Spot welding vs tack welding are now two most popular welding techniques for joining metal components. While both methods aim to fuse metals together, they differ in their principles, applications, and resulting joint characteristics. By distinguishing between spot welding and tack welding, we can choose the right technique for each specific project.

The article will introduce the primary differences between using spot welding vs tack welding, exploring their unique strengths and limitations in different materials and industries.

What is Spot Welding?

Spot welding is a resistance welding process that welds two or more metal sheets together. It works by applying pressure and electric current to the spot-weld point, and the heat is created by the metal’s internal resistance to the electric current.

The electric current and pressure are delivered through copper alloy electrodes that have tips to place on the opposite sides of the metal pieces. The resulting heat melts the metals, and then the electrodes squeeze the pressure on the molten metal to create a weld.

In other words, the spot welding process relates to modifying pressures, electrical inputs, and current durations. Different modifications can work for different metal types and thicknesses to create different spot-welding results. Below are the details:

  • Electrode Force: Substantial force electrode force squeezes the metal sheets together can form a quality weld. The more force, the lower the resistance with better contact, and less heat is created.
  • Squeeze Time: This is the interval between applying the electrode force and initiating the current flow. This delay time is essential to obtaining the proper electrode force and also supports electrode wear, arcing, and interfacial expulsion.
  • Welding Time: It is the time for active electric current to flow through the metal pieces that are computed by line voltage cycles. It is hard to determine welding time as it depends on the welding spot reaction.
  • Holding Time: This period takes place after welding time while the electrodes are still applied to the metal, allowing the weld nugget to solidify.

What is Spot Welding?

What is Tack Welding?

Tack welding is a temporary welding process used before the final welding. It involves using low heat and a very short arc to join metal pieces together temporarily. The purpose is to hold the metal parts in place so they don’t move during the main welding process. Without tack welds, it would be extremely difficult to weld accurately to the required specifications.

There are different types of tack welding, each suited for specific situations:

  • Standard Tack Weld: This is ideal when the final weld needs support. The tack weld is large enough to handle the weight of the materials and prevent them from breaking apart.
  • Bridge Tack Weld: This is effective in bridging the gap between the materials after they are fitted together, often due to cutting mistakes or material distortion.
  • Thermit Tack: This involves mixing magnesium metal, aluminum powder, and iron oxide powder to create an extremely hot reaction above 4000°F.
  • Hot Tacking: Excess filler metal is deposited in the gap area between parts to bridge the gap when contact is needed, but other factors prevent it.
  • Ultrasonic Tack: An electric or gas welding machine creates several rapid push-pull motions at ultrasonic speeds to join the parts temporarily.

What is Tack Welding?

Differences Between Spot Welding vs Tack Welding

Below is the comparison table presenting the main differences between spot welding vs tack welding:

Spot Welding Tack Welding
Purpose A permanent joining method that is suitable for thin metal sheets with no need for filler material. Temporary joining technique to position workpieces with correct alignment for final welding.
How it works Spot welding applies pressure and heat to the weld area through shaped alloy copper electrodes, which transport an electrical current through the weld pieces. The melt material fuses the parts together while the electrodes apply pressure, and the molten nugget solidifies, finishing the joint. Tack welds are often small beads spaced out along the workpieces. They are quite similar to final welding, but they employ a smaller stud or torch and electrode to put small metal dots on the workpiece.
Weld strength Welds are strong and permanent, which are not easy to adjust. Welds are temporary and can be reapplied for correct alignment.
Weld appearance The resulting welds are small and localized. The final weld is normally inconspicuous.
Weld positions Work in different positions, including flat, vertical, and overhead. Normally only work in flat or horizontal positions.
Application Widely applied in manufacturing and automotive industries Best to apply for precise work and small projects
Filler material The spot welding process does not need flux or filler material to create a joint. Do require filler material to ensure the quality.
Equipment Specialized spot welding machines with electrodes are necessary. Can work with various welding equipment, including MIG, TIG, or arc welders.
Distortion Spot welding requires higher heat input, so causing more distortion or warping of the metal components, especially for thin or delicate materials Tack welding reduces distortion by forming small, localized welds with less heat input.

Differences Between Spot Welding vs Tack Welding

Tips to Assess Quality of Spot Welding and Tack Welding

How to evaluate the quality of the spot welding vs tack welding process? They might require different techniques and practices.

Spot Welding Quality Assess

Spot weld quality can be controlled by both destructive and non-destructive methods. Destructive testing is more popular but requires more time and expenses than non-destructive testing. In the destructive testing for high-strength steel sheets, the metal sheets are destroyed after the assessment.

Below are the common methods of spot welding quality assessment:

  • NDT of spot welds
  • Defects in spot welds
  • Automatic quality control of spot welds
  • Visual Inspection
  • Penetrant testing
  • Eddy Current Testing
  • Ultrasonic Testing
  • Magnetic particle testing
  • Radiography testing (X-ray)
  • Acoustic emission
  • Digital shearography
  • IR-thermography

Tack Welding Quality Assess

These evaluations provide valuable solutions for the strength and integrity of welds:

  • Visually inspect: Recognize any defects or anomalies in the weld bead.
  • Liquid Penetrant: The liquid penetrant testing is applied to the weld surface to check for cracks or discontinuities.
  • Magnetic particle: Use magnetic particles to inspect surface and near-surface defects in ferromagnetism materials.
  • Ultrasonic: This testing employs high-frequency sound waves to detect internal inconsistencies of flaws with weld joints.
  • Tensile: A tensile force is applied to the weld specimen to ascertain its final tensile strength and elongation properties.
  • Macroscopic: Cut and polish welds to be able to see their internal structure, identify defects, and recognize any inconsistencies or discrepancies.

Tack Welding Quality Assess

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Fischer ASIA is backed by state-of-the-art equipment, quality control measures, and a commitment to adhering to industry standards for both spot welding vs tack welding. We can handle welding for prototypes, small-batch production, or even large-scale manufacturing in efficient, reliable, and cost-effective solutions.

Consult with our welding experts today to discuss your specific requirements and explore the best welding solutions!

FAQs About Spot Welding vs Tack Welding

This final section will provide answers to frequently asked questions about two welding methods: tack welding vs spot welding.

  1. How to ensure the quality of tack welding?

To enhance the quality of tack welding, pay attention to the direction and sequence of the welds to reduce distortion. The resulting welds must be strong enough to resist transverse shrinkage and form adequate weld penetration. Post-weld heat treatments, high heat input efforts, and preheating are also essential to prevent cracks.

  1. How to ensure the quality of spot welding?

The first thing to consider is the material grade and surface coatings (like type and thickness) should meet specifications. The material should also be free from contamination like dirt or rust. Another important factor is the presswork or how well the parts fit together. Poor fit-up of the parts or having the sheet edges too close together can lead to poor or inconsistent weld quality.

  1. What can weaken a weld?

The three main things that can make a weld weaker are:

  • Impurities – Having contaminants or unwanted materials in the weld area.
  • Improper heating – Not applying the right amount of heat during the welding process.
  • Porosity – Having tiny holes or pockets in the weld.

Over time, spot weld vs tack weld can also become weaker because of oxidation (rust/corrosion) on the metal surfaces.