EP0732419A1

EP0732419A1 - Re-melt processing method on metal surface

Info

Publication number: EP0732419A1
Application number: EP95200610A
Authority: EP
Inventors: Ken Isuzu Viewlight Kohoku Moizumi
Original assignee: Isuzu Motors Ltd
Current assignee: Isuzu Motors Ltd
Priority date: 1995-03-14
Filing date: 1995-03-14
Publication date: 1996-09-18

Abstract

In a method of the invention, a metal plate is improved by applying arc on an outer portion for melting a part of the outer portion, and quickly cooling and solidifying the molten part. In the method, the arc is moved by a combination of a first movement in a direction of processing, and a second cyclic movement parallel to an outer surface of the outer portion and oriented in a direction different from the processing direction. Thus, a part of the outer portion is melted for a width more than a diameter of the arc. It is possible to accurately form the width and to suppress generation of blowing holes. Also, the processing can be made effectively by using an inexpensive TIG torch.

Description

Background of the Invention and Related Art Statement

The present invention relates to a re-melt processing method for locally improving characteristics of a part of a metal surface portion, such as an aluminum alloy cast.
A product with a complicated shape, such as a cylinder head, is generally formed by casting. However, the casting product may contain an inner defect, such as pores, and its texture is fairly large. Thus, it can not form products excellent in strength against fatigue when comparing with the products made by forging and pressing.
In order to improve the strength against fatigue in the casting products, the inner defects may be reduced, and crystal grains or particles may be formed finely. In order to form minute crystal grains, cooling speed when solidifying a molten material may be increased. However, when the cooling speed is increased, shaping ability of the molten material, such as flow and run of the molten material, is reduced, and the inner defect is solidified and remains inside the product.
It was proposed in Japanese Patent Publication (KOKAI) No. 61-193733 that in forming a cylinder head by an aluminum alloy, after the cylinder head is formed by casting, high density heating energy, such as TIG arc, laser beam, electron beam or plasma arc, is applied to a part where high strength against fatigue is required, to re-melt the surface of the part followed by immediate cooling, so that quality of the part of the cylinder head is improved. The local surface improving method is called as "re-melt processing".
In Japanese Patent Publication (KOKAI) No. 61-193733, it was disclosed that high density heating energy is applied to a necessary part of an aluminum alloy cylinder head for an internal combustion engine to re-melt the part. As a result, the defect structure formed inside the part, such as tiny holes, is pushed outwardly, and the metal crystal grains are divided finely by quick cooling, so that the mechanical characteristics are improved.
In the re-melt processing, metal is melted generally by TIG arc. This is because a machine for TIG arc is inexpensive, and generation of holes by blowing at the time of melting the metal is not so great. However, the re-melt processing by TIG arc has a disadvantage of accuracy of the size or width. Especially, a product having a complicated shape like a cylinder head must be formed precisely because of the size limitation by valve sheets and a water jacket.
Figs. 7A and 7B show limitation of the size for the cylinder head. Fig. 7A is an explanatory view for showing a surface condition of valve portions of the cylinder head, wherein a re-melt processing portion 31 contains a size limiting portion requiring high accuracy, as indicated by an arrow, at a middle portion between an exhaust valve sheet 32 and an intake valve sheet 33. Fig. 7B is an explanatory view showing a section where a water jacket in the cylinder head is formed, wherein a re-melt processing portion 31 has a size limiting portion in the direction of depth, as indicated by an arrow, according to the thickness 35 of the cylinder head defined by the shape of the water jacket 34.
In order to improve accuracy of the size by the TIG arc molten method, it is required to increase the density of an energy beam from a heat source to be used. Namely, the applied heat energy must be completely used for melting the metal without wasting. The diameter of arc generally used is 5-8 mm. In case the re-melt processing is made for forming width more than 5-8 mm, the diameter of arc may be increased, but it is difficult to accurately form the size.
Figs. 8A and 8B show a relationship between high and low densities of the energy beam from the heat source and the condition of the molten portion corresponding thereto. In Fig. 8A(a), a curve 36 showing an energy distribution discloses that the diameter of arc is small, and the density of the energy beam is high. The shape of the molten portion 38 obtained by the above is shown in Fig. 8A(b), by which it is shown that a heat energy by radiation of heat is not wasted, and accuracy of the size is kept constant. In Fig. 8B(a), a curve 37 showing an energy distribution discloses that the diameter of arc is large, and the density of the energy beam is low. The shape of the molten portion 39 obtained by the above is shown in Fig. 8B(b), by which it is shown that a heat energy by radiation of heat is likely to be wasted, and it is difficult to keep the accurate size.
In the heat source for high density energy beams, there are laser beam, electron beam and so on. In the density of the energy beams, while TIG arc has 10^-4-10⁵ W/cm², the laser beam has more than 10⁷ W/cm², and electron beam has more than 10⁶ W/cm².
However, the machines for these heat sources are expensive, and in addition, a large number of blowing holes may be generated according to contents in a product where re-melt processing is made. Since the blowing holes largely affect heat tearing, the blowing holes should not be formed. Now, this remains as a serious problem.
Also, as a method of improving accuracy of the size in the TIG molten method, there is a pulse molten method. In the pulse molten method, an input electricity for an energy supplied to a torch or electrode is in a form of pulse with a predetermined cycle. When peaks of electricity of the pulse flow, a portion to be melted is melted, while when bases of electricity flow, the torch is moved and the molten portion or molten pool is cooled. Namely, the molten pools cyclically formed as spots are joined.
Figs. 9A and 9B are explanatory views for the pulse molten method. Fig. 9A shows a wave 40 of an input electricity in the form of a pulse to be supplied to the torch, wherein the peaks of electricity and the bases of electricity are alternately supplied to the torch. Fig. 9B shows a section of the part made by the re-melt processing by the pulse molten method. Molten beads 41a are formed by joining the molten pools formed according to the peaks of the electric wave 40 while the torch is moving. The molten shape is shown in section 41b, wherein a boundary 41c between the molten layer and a main material not melted has a wave shape.
In this method, since the melting is made instantaneously, efficiency is good, and the size is accurate. However, there are following defects:

(1) The boundary between the molten layer and the main material not melted becomes a wave shape.
(2) Since the depth of melting at the peak of electricity is shallow, a large electric source capable of supplying large electricity than that required for the regular TIG method is required.
(3) Troublesome noise is generated.
(4) The width of the molten beads is narrow.

As a countermeasure for the wave shape at the boundary between the molten layer and the main material not melted, cycle of a pulse may be increased. However, if the cycle is increased, the depth of melting becomes shallower, and the width of the molten bead is narrower. Further, a large noise is generated. Also, if the cycle of the pulse is increased too much, sufficient cooling can not be made. Thus, the melting like a spot can not be made, and it is almost the same as the continuous regular TIG melting.
As a countermeasure for the shallow molten depth, moving speed of the torch may be reduced without increasing the electricity. However, the melting time is prolonged, so that heat efficiency which is the basic object is lowered, and the processing time is increased.
The present invention is directed to a re-melt processing method by applying high density energy onto a metal surface while moving, re-melting the metal surface and quickly solidifying the molten surface to thereby locally improve the characteristics of the surface layer, wherein an object of the invention is to provide a re-melt processing method effectively using an inexpensive TIG arc method, and wherein when the processing is made for a size more than the width of arc forming high density heat energy, accuracy of the size is maintained, and generation of blowing holes is suppressed without using expensive machine, such as laser beam, electron beam or TIG pulse method.

Summary of the Invention

In the present invention, a metal surface portion is re-melted by applying TIG arc followed by quick solidification to thereby improve the surface portion, wherein when the TIG arc is moved, the arc is moved by a combination of a cyclic movement which is parallel to the metal surface and is oriented in a direction different from the moving direction of the processing, and a movement in the moving direction of the processing, so that the processing is made for a size more than the width of the arc.
In the invention, it is possible to use inexpensive TIG arc with a diameter or width smaller than an area for the re-melt processing. Also, heat radiation is small, and the accuracy of a size is well maintained. Further, melting and cooling are quick. Thus, crystal grains or particles of the metal texture are apt to be finely divided to thereby increase the advantages of the re-melt processing.
Also, since the molten portion or pool is agitated by the combined movements by the arc, harmful inner blowing holes in the processed portion float to the surface to thereby remove the blowing holes.

Brief Description of the Drawings

Fig. 1A is an explanatory view of a first embodiment of the present invention;
Fig. 1B(a)-1B(c) are explanatory views for showing changes of offset amounts;
Figs. 1C and 1D are explanatory views for showing changes of trails of a torch;
Fig. 2 is an explanatory view for showing a trail of a torch of the first embodiment;
Fig. 3 is an explanatory view of a second embodiment of the invention;
Fig. 4 is an explanatory view of a third embodiment of the invention;
Fig. 5 is an explanatory view of a fourth embodiment of the invention;
Figs. 6A and 6B are explanatory views of a fifth embodiment of the invention;
Figs. 7A and 7B are explanatory views for the shapes of the processed portions;
Figs. 8A and 8B are explanatory views for showing an arc energy distribution and a melting portion; and
Figs. 9A and 9B are explanatory views for showing an electric wave and a melting portion in the prior art.

Detailed Description of Preferred Embodiments

Figs. 1A and 1B are explanatory views of a first embodiment for re-melt processing of the present invention. Fig. 1A shows a re-melt processing applied to a work 2 by using a robot 3 for moving a TIG torch 1.
The TIG torch 1 is attached to a tip of a robot arm. One output of an electric source 8 is fixed to the TIG torch 1 for providing arc, and the other output is attached to an earth 4 for the work 2. The TIG torch 1 is moved relative to the work 2 along a trail or path taught already by means of the robot controlled by a control device 7 for the robot, so that re-melt processing is performed.
In the robot, the trail or path to a point inputted already is changed to numerical values, and the robot is moved to the point or plot (x, y, z) changed to the numerical values. In this machine, a voltage with a sine wave generated by an alternating current oscillator 5 is outputted in an analogue form, which is changed to digital value by a calculator 6, such as a computer. Thereafter, the value is inputted to the control device 7 for the robot as an offset amount Δx in the direction of an x-axis, and the plot that the robot must be drawn or moved is changed to (x+Δx, y, z).
Fig. 1B is an explanatory view for showing changes to an offset amount Δx to be inputted to the control device 7. Fig. 1B(a) shows a voltage wave generated by the alternating current oscillator 5, wherein the ordinate shows a voltage and the abscissa shows a time. Fig.1B(b) shows a relationship between the generated voltage V from the alternating oscillator and the offset amountΔx. It indicates that when the equation shown in the oblique line is defined, the generated voltage V can be changed to the offset amount Δx (mm).
In Fig. 1B(c), the ordinate shows the offset amount Δx, and the abscissa shows a time. It indicates that the voltage waveform generated by the current oscillator 5 is changed to the offset amount. The voltage waveform is changed to the offset amount Δx by means of the calculator 6.
Thus, the plot at the tip of the robot arm, i.e. the plot of the TIG torch 1, is controlled by the control device 7 with reference to the offset amount obtained from the calculator 6. For example, the plot changes from a linear line 9 from a beginning point a to an end point b as shown in Fig. 1C to a zigzag line 10 from a beginning point c to an end point d as shown in Fig. 1D.
In the present invention, the alternating current oscillator 5 may be installed inside the control device for the robot. On the other hand, analogue data which can be freely set by the robot may be outputted to an outer calculator, such as a computer, and the setting of an oscillator, such as oscillating width of voltage, cycle and duty (output characteristic), may be controlled by the data. Accordingly, it is possible to perform re-melt processing while changing the zigzag width by the torch in one molten bead.
Next, actual embodiments are explained.
A re-melt processing is performed for a portion between lower valve sheets of a cylinder head formed by casting an AC2B material (aluminum cast material by Japanese Industrial Standard) in accordance with a TIG torch of the present invention. The trail for this processing is shown in Fig. 2. Arc for the TIG torch is started at a point a, and in order to stabilize the arc, offset input signals from the oscillator are cut to a point b. The offset input signals are applied to the control device from the point b to change the trail 10 of the TIG torch in the zigzag form. When the torch reaches a point c, the offset input signals from the oscillator are stopped. The trail of the TIG torch is returned in the condition before the input signals, and the processing is continued to a point d.
In the portion of the zigzag form, the TIG torch has the width of 2.5 mm, cycle of 2 Hz and duty of 50%. Also, the re-melt processing was made in three stages such that electricity in the operating section is changed 350 → 430 → 300 (A), and 44.5 → 45 → 45 (V), while the speed of the torch is changed 8 → 7 → 11 (m/min). The favorable results were obtained.
In the present invention, the movement of the arc is fast due to its trail, so that melting and cooling are repeated fast. Thus, it is possible to improve accuracy of the size at the molten portion by preventing application of unnecessary heat to the molten portion. In the conventional linear TIG re-melt processing, the size accuracy is ± 4 mm in width and ± 3 mm in depth, while in the present invention, it is possible to form within ± 1 mm in both width and depth.
In the first embodiment, the TIG torch is moved by the electric signals, but it is possible to mechanically move the TIG torch. Figs. 3, 4 and 5 show the embodiments thereof.
In Fig. 3, a torch 1 having a cam follower and its moving device (both not shown) is formed near a surface of a cam 11, so that a trail 10 moving along the surface of the cam can be obtained.
In Fig. 4, a pair of a pinion 12 and a gear 13 engaging together is used, and a TIG torch 1 is formed at a bottom of a tooth of the gear 13. The gear train is held and is rotated while moving, so that the torch 1 can draw a trail 10 as shown in Fig. 4.
In Fig. 5, a cam 15 rotates around an axis 16, and a follower bar 14 abutting against the cam 15 moves parallel. A TIG torch 1 is movably situated on the follower bar 14. When these members are assembled and are moved properly, a trail 10 as shown in Fig. 5 can be drawn.
Further, since the TIG arc is a flow of electricity, it is possible to vibrate the arc by a magnetic field generated by an electric magnet. Figs. 6A and 6B show the explanation thereof.
In Fig. 6A, 17 is an electrode for TIG arc, and the arc is ejected in the direction perpendicular to the sheet of the drawing. Electricity is applied to a coil 19 for the electric magnet, so that magnetic flux is generated in the direction 20a. By the influence of the magnetic field generated by the magnetic flux, the arc flowing from the electrode biases as shown in 18a. Fig. 6B shows a condition that the electric flow direction applied to the coil of the electric magnet is made opposite as shown in Fig. 6A. The direction of generated magnetic flux 20b is opposite, so that arc 18b biases in the opposite direction relative to the electrode.
Accordingly, when the amount and direction of electricity applied to the coil, and retaining and movement of the coil and the TIG torch are controlled properly by a machine (not shown), it is possible to allow arc of the TIG torch to draw a desired pattern or trail.
In the re-melt processing for locally improving a surface portion in the present invention, when a processing is made for the width more than the diameter of the arc or electrode, it is possible to accurately form the width and to suppress generation of the blowing holes. Also, the processing can be made effectively by using an inexpensive TIG torch.

Claims

A method of improving a metal plate by applying arc on an outer surface for re-melting a part of the outer portion, and quickly cooling and solidifying the molten part,
said method being characterized in that the arc is moved by a combination of a first movement in a direction of processing, and a second cyclic movement parallel to an outer surface of the outer portion and oriented in a direction different from the processing direction, to thereby melt a part of the outer portion more than a diameter of the arc.
A method of improving a metal plate according to claim 1, wherein said arc is TIG arc.
A method of improving a metal plate according to claim 1, wherein said second movement is directed perpendicular to the first movement.
A method of improving a metal plate according to claim 1, wherein the arc is moved on the metal plate so that a re-melt strip having a width more than the width of the diameter of the arc is formed.
A method of improving a metal plate according to claim 4, wherein the arc is ejected from a torch, said torch being mechanically moved in the direction of the second cyclic movement.
A method of improving a metal plate according to claim 4, wherein the arc is ejected from a torch, said arc ejected from the torch being cyclically oriented by a magnetic field.
A method of improving a metal plate according to claim 2 wherein said arc is moved by a magnetic field.
A method of improving a metal plate according to any one preceding claim wherein said metal plate initially has a cast structure.