Different materials are spot welded together?

The immediate reply is yes, but it may be neither easy nor advisable in certain cases if brittle structures are produced by the different metals mixing in the molten nugget.

In exacting austenitic stainless steel and carbon steel is not generally spot welded jointly since the resulting nugget arrangement risks being hard and brittle, though it can be studied and modified using the Schaeffer diagram and particularly conceived warmth treating cycles.

Materials having widely dissimilar properties need that a warmth balance be achieved by compensation. The additional conductive material, electrically and thermally, have to be heated extra as it provides fewer resistive warmth, and the warmth is lost more easily by conduction.

An ordinary method uses an electrode of smaller face diameter and superior resistivity facing the additional conductive material, or by inserting a foil of poorly conductive material among them.

Concerning the number of sheets weldable with a single nugget, usual practice advices not more than three layers, although four sheets are infrequently spot welded jointly. In any case the ratio of the thickest to the thinnest sheet should not go beyond three.

Arc, Spot Welding Techniques:

Welding has become an integral part of the lives of many people. Just like there are various different welding machines available, all the way from Diesel Welders to Pro-Arc's, there are also numerous welding techniques that can be used. In this article, we will cover some of the popular welding techniques like:

(1). Arc (2). Spot

Here, we will talk about why Arc welding is important. Arc welding is usually done by making use of a diesel welder. In arc welding, high electric voltage is used, instead of a gas torch. The arc, or the spark, created from the electrode rod, used by the welder, is transferred from one metal to the other metal. This in turn causes the metals to heat up and melt together which creates a bond or a weld amongst them. Arc welding is known to create cleaner, smoother, accurate and stronger welds.

The second technique to be discussed is the Spot technique. In this method, the two metal sheets are held together tightly by the electrodes. The surface area of contact between the electrodes and the materials is very small, and hence it is called 'spot' welding. When a large amount of current is passed through the electrodes, it heats up the spots to the point of melting, which results in a sturdy weld. This technique ensures that the rest of the metal isn't heated up unnecessarily and also takes lesser time.

Instructions of Spot Welding:

1). Slide the sheet metal gauge onto one edge of the sheet metal to determine the thickness of the metal. When the sheet metal fits snug in a sheet metal gauge slot, read the number above the slot. This will show the thickness of the metal.

2). Set the hold, weld, squeeze and off-time of the spot welder by locating the thickness of the sheet metal on the welding chart affixed to the spot welder.

3). Clean the tips of both the top and bottom electrode by sliding the file over each tip. To weld correctly, the tip of each electrode should have no black carbon deposits.

4). Put on your leather work gloves.

5). Turn on the spot welder and open the cold water supply valve to flow water through the electrodes.

6). Stack the two pieces of sheet metal on top of one another.

7). Set the stacked pieces of sheet metal on the bottom electrode of the spot welder.

8). Depress the pedal of the spot welder to lower the top jaw of the spot welder and weld the sheet metal.

9). Twist the welded sheet metal a quarter-turn before releasing the pedal to avoid pulling a welding electrode from its socket.

10). Attempt to pull the sheet metal apart. If the metal bends rather than pulls apart, the spot welder is set correctly. If the spot weld breaks, increase the weld time of the spot welder and repeat the listed steps until you achieve a proper spot weld.

Inert Gas Welding and Spot Welding Difference:

Unlike inert gas welding, spot welders do not require a shielding gas to eliminate weld contamination or filler metal to build up the weld.

A spot weld is a resistant weld that sits within the two pieces of metal. The finished weld is flush or slightly recessed to the exterior of the base metals and has a silvery or coppery appearance. The color of the spot weld will vary by the alloy of the metal being welded and the type of electrodes used to create the spot weld.

An inert gas weld is built up in a gap between two pieces of metal. The finished weld is humped up above the base metal and is easily visible. The high amount of heat generated by inert gas welding can cause the base metal to warp and twist.

Spot Welder Advantages Limitations & Applications:

Advantages:

* The procedure involves smaller amount maintenance money.
* Ability of the worker does not influence the quality of spot welds obtained by this procedure.
* The process is usually free from burn and splash.

Limitations:

* To achieve small cost of building, the weld manufacture cost is not very high.
* The procedure proves uneconomical as compared to resistance spot welding where the latter are able to be simply employed.

Applications:

The use of physically operated arc spot welding gun is an ordinary thing in a variety of industries. Arc spot welding is particularly applicable for situations where resistance spots welding wants high pressure, high currents, does not have simple access and therefore fails to weld.

Arc spot welding torch be able to also be working to make holes in plates up to 3 mm broad or consequently. After the arc has impinged on the plate for a exact time, high force inert gas blows away the molten metal leaving a hole in the plate. Additional applications of, the process are in:

1. Automobile manufacturing.
2. Manufacture of sheet metal goods.
3. Assemble sheet metal to steel structures.
4. Manufacture of pulleys.
5. Joining killed carbon steel, low alloy steel, high alloy steel, titanium, etc.

Tips for Spot Welders:

Spot welding tips require be sizing and matching to the exact thing that is being welded. The many styles of tips permit for the welding of a lot of sizes and types of metal. The key to finding a best brand is finding a tip that has best conductivity and is completed from a very pure alloy. This will permit the tip to reach the maximum temperatures and not deform.

The most common spot welding tips types:

Truncated cone, radius faced angled straight tip, offset nose, sharp nose, dome nose and flat nose. Truncated cone tips seem like a 45-degree cone with the top compressed out. Radius face tips are rounded crossways the entire tip in a 45-degree arc. Angled straight tips are tips that come to a in a straight line point and contain a 30-degree viewpoint cut crosswise them. Offset tips have one in a straight line edge that come flat nose tip with 45-degree viewpoint. A point nose tip comes to a point with a flat welding face. A dome nose tips have a half circle tip with a very little welding face. Flat tips are totally flat with the complete tip being the welding face.

Spot Welder Parts:

A spot welder normally has just a small number of parts. These include the electrodes which are most usually complete of an alloy metal similar to copper or silver. These electrodes require to be clamped on a sheet of metal to apply the current. This is complete by handler or tool holds which place the electrodes. These can be very lengthy arms that are attached to a machine or automaton which can shift the electrodes approximately a surface. The additional thing a spot welder has is a coolant. This is typically water or several additional type of combination depending on the application. Since a spot welder generates a lot of power it creates a lot of high temperature and so wants coolant.

Spot Welding Overview

Spot welding is used to when full joins are not necessary, and particularly on plate and sheet metal, where the warping caused by other welding methods would be undesirable. Most metal framework are spot welded. Two electrodes, normally copper, are placed outside two directly abutted pieces of sheet. Electricity passes through the electrodes to the sheet, where the resistance of the metal guides to heat, causing melting and joining. Spot welding process is also the type resistance welding.

Spot Welder

Resistance spot welding delivers an electric current to join materials through heating. The four stages in the welding process are squeeze, weld, hold, and release. During the first stage the materials are brought into contact through physical force. The welding current is then delivered and the materials are brought to their plastic state. By holding the materials together after the weld pulse the weld nugget solidifies and then the force can be released. The more control a welding instrument provides over each of these stages the greater the quality of weld that can be produced. Ideally, an instrument allows the operator to program weld cycles with specified currents and pressures.

When executing a typical single-pulse weld, the current travels through the points of initial contact at the junction. Due to the presence of the oxide layers at this interface the current will flow through the peaks created by the oxide layer forming a brittle weld between those peaks. In a dual pulse spot weld, a pre-pulse, prior to the weld pulse removes the oxide layer from the materials and increases surface contact to create an effective weld.

Variables that must be controlled by spot-welding instruments are energy, time, and physical force. The new High-Frequency Direct Current inverter (HFDC) systems provide highly programmable instruments that use feedback loops to accurately deliver the specifications of a welding cycle. Controlling the position of the welding tips and the physical force applied to the weld site, are necessary to achieve quality welds. Hand-held weld heads rely on operators' ability to choose, and maintain throughout the weld cycle, the position of the tips and the level of the force. Technological improvements such as air-actuated electrodes control the force with which materials are brought into contact, reduce operator variability and increase reproducibility of quality welds. The required force is set by the operator and the weld-head is driven by an air cylinder that maintains the preset force. Application of the required force as materials change volume during the weld cycle is central to the production of strong spot-welds. Another advantage of the air-actuated weld held is the decrease in crystal damage because the polished crystal face is not placed down face down during welding. The operator orients the crystal and other metal between the tips of the electrodes and uses the bi-level foot switch to apply force to the materials and then deliver the weld cycle.

Welding Techniques

• Hold torch approximately 75-80 degrees from horizontal and the filler rod 15-20 degrees from horizontal.
  
  
• Hold the tip of the electrode about 1/8” away from piece and press the pedal down until an arc is created between the electrode and work piece.
  
  
• Keep peddle depressed until puddle is created.
  
  
• Ensure the tip of the electrode does not touch the molten weld puddle. This cause electrodes to become contaminated with weld material.
  
  
• After puddle is created use a circular motion or repeated crescent motion to move puddle while adding filler rod.
  
  
• When welding thinner materials a heat sink, consisting of a metal block, can be used to dissipate excess heat. Place this heat sink underneath material when welding.

Gas Tungsten arc welder

The team’s first experiments examined titanium welds. Titanium is popular in manufacturing because of its corrosion resistance and light weight. Also, titanium has two well-characterized solid-phase transitions at ambient air pressure before it melts. In pure titanium, the alpha phase exists from room temperature to 882°C. At these temperatures, titanium has a hexagonal-close-packed crystalline structure.

Using the experimental setup shown in the figure below, a metal bar rotates under a Gas Tungsten arc, taking 6 minutes for a full revolution. An intense x-ray beam from the synchrotron source passes through a pinhole to allow researchers to resolve features as small as 180 micrometers. During welding, the x-ray beam is aimed at specific points around the heat source. A silicon photodiode linear array detector records the diffraction patterns during the experiment.

Phase mapping experiments performed using the SRXRD method are useful for observing phase changes under quasi-steady-state heating and cooling conditions. The next step was to examine the changes that occur at a single spot as a function of time. Wong developed a time-resolved x-ray diffraction (TRXRD) technique that takes a set of x-ray diffraction patterns at a single location adjacent to or within a stationary spot weld.

When the detector is clocked for durations of tens to hundreds of milliseconds, phase transformation may be observed on a much shorter time scale than is possible with moving welds. Changes in the diffraction pattern show directly how phase changes are taking place as a function of time and temperature. As the temperature goes up and then down, the metal at the weld becomes liquid and then solidifies. With TRXRD, the Livermore team has been able to examine the solidification and subsequent solid-state phase transformations in a number of different materials for the first time.

Spot- Weldings

Spot welding is a type of resistance welding in which pieces of metals are pressed together and an electric current is passed through them. Spot welders are at risk of contact with some potentially hazardous agents but there are few studies about the respiratory effects of spot welding.

AIM: Our objective was to study lung function and respiratory symptoms among spot welders and office workers at an automobile assembly factory in Iran.

METHODS: This was a cross-sectional study of 137 male spot welders and 129 office workers. We used a questionnaire to record demographic data, smoking habits, work history and respiratory symptoms. Spirometry was performed to assess lung function status. Metal fume samples from the respiratory zone of spot welders were analysed.

The concentrations of metal fume were less than the American Conference of Industrial Hygienists (ACGIH) threshold limit values. There were significantly lower values for average forced expiratory volume in 1st second (FEV(1)), FEV(1)/forced vital capacity and 25-75% forced expiratory flow in spot welders compared to controls. There was also a significantly raised prevalence of respiratory symptoms (sputum and dyspnoea) in spot welding. Fifteen per cent of spot welders and 1% of controls had an obstructive pattern in spirometry.

CONCLUSION: Our survey suggests that spot welders are at risk of developing respiratory symptoms and decreasing pulmonary function values despite their exposure to components of welding fume being within ACGIH guidelines.