There are many process parameters affecting the weld quality, strength and mechanical characteristics. The parameters need to be selected according to the kind of elements to be welded, paying special attention to the preparation of elements before welding. The welding current flow, and hence resistance heating, must take place on the whole contact surface of the welded elements. In order for the phenomenon to be correct, it is very important to prepare the elements before welding. Any contaminations on the sections of welded elements and inappropriately matched contact surfaces cause non-uniform current flow, non-uniform contact heating, local overheating and oxidation of a part of the welded section. If the contact surface is too small or limited by contamination (grease, rust), the current does not flow uniformly and fixing areas are overheated, which is unfavourable for the welding process and for the welder.
Basic parameters of butt-upset welding include the following:
- current intensity
- pressure force
- fixing length
- upset allowance
- welding current flow time
|Welded material||Diameter range||Unit power [kVA/mm2]||Welding current flow density [A/mm2]||Welding current flow time [s]||Unit pressure [MPa]||Fixing length [l]||Upset allowance [mm] |
|C-Mn steel||0.5-5||0.12-1.5||20-900||0.1-15||5-50||(0.5-1.5)d||(0.15-0.7)d |
Recommended parameters of butt-upset welding
The relevant and optimum welding parameters are identified based on nomograms obtained according to our experience and tests performed on the same or similar welds. Producers often enclose nomograms with welders.
It is recommended to use upper values of current density or unit power and short welding times. It is determined by greater heat losses due to a higher circumference-section ratio at a decreasing bar diameter. The current flow intensity has a great impact on the welding speed and determines the weld formation mechanism. High intensity and short welding time are used for welding materials with good thermal conductivity (copper alloys, aluminium and its alloys, low-carbon steel). It is opposite for steel with good hardening capacity, where a long heating time is required.
An advantage of using high current intensity and short welding time is the weld self-cleaning of contaminations by their removal of the molten material as a flash. It contributes to the formation of a narrow welding zone and HIZ with a fine-grain structure.
The pressure force is selected during welding according to the type, shape and section of the materials to be welded. A current pressure force ensures uniform heating in the weld, proper metal upsetting in the weld area and pressing out of liquid metal with contaminants in a form of flash. If the pressure force is too low, adhesion occurs in the weld, while too low-pressure force may cause buckling or displacement of the welded elements against one another.
The fixing length affects the value of Rp resistance of the welded workpieces and the heat balance during welding. Too small contact surface between the welded elements and the electrodes causes their excessive heating, which deteriorates the process efficiency. If the fixing length is too high, the Rp resistance can become so high that the fixed workpieces will be subjected to overheating, buckling or displacement under welding pressure, which will make welding impossible.
The upsetting allowance (ΔS) is a value by which the workpieces will be shortened during welding. The allowance is a part of the material which becomes plasticised under resistance heating and pressure force. The parameters determine material deformation degree and size of the flash with which contaminations formed during welding are removed.
The welding time is a resulting parameter and depends on the upsetting allowance under ΔSp current and mean upsetting speed.
The upsetting speed (Vs) has an important impact on the weld quality. It is a function of workpieces heating speed and their mechanical characteristics. The welding speed is affected by the welded material resistance. In the majority of the currently produced butt-upset welders, the upsetting speed and welding time are adjusted directly by appropriate positioning of the limit switch.
Range of butt-upset welders from ASPA
Manual butt-upset welders from ASPA are used for welding bars with a circular, square or rectangular section, made of low-carbon and alloy steel, copper and aluminium, with a diameter ranging from 0.3 mm to 20 mm. A common feature of this group of welders is their simple construction, reliable operation and high quality of welds. The welders are featured with simple mechanical units for adjusting the welding current and pressure force, which facilitates setting the relevant welding parameters. Each welder comes with a weld annealing unit. The group of butt welders includes special welders such as ZDZ-3PW dedicated to welding of cutting tapes, and ZDZ-0.6S with a grinding machine to remove flashes.
Pneumatic butt-upset welders from ASPA are used for welding of bars with a circular, square or rectangular section, made of low-carbon and alloy steel, copper and aluminium, with diameters ranging from 2 mm to 25 mm. Their constructions are based on:
- welding transformers with the rated power P50% – 4,7,16 and 40 kVA;
- pressure units with the pressure force between 10 and 900 daN;
- ASM-911 type control system which allows an automatic compensation of changes in the welder supply voltage, programming and remembering eight welding technologies as well as troubleshooting.
Course and results of tests
Tests of welding 55 carbon steel bars with a diameter of φ 6.5 mm were performed using the ZDZ-2.5 device from ASPA.
The quality of butt-upset welds is evaluated based on the following:
- tensile strength test
- metallographic tests
- hardness arrangement measurement
The weld flash should be narrow and uniformly distributed along the whole weld circumference, which is the evidence of thorough penetration of the welded material. The tensile strength of butt-upset welds can be from 10% to 20% lower than the tensile strength of the original material. It is caused by a presence of contaminants in the weld and grain growth in the relatively wide heat impact zone (HIZ). When you weld carbon steel containing over 0.3% C or metals requiring heat treatment after welding, an additional flow of one or more current pulses is used. It causes material tempering in the heat impact zone, and consequently approximation of the weld structure to the original material and improvement of the weld quality. The samples of welded bars were subjected to a tensile strength tests. They confirmed that the welds had been made correctly.
The results confirm the high quality of equipment for butt-upset welding produced by Przedsiębiorstwo Aparatury Spajalniczej ASPA. The company has a long tradition of such equipment production and has been appreciated by domestic and foreign customers, which makes it a leader in its sector.
The paper was published in “Spajanie” magazine.