How does welding join metal?

1. Rosa is a welder. She needs to join two steel bars for a gate. Glue would be too weak, and bolts could work loose. She is going to melt the bars together.
2. Rosa wears a helmet with a dark window, thick gloves and a heavy jacket. She squeezes the trigger, and electricity leaps from a thin wire to the steel. This bright, hot spark is called an arc.
3. The arc melts the edge of each bar, and the tip of the wire too. All three flow together into a small glowing puddle called the weld pool.
4. Hot liquid metal soaks up air, and that leaves bubbles and weak spots. So the torch blows a blanket of special gas over the puddle to push the air away.
5. Rosa moves the torch steadily along the gap. The puddle follows the arc. Behind it the metal cools and hardens into a rippled line called a bead.
6. When it cools, there are no longer two bars with a gap. There is one piece of steel. A good weld is as strong as the metal around it.

The torch and the steel are cut open so you can see what happens. Tap each part.

• Welding helmet. The arc is far too bright to look at. The helmet's window turns dark the instant the arc starts, and the shell keeps sparks off the welder's face.
• Torch nozzle. The welder holds the torch and points its nozzle at the gap. The wire comes out of the middle and the gas flows out all around it.
• Wire. A motor pushes a thin steel wire out through the torch. The wire carries the electricity, and its tip keeps melting off to add extra metal to the puddle.
• Arc. The arc is electricity jumping through the air from the wire to the steel, like a tiny bolt of lightning that stays on. It is hot enough to melt steel in a blink.
• Shielding gas. Gas from a tall bottle flows out of the nozzle and covers the puddle like a blanket. It keeps air out so no bubbles form in the hot metal.
• Weld pool. Right under the arc is a small puddle of liquid metal. It is made from the melted edges of both bars and the melted wire, all mixed together.
• Weld bead. Behind the torch the puddle cools and hardens into a rippled line called the bead. The ripples show where the puddle froze, a little at a time.
• Base metal. The parts being joined are called the base metal. Their edges must melt, or the new metal will only sit on top like a blob of wax.

Three pairs of bars are waiting: one set to low heat, one to medium and one to high. Drag the torch along the gaps, then cool them, cut them open and pull.

Experiment: Which heat makes the strongest joint in these bars?

1. Drag the torch along all three gaps, or press Weld.
2. Press Cool and test. Look at each joint cut open.
3. Watch the pull test. Which joints snap, and why?

What we found: With low heat the edges of the bars never melted, so the new metal just sat on top and the joint snapped. With too much heat the puddle fell right through and left holes. With medium heat both edges melted and mixed with the wire, and the joint was as strong as the bar itself.

Turn on JavaScript to try it yourself.

Weld three joints: thin sheet, a medium bar and a thick plate. Choose the heat for each one. All three must pass the pull test.

There are many ways to join metal. You can bolt it, glue it or squeeze it together with rivets. In all of those, the two parts stay two parts. Welding is different. It melts the edges of both parts so that they flow together, and when the metal cools there is only one part left.

To melt steel you need a lot of heat in a very small spot. An arc welder makes that heat with electricity. The machine pushes electric current down a thin wire, and the current jumps from the tip of the wire to the steel as a bright, hot arc, like a tiny bolt of lightning that stays switched on.

The arc melts the edges of the two parts and the tip of the wire into a small puddle of liquid metal. Gas blowing out of the torch keeps the air away from it, because air would leave bubbles in the metal. As the welder moves along, the puddle behind the torch cools and hardens into a rippled line called a bead.

Getting the heat right is the skill. With too little heat the parts do not melt, and the new metal only sits on top. With too much, the puddle falls through and leaves a hole. Welding is a job for trained grown-ups in helmets, gloves and jackets, because the arc is hot enough to burn and far too bright to look at.

Key words

Arc

Electricity jumping through a gap, making bright light and great heat.

Weld pool

The small puddle of melted metal under the arc.

Bead

The line of hardened metal left behind as the weld pool cools.

Base metal

The metal of the parts that are being joined.

Shielding gas

Gas blown over the hot metal to keep air away from it.

Heat input

How much heat goes into each little bit of the joint.

Check your thinking

1. How is welding different from gluing?

   Show the answer

   Welding melts both parts so they become one piece. The melted edges mix and harden into a single piece of metal.

2. Why must nobody look at a welding arc?

   Show the answer

   Its light is bright enough to hurt your eyes. The arc gives off very bright light. Welders use a dark filter, and everyone else looks away.

3. A welder uses too little heat. What does the joint look like inside?

   Show the answer

   New metal sits on top, but the bars underneath never melted together. If the edges of the bars do not melt, nothing mixes. The blob on top can snap off.

4. What is the gas from the torch for?

   Show the answer

   It keeps air away from the melted metal. Air would leave bubbles in the hot metal. The gas pushes the air away.

5. Which needs more heat to weld: thin sheet or a thick plate?

   Show the answer

   The thick plate. Thick metal carries heat away quickly, so it takes more heat to melt its edges.

6. At 20 volts, which settings give the highest heat input?

   Show the answer

   200 amperes at 4 millimeters per second. 20 times 200 divided by 4 is 1,000 joules per millimeter. The other two both give 500.