GB1580474A - Method and apparatus for pressure welding metal workpieces - Google Patents

Description

(54) METHOD AND APPARATUS FOR PRESSURE WELDING METAL WORKPIECES (71) We, ALFORGE METALS CORPORA TION LIMITED, a Canadian Company, of 56 Third Street, Orangeville, Ontario, Canada, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to pressure welding of metal workpieces of aluminium or magnesium alloy.

The pressure welding of metals by cold welding and hot welding techniques is well known. Each technique has certain limitations in that cold pressure welding requires a large amount of deformation and extremely high pressures to obtain a weld and, in many cases, it has been found impossible to obtain welds at room temperature because of excessive cracking and insufficient flow of the metal. Hot pressure welding of metals such as aluminum, on the other hand, normally is conducted at a temperature in excess of 4800C with relatively little deformation and upset. However, known hot pressure welding methods have the inherent disadvantage of overaging, recrystallizing and annealing the metals welded resulting in low joint strengths.

The method of the present invention differs from the aforesaid cold and hot pressure welding techniques in that only sufficient heat is provided to the welding operation to allow the metals to flow without cracking during the welding process, without harmful effects on the metals of overaging, recrystallization and annealing, and high joint strengths in the welding of aluminum and magnesium alloys can be obtained.

Canadian Patent 719,855 issued October, 1965 discloses a method of pressure welding aluminum and magnesium alloys at elevated temperatures under conditions which will enhance plastic flow of the metal during the welding operation but which will preclude overaging recrystallization or partial annealing of the metal. The method of the present invention constitutes an improvement over the method taught in this patent.

Canadian Patent 947,539 issued May, 1974, is directed to a welding machine for pressure welding flat sheet metal workpieces together. The apparatus of the present invention constitutes an improvement over this structure.

According to the present invention, there is provided a method of pressure welding together metal workpieces of aluminium or magnesium alloy comprising juxtaposing a longitudinal edge of a first workpiece gripped between a pair of dies into substantially parallel spaced alignment with a longitudinal edge of a second workpiece gripped between a second pair of dies maintaining the longitudinal edge of the first piece between 0.8 mm and 2.5 cm away from the opposed longitudinal edge of the second workpiece so as to form an elongated open rectangular channel therebetween, introducing a uniform stream of heated gases into the proximity of the opposed longitudinal edges and causing said stream of heated gases to flow through said elongated open rectangular channel, maintaining the flow of heated gases through said elongated open rectangular channel for a time sufficient to heat and soften the surfaces of said opposed edges to a temperature within the range of from 93 to 4800C while retaining a core of relatively cooler and harder material within each edge, eliminating said elongated open rectangular channel by moving the longitudinal edges of said workpieces into abutting relationship with each other to obviate the flow of said heated gases therethrough, immedately applying pressure to the abutting heated longitudinal edges of said workpieces while they are at a temperature of between 93 to 4800C to create a solid phase weld bond between the cores and to upset a portion of heated adjacent surfaces and cause heated metal to be substantially displaced out of the plane of said pressure welded interface.

Also according to the present invention there is provided a pressure welding press for joining two metal workpieces together by pressure welding comprising at least one pair of stationary C-shaped metal plate frames arranged in a parallel side-by-side spaced-apart relationship, a laterally movable C-shaped metal frame mounted between each pair of stationary C-shaped metal plate frames, a first pair of die holders associated with each pair of stationary Cshaped metal plate frames which die holders are adapted to grip a workpiece therebetween, at least one of said die holders being mounted for reciprocal movement in a direction towards and away from the other, a second pair of die holders associated with said movable C-shaped metal frame which die holders are adapted to grip a workpiece therebetween, at least one of said die holders being mounted for reciprocal movement in a direction towards and away from the other, means for moving said movable Cshaped metal frame between said stationary C-shaped metal plate frames whereby die holders associated with said laterally movable C-shaped metal frame can be moved towards and away from the die holders associated with said stationary C-shaped metal plate frames such that workpieces gripped therebetween can be spaced with opposed edges a fixed distance apart in alignment with each other prior to abutment of said workpieces for pressure welding, means for heating the opposed edges of said workpieces to a uniform temperature within the range of from 93 to 4800C while said workpieces are spaced a fixed distance apart immediately prior to abutment for pressure welding, and comprising a heat source uniformly disposed along the length of the cavity formed within the C-shaped frames to achieve uniform heating of the said opposed edges, the heat source comprising a manifold assembly having a plurality of outlets disposed at equal intervals along its length and adapted to introduce a flow of a combustible mixture of gases.

Reference is now made to the accompanying drawings, in which: Figure 1 is a perspective view of a portion of the machine of the present invention showing the relative layout of component parts of the machine; Figure 2 is a side elevation, partly cut away, of the machine illustrated in Figure 1; Figure 3 is a plan view of the machine illustrated in Figure 1; Figure 4 is a front view of the machine illustrated in Figure 1; Figure 5 is an enlarged fragmentary view of the die holders and dies of the machine of the present invention in their retracted, fully opened positions, showing the heating manifold, taken along the line 5-5 of Figure 1; Figure 6 is an enlarged fragmentary view of the die holders with dies shown in Figure 6 in a first position in which the workpieces are aligned, taken along the line 5-5 of Figure 1; Figure 7 is an enlarged fragmentary view of the said die holders and dies in a second position in which the workpieces are retracted to form a heating channel; Figure 8 is an enlarged fragmentary view of the die holders and dies in their final pressure welding position in which the workpieces are pressure welded together; Figure 9 is a cross-sectional view of a preferred configuration of workpieces used according to the method of the present invention; Figure 10 is a graphical presentation of the forging pressure, detent pressure, heat supply temperature and workpiece temperaperature during each pressure welding cycle; and Figure 11 is a cross-sectional view of the workpiece after completion of the pressure welding cycle indicating the disposition of heated workpiece metal upset as flash.

With reference now to Figures 1--4, the pressure welding press of the present invention comprises a plurality of stationary Cframes 10 arranged parallel to each other a uniformly spaced distance apart by tie-bolts 12 and spacers 14 located at the corners of the C-frames. A movable C-frame 16 is mounted between each pair of adjacent stationary C-frames 10 for reciprocal sliding travel on pillow blocks 17 guided by alignment rods 18 rigidly secured to spacer blocks 20 disposed between adjacent stationary C-frames 10.

This arrangement of the interconnected stationary frames 10 results in a modular type of machine which may be made any desired length by simply bolting together the desired number of stationary frames 10 and movable frames 16, the present embodiment showing six movable C-frames operating in unison as will be described in detail hereinbelow.

With particular reference to Figure 2, a plurality of sets of four die holders 22, 24, 26 and 28 are positioned within the cavity defined by the jaws of stationary and movable C-frames 10, 16. Die holders 22, 24 are mounted one above the other in stationary C-frames 10, die holders 22 each being rigidly secured to a lower jaw portion 30 of each frame 10 and die holders 24 each being supported by a hydraulic cylinder 32 mounted in the upper jaw portion 34 of each frame 10 by bridging plates 35 bolts thereto by bolts 37. Each die holder 24 is recirocally vertically movable relative to a fixed die holder 22 by rod 36.

Die holders 26 are rigidly secured to lower jaw portions 38 of movable C-frames 16 and die holders 28 are each supported by a hydraulic cylinder 40 mounted in the upper jaw portion 42 of each frame 16 by flange 43 and bolts 45. Die holders 26, 28 thus are movable with each movable C-frame 16, each die holder 28 being reciprocally movable vertically relative to die holder 26 by rod 44.

Each of movable C-frames 16 is laterally reciprocal by a double-acting high-pressure forging cylinder 46 connected thereto by rod 47 such that the die holders 26, 28 can be extended to and retracted from opposed die holders 22, 24. Each forging cylinder 46 is mounted between a pair of adjacent stationary C-frames 10 and secured thereto by bolts 48 through flanges 49. A hydraulic system, well known in the art, supplies a hydraulic fluid uniformly to cylinders 46 such that all cylinders work together in unison, as will be described.

Die holders 22, 24 interact to clamp a workpiece between upper and lower jaw portions 34, 30 of stationary C-frames 10 and die holders 26, 28 interact to clamp a workpiece between upper and lower jaw portions of laterally reciprocal C-frames 16 for abutment of the workpieces together for forge welding upon extension of forging cylinders 46.

Three detent cylinders 50 mounted on the lower jaw portions 30 of stationary clamps 10 by bolts 52 through cylinder flanges 53 extend between certain stationary damps 10 for abutment of guide pins 51 against lower die holders 26 to positively space die holders 26 a predetermined parallel spaced distance from opposed stationary lower die holders 22 whereby workpieces clamped between dies 221, 241 and 261, 281 respectively can be spaced apart to define an elongated open rectangular channel 19 (Fig. 7) therebetween.

A segmented heater 54 extending the length of the lower dies 221, 261 is disposed in the cavity 56 formed below said lower dies and preferably secured to stationary Cframe 10, as shown most clearly in Figures 5 through 8, through die holder 22 or alternatively secured to die 221. Heater 54 consists of a plurality of six inch burner segments interconnected to form manifold 60 with outlets 58 equispaced at 3 mm intervals adjacent the length of the lower dies for introducing a uniform flow of a combustion mixture of gases to achieve uniform heating of the edges of workpieces clamped by the dies. A mixture of oxygen and fuel gas such as propane has been found satisfactory to provide heat requirements of 30000C to achieve a workpiece temperature of from 93 to 4800C.

Heater 54 is adapted by valving to provide heat at a predetermined portion of the pressure welding cycle when the aforementioned rectangular channel is defined at a preset width between workpieces, as represented by the sequence of steps illustrated in Figure 10.

We have found optimum heating and temperature control of workpieces can be achieved by providing an air curtain designated by arrows 75 in Figure 7 between combustible gas outlets 58 and die 221 for deflection of the heating gases through the rectangular channel 19.

Referring now to Figure 5, the air curtain is provided by a plurality of jets of air discharged under pressure into cavity 56 from manifold 62 extending parallel to heater manifold 60 through the outlets 64 equispaced at a linear spacing of 6 mm. Relative heating of the exposed edges of workpieces defining channel 19 can be controlled by passing a desired volume of air between the source of heat from burner outlets 58 and the workpiece edge carried by dies 221, 241 such that said edge is shielded from the heat supply permitting the opposed edge to be heated more rapidly. Equalization of the temperature of workpiece edges can thus be attained when the workpieces carried by dies 221, 241 is normally heated to a higher temperature than the temperature of the workpiece carried by dies 261, 281 due to gas flows in cavity 56. Reduction of the edge temperature of the workpiece carried by dies 221, 241 relative to the temperature of the opposed workpiece may be desired when the workpieces are of different thickness or when the workpieces are of different alloy composition.

Cooling of the dies is provided by water passages in each of the upper and lower dies.

A single passage 97, 98 shown formed longitudinally in each of the upper dies 241, 281 was found to return the dies to initial die temperature of 13"C within 20 seconds of cessation of heating and hence before initiation of the next heating cycle. A pair of water passages 100, 102, shown formed longitudinally in each of lower dies 221, 261, in lieu of a single passage, can be provided if desired to accelerate cooling of the dies.

Workpieces 11 illustrated in Figure 9 may be formed by extrusion and each workpiece edge 66 has a nose configuration which comprises flattened tip 68 and bevelled side faces 70 to provide a cross-section with material thickness "T", root dimension "R", length dimension "L" and end dimension "E".

We have found that a ratio of L: R of at least about 1.1:1, preferably in the range of from 1.1:1 to 1.5:1, and a ratio of E:R of at least about 0.4:1, in the range of from 0.4:1 to 1:1, preferably about 0.5:1, are important to obtain satisfactory abutment of core material 72 with upset of surface oxides and softened metal as flash. Shoulders 86, 88 formed on each workpiece for engagement by the dies can be formed, as shown, one or both shoulders spaced rearwardly remote from the nose of the workpiece, in which case R could equal T, or formed on one side only of the workpiece.

Although it will be understood we are not bound by hypothetical considerations, we believe the nose configuration aids rapid and controlled heating of workpiece edges to be pressure welded to provide a temperature gradient such that the heated edge tip 68 and side faces 70 can be laterally deformed and upset, as shown with reference to Figures 6 to 8 and Figure 11, with abutment of cooler and harder inner core material 72 of each workpiece which has not had its mechanical properties adversely altered by the heat.

The steep temperature gradient provides a a peripheral zone of plastic material at relatively high temperature, i.e. up to 4250C, surrounding a core of harder material at lower temperature, i.e. up to 2300C. During the pressure welding process the harder core material functions as a "spear" and divides and expels the soft overheated material from the weld joint. The oxide layer coating the workpieces thus is ruptured and expelled by extrusion with overheated material softened by annealing during the heating stage into the flash cavity as flash 71, to preferably increase the interface thickness at least 1.6 times the cross-sectional area of the workpiece metal. The minimum L : R ratio of 1.1:1 provides sufficient material upset for the formation of a strong weld under forging pressures unaffected by deleterious materials. A ratio of L: R of greater than 1.5:1 would result in the abutment and expulsion of excess harder core material 72 with resulting slippage of dies and inconsistency of weld uniformity. The E:R ratio range of from about 0.4: 1 to 1:1, preferably about 0.5:1, provides the desired steep temperature gradient illustrated in Figure 9, with resultant desired flow characteristics shown in Figure 11.

In operation, a shaped workpiece 11, having a longitudinal edge on each side as shown most clearly in Figure 9, is fed into machine cavity 13 from the side while the machine is in its open position, i.e. upper die holders 24, 28 are in their raised positions and the plurality of laterally movable C-frames 16 have been retracted, to the left as viewed in Figure 2. With reference to Figures 5 through 8, one workpiece 11 is clamped between die holders 22, 24 of the stationary C-frames such that shoulders 82, 84 formed in dies 221, 241, respectively, can engage shoulders 86, 88 along one edge of workpiece 11. A second workpiece 11 is fed into cavity 13 to be clamped between die holders 26, 28 of the movable C-frame 16 in like manner, shoulders 90, 92 in dies 261, 281 adapted to engage shoulders 86, 88 along the opposed edge of the workpiece.

Upper die holders 24, 28 are lowered to clamp the two workpieces 11, as indicated in Figure 6, under a light clamping cylinder pressure of 14--21 kg/cm2 and the opposed edges of the workpieces forced together under light pressure of from 14-35 kg/cm2 of the forging cylinders to obtain alignment thereof relative to each other and to the forging dies. The final cylinder clamping pressure of about 155 kg/cm2, dependent on the nature of the metal welded and its thickness, is immediately applied to the die holders and maintained to the completion of pressure welding. Figure 10 illustrates graphically the steps of the method of our invention with reference to forging cylinder pressure, detent cylinder pressure, heat supply temperature and workpiece temperature. An increase of forging cylinder pressure will be noted during the positioning step of the cycle.

The detent cylinders 50 are then actuated at a pressure above 17.5 kg/cm2 sufficient for extension of guide pins 51, which function as stops in opposition to the forging cylinders, concurrent with reduction of the forging pressure, to space the workpieces a predetermined distance of from 0.8 mm to 2.5 cm apart, preferably 3 mm apart, to define an elongated open rectangular channel 19 shown most clearly in Figure 7.

Control valves to heater 54 are then opened, by solenoids not shown, to introduce a uniform flow of heating gases, such as oxygen and propane ignited by a pilot flame and combusted in situ, to rapidly heat the workpiece nose configurations as shown in Figure 9. Heating of the workpieces has been found to take typically 3 to 8 seconds for a metal workpiece thickness of 6 mm.

Heat times in excess of 8 seconds have often resulted in loss of metal properties, inconsistent welds and apparent oxide inclusions at the weld joints. Heat times of 4 to 6 seconds have resulted in a consistent and level hardness profile through the weld section with no loss of properties.

Detent cylinder pressure is then released and, substantially concurrent with cessation of heating of the workpieces, the forging pressure can be applied to eliminate the heating channel and bring the workpieces into abutment for about 2 to 5 seconds, normally about 3 seconds, with resultant pressure welding of the workpieces and simultaneous expulsion of flash 71 as shown in Figures 8 and 11 at an effective edge pressure of at least 3500 kg/cm2.

The forging pressure is then relieved, cylinder 32 retracted to raise upper die holder 24 and release the welded workpieces from the front dies 22', 241, and the forging cylinders 46 and movable C-frames 16 connected thereto partially retracted to separate dies 261, 281 from dies 221, 241 and to free the resulting panel from lower die 221.

Cylinder 32 is again actuated to pinch the panel between dies 22l 241 while rear dies 26l, 281 are separated by retraction of upper die 281 by raising die holder 28 through cylinder 40, and forging cylinders 46 then fully retracted to free the panel from die 261.

A walking beam table, not shown, raises the panel 1.3 cm inch above the lower dies 221, 261 and partially draws the panel out of the machine so that the rearward edge of the panel can be pressure welded to the next workpiece fed to the machine by a repeat of the forge welding cycle.

It is essential for successful pressure welding that machine alignment be maintained throughout each welding cycle, i.e. the front and rear lower dies be maintained in the same horizontal plane, each of the pairs of rear upper and lower dies and the front upper and lower dies be maintained in their respective vertical planes and symmetry of die faces be provided along the length of the machine. We have found the use of alignment rods 18 sliding within pillow blocks 17 effectively aligns the front and rear lower dies in the same plane. Vertical alignment of each pair of upper and lower dies is provided by the configurations of the 'stationary and movable C-frames and symmetry of die faces is ensured by abutment of front and rear dies with each other as indicated in Figure 8, or by abutment of lower die holders with each other, not shown.

Pressure welding of aluminum workpieces having "T" thicknesses of 3.5, 5.3 and 5.6 mm thickness has been conducted successfully on aluminum alloys designated 6063 T6, 6061-T6, 6351-T6, 5456-H111, 7005-T53 and 7075-T6 and, with the exception of 7075-T6, 100% joint efficiencies have been obtained throughout the weld. Joint efficiency of about 90% was obtained for the 7075-T6 alloy and tests have indicated 100% joint efficiency can be obtained for this alloy by increasing the forging capacity of the machine or reducing the cross section of the workpiece.

Tests conducted on the workpiece shown most clearly in Figure 9, extruded from the aluminum alloy 6061-T6, with shoulders 86, 88 formed thereon for engagement by the dies, as has been described, are exemplary of tests conducted on the aforementioned alloys. A shoulder 86, 88 of 1.5 mm thickness on each side of the workpiece, to provide a resulting root dimension R of 8.3 mm for material thickness T of 5.3 mm, resisted slippage of the workpieces in the dies and permitted satisfactory welds. Optimum expulsion of flash 71 with removal of oxides and of material heated over 260"C, as shown most clearly in Figure 11 was obtained by a length dimension L of 9.5 mm and end dimension E of 3.9 mm, providing an L:R ratio of 1.14:1 and an E:R ratio of 0.47:1.

The present invention provides a number of important advantages. Workpieces of high strength aluminum or magnesium can be pressure welded together by the method and apparatus of the invention to provide a solid-phase weld bond having physical characteristics substantially equal to the characteristics of the parent metal welded.

Not only are undesirable effects from the use of excess heat from conventional forge welding obviated, but weld strengths greater than the strength of the parent metals welded can be obtained. Heating and pressure welding of workpieces can be quickly effected in less than 10 seconds, with a complete welding cycle, including assembly and alignment of workpieces and removal of the finished panel, taking place in about one-half minute. Complex extrusions of either open or closed sections, having suitable welding edges, can be joined to form integral panels. Thus workpieces in desired shapes can be formed using relatively inexpensive extrusion presses and a multiplicity of shaped workpieces quickly pressure welded together to form continuous panels of desired length and structural configurations without loss of physical properties of the parent metal welded.

Panels formed of pressure welded aluminum extrusions have been successfully incorporated in dump trailers and dump bodies to increase load capacity by lowering vehicle weight.

Attention is drawn to the existence of British Patent No. 1,350,972.

WHAT WE CLAIM IS:- 1. A method of pressure welding together metal workpieces of aluminium or magnesium alloy which comprises: (a) juxtaposing a longitudinal edge of a first workpiece gripped between a pair of dies into substantially parallel spaced alignment with a longitudinal edge of a second workpiece gripped between a second pair of dies, (b) maintaining the longitudinal edge of the first workpiece between 0.8 mm to 2.5 cm away from the opposed longitudinal edge of the second workpiece so as to form an elongated open rectangular channel therebetween, (c) introducing a uniform stream of heated gases into the proximity of the opposed longitudinal edges and causing said stream of heated gases to flow through said elongated open rectangular channel, (d) maintaining the flow of heated gases through said elongated open rectangular channel for a time sufficient to heat and soften the surfaces of said opposed edges to a temperature within the range of from

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. panel between dies 22l 241 while rear dies 26l, 281 are separated by retraction of upper die 281 by raising die holder 28 through cylinder 40, and forging cylinders 46 then fully retracted to free the panel from die 261. A walking beam table, not shown, raises the panel 1.3 cm inch above the lower dies 221, 261 and partially draws the panel out of the machine so that the rearward edge of the panel can be pressure welded to the next workpiece fed to the machine by a repeat of the forge welding cycle. It is essential for successful pressure welding that machine alignment be maintained throughout each welding cycle, i.e. the front and rear lower dies be maintained in the same horizontal plane, each of the pairs of rear upper and lower dies and the front upper and lower dies be maintained in their respective vertical planes and symmetry of die faces be provided along the length of the machine. We have found the use of alignment rods 18 sliding within pillow blocks 17 effectively aligns the front and rear lower dies in the same plane. Vertical alignment of each pair of upper and lower dies is provided by the configurations of the 'stationary and movable C-frames and symmetry of die faces is ensured by abutment of front and rear dies with each other as indicated in Figure 8, or by abutment of lower die holders with each other, not shown. Pressure welding of aluminum workpieces having "T" thicknesses of 3.5, 5.3 and 5.6 mm thickness has been conducted successfully on aluminum alloys designated 6063 T6, 6061-T6, 6351-T6, 5456-H111, 7005-T53 and 7075-T6 and, with the exception of 7075-T6, 100% joint efficiencies have been obtained throughout the weld. Joint efficiency of about 90% was obtained for the 7075-T6 alloy and tests have indicated 100% joint efficiency can be obtained for this alloy by increasing the forging capacity of the machine or reducing the cross section of the workpiece. Tests conducted on the workpiece shown most clearly in Figure 9, extruded from the aluminum alloy 6061-T6, with shoulders 86, 88 formed thereon for engagement by the dies, as has been described, are exemplary of tests conducted on the aforementioned alloys. A shoulder 86, 88 of 1.5 mm thickness on each side of the workpiece, to provide a resulting root dimension R of 8.3 mm for material thickness T of 5.3 mm, resisted slippage of the workpieces in the dies and permitted satisfactory welds. Optimum expulsion of flash 71 with removal of oxides and of material heated over 260"C, as shown most clearly in Figure 11 was obtained by a length dimension L of 9.5 mm and end dimension E of 3.9 mm, providing an L:R ratio of 1.14:1 and an E:R ratio of 0.47:1. The present invention provides a number of important advantages. Workpieces of high strength aluminum or magnesium can be pressure welded together by the method and apparatus of the invention to provide a solid-phase weld bond having physical characteristics substantially equal to the characteristics of the parent metal welded. Not only are undesirable effects from the use of excess heat from conventional forge welding obviated, but weld strengths greater than the strength of the parent metals welded can be obtained. Heating and pressure welding of workpieces can be quickly effected in less than 10 seconds, with a complete welding cycle, including assembly and alignment of workpieces and removal of the finished panel, taking place in about one-half minute. Complex extrusions of either open or closed sections, having suitable welding edges, can be joined to form integral panels. Thus workpieces in desired shapes can be formed using relatively inexpensive extrusion presses and a multiplicity of shaped workpieces quickly pressure welded together to form continuous panels of desired length and structural configurations without loss of physical properties of the parent metal welded. Panels formed of pressure welded aluminum extrusions have been successfully incorporated in dump trailers and dump bodies to increase load capacity by lowering vehicle weight. Attention is drawn to the existence of British Patent No. 1,350,972. WHAT WE CLAIM IS:-

1. A method of pressure welding together metal workpieces of aluminium or magnesium alloy which comprises: (a) juxtaposing a longitudinal edge of a first workpiece gripped between a pair of dies into substantially parallel spaced alignment with a longitudinal edge of a second workpiece gripped between a second pair of dies, (b) maintaining the longitudinal edge of the first workpiece between 0.8 mm to 2.5 cm away from the opposed longitudinal edge of the second workpiece so as to form an elongated open rectangular channel therebetween, (c) introducing a uniform stream of heated gases into the proximity of the opposed longitudinal edges and causing said stream of heated gases to flow through said elongated open rectangular channel, (d) maintaining the flow of heated gases through said elongated open rectangular channel for a time sufficient to heat and soften the surfaces of said opposed edges to a temperature within the range of from

-93 'to 4800C while retaining a core of relatively cooler and harder material within each edge, (e) eliminating said elongated open rectangular channel by moving the longitudinal edges of said workpieces into abutting relationship with each other to obviate the flow of said heated gases therethrough, (f) immediately applying pressure to the abutting heated longitudinal edges of said workpiec'es while they are at a temperature -of between 93 to 4800C to create a solidphase weld bond between the cores and to upset a portion of heated adjacent surfaces and cause heated metal to be substantially displaced out of the plane of said pressure welded interface.

2. A method as claimed in Claim 1, wherein the longitudinal edge of the first workpiece is maintained 3 mm away from the longitudinal edge of the second work -piece so as to form the elongated open channel.

3. A method as claimed in Claim 1 or 2, wherein said metal workpieces are of high strength aluminium or magnesium alloy.

4. A method as claimed in Claims 1, 2 or 3, including juxtaposing the longitudinal edges of the workpieces by bringing the said edges of the workpieces into continuous abutment and into opposed alignment with 'each other and thereafter retracting one of the said workpieces a desired predetermined distance.

5. A method as claimed in any of Claims 1 to 4 including introducing said uniform stream of heated gases by discharging a combustible mixture of gases through a plurality of equispaced apertures positioned on one side of said workpieces adjacent the spaced opposed edges and igniting said -mixture of gases for heating.

6. A method as claimed in Claim 5 including introducing a uniform flow of the combustible mixture of gases along the plurality of equispaced apertures at 3 mm intervals to achieve uniform heating of the opposed edges.

7. A method as claimed in Claim 5 or 6 providing an air curtain between one of the dies and the plurality of equispaced aper tures for directing the combustible mixture of gases through the rectangular channel.

8. A method as claimed in any of Claims 1 to 7 including applying pressure to said abutting edges by forcing said edges together at a pressure of at least 3500 kg/cm2 where by upset metal causes a substantial increase in the interface thickness.

9. A method as claimed in Claim 8, wherein said increase of interface thickness is at least 1.6 times the original thickness of the workpiece metal.

10. A method as claimed in any of Claims 1 to 9 including maintaining the flow of heated gases through the elongated open rectangular channel for 3 to 8 seconds.

11. A method as claimed in any of Claims 1 to 9 including maintaining the flow of heated gases through the elongated open rectangular channel for 4 to 6 seconds.

12. A pressure welding press for joining two metal workpieces together by pressure welding comprising: (a) at least one pair of stationary C-shaped metal plate frames arranged in a parallel side-by-side spaced-apart relationship, (b) a movable C-shaped metal frame mounted between each pair of stationary C-shaped metal plate frames, (c) a first pair of die holders associated with each pair of stationary C-shaped metal plate frames which die holders are adapted to grip a workpiece therebetween, at least one of said die holders being mounted for reciprocal movement in a direction towards -and away from the other, (d) a second pair of die holders associated with said movable C-shaped metal frame which die holders are adapted to grip a workpiece therebetween, at least one of said die holders being mounted for reciprocal movement in a direction towards and away from the other, (e) means for moving said movable Cshaped metal frame between said stationary C-shaped metal plate frames whereby die holders associated with said laterally movable C-shaped metal frame can be moved towards and away from the die holders associated with said stationary C-shaped metal plate frames such that workpieces gripped therebetween can be spaced with opposed edges a fixed distance apart in alignment with each other prior to abutment of said workpieces for pressure welding, (f) means for heating the opposed edges of said workpieces to a uniform temperature within the range of from 93 to 4800C while said workpieces are spaced a fixed distance apart immediately prior to abutment for pressure welding, and comprising a heat source uniformly disposed along the length of the cavity formed within the Cshaped frames to achieve uniform heating of the said opposed edges, the heat source comprising a manifold assembly having a plurality of outlets disposed at equal intervals along its length and adapted to introduce a flow of a combustible mixture of gases.

13. A press as claimed in Claim 12 which additionally comprises means for introducing a curtain of air for directing the flow of heating gases towards the channel defined between the opposed edges of the workpieces.

14. A press as claimed in Claim 12 or Claim 13 in which said means for moving said movable C-shaped metal frame between said stationary C-shaped metal plate frames comprises a double-acting forging cylinder rigidly secured to the stationary C-shaped metal plate frames and operatively connected to said movable C-shaped metal frame, said press additionally comprising a detent cylinder rigidly secured to said stationary C-shaped metal plate frames and operatively connected to said movable C-shaped metal frame in opposition to the forging cylinder whereby the workpieces can be readily positioned a desired predetermined distance apart.

15. A press as claimed in any of Claims 12 to 14 in which each of said die holders has a die attached thereto for engaging a workpiece, each said die having a shoulder formed thereon for engaging a shoulder on the workpiece.

16. A method of pressure welding substantially as herein described with reference to the accompanying drawings.

17. A pressure welding press substantially as herein described with reference to and as shown in the accompanying drawings.