kingston ). the resulting sheet was solutionized at 560 ° c ., water quenched , pre - aged for 8 hours at 85 ° c . to produce the t4 temper , and then naturally aged for a minimum of 7 days prior to tensile and bend testing . after rolling to final gauge , the clad sheet had a clad layer thickness of 20 μm on each side . solutionizing recrystallized the sheet and the resulting microstructure and grain structure are shown in fig2 a and 20b for the sheet with the aa3003 cladding . the tensile properties of the two materials are given in table 2 . the tensile properties in both the t4p and the paint bake t8x tempers are in the range expected for aa6111 , so the thin cladding did not result in any significant degradation of tensile strength or ductility . the sheet with the 3003 cladding was slightly weaker than that clad with 3104 , but the differences were very small . the material was not given an inter - anneal to reduce roping , and the cladding was too thin to influence the roping behaviour . however , the bendability was significantly affected . fig2 a shows the variation in bendability with pre - strain , and fig2 b shows the same data in terms of the flow stress . the bendability deteriorates with increasing pre - strain and increasing flow stress , which is expected , and the slightly lower strength sheet has the better bendability . however , the bendability of the clad sheet was far superior to conventional aa6111 . fig2 a and 23b compare the clad sheet with bare aa6111 , all for bending in the transverse direction ( i . e . the bend axis is in the rolling direction ). the significant improvement in bendability is very apparent . there was no evidence of delamination at the clad / core interface in any of the tests . however , “ roping ” was not influenced by the cladding ; i . e . it was not improved . in summary , cladding aa6111 sheet with a 20 μm layer of higher bendability aa3000 sheet significantly improves the bend performance , and this improvement is maintained with pre - strain . this means that the hemming of formed sheet will be improved . the improvement in bendability is achieved without any significant decrease in t4p or t8x strength .