Stainless steel is easier to weld than aluminum alloy. Because the melting point of stainless steel is higher than that of aluminum alloy, which makes it more stable during the welding process, aluminum alloy is prone to the risk of being burned through due to its low melting point during the welding process. In addition, aluminum alloys can easily form an aluminum oxide film on the surface in the air. The melting point of the aluminum oxide film is much higher than the aluminum itself, making it more difficult to weld.

The weldability of metal materials refers to the ability of metal materials to obtain excellent welding joints under the conditions of using certain welding processes, including welding methods, welding materials, welding specifications and welding structural forms. If a metal can obtain excellent welding joints using more common and simple welding processes, it is considered to have good welding performance. The weldability of metal materials is generally divided into two aspects: process weldability and application weldability.

In fact, the welding performance of metal materials is determined by many factors, such as materials, processes, structures, etc., including the selected welding process and welding conditions.

Material factors

Materials include base metal and welding materials. Under the same welding conditions, the main factors that determine the weldability of the base metal are its physical properties and chemical composition.

Physical properties:

Factors such as the melting point, thermal conductivity, linear expansion coefficient, density, heat capacity and other factors of the metal all have an impact on processes such as thermal cycle, melting, crystallization and phase change, thereby affecting weldability. Materials with low thermal conductivity such as stainless steel have large temperature gradients, high residual stress, and large deformation during welding. Moreover, due to the long residence time at high temperature, the grains in the heat-affected zone grow, which is detrimental to the joint performance. Austenitic stainless steel has a large linear expansion coefficient and severe joint deformation and stress.

Chemical composition:

Among them, the carbon element has the greatest impact, which means that the carbon content of the metal determines its weldability. Most of the other alloying elements in steel are not conducive to welding, but their impact is generally much smaller than that of carbon. As the carbon content in steel increases, the hardening tendency increases, the plasticity decreases, and welding cracks are prone to occur.

Usually, the sensitivity of metal materials to cracks during welding and the changes in mechanical properties of the welded joint area are used as the main indicators to evaluate the weldability of materials. Therefore, the higher the carbon content, the worse the weldability. Low carbon steel and low alloy steel with a carbon content of less than 0.25% have excellent plasticity and impact toughness, and the plasticity and impact toughness of the welded joints after welding are also very good. Preheating and post-weld heat treatment are not required during welding, and the welding process is easy to control, so it has good weldability.

In addition, the smelting and rolling state, heat treatment state, organizational state, etc. of steel all affect weldability to varying degrees. The weldability of steel can be improved by refining or refining grains and controlled rolling processes.

Welding materials directly participate in a series of chemical metallurgical reactions during the welding process, which determine the composition, structure, properties and defect formation of the weld metal. If the welding materials are improperly selected and do not match the base metal, not only will a joint that meets the usage requirements not be obtained, but defects such as cracks and changes in structural properties will also be introduced. Therefore, the correct selection of welding materials is an important factor in ensuring high-quality welded joints.

Process factors

Process factors include welding methods, welding process parameters, welding sequence, preheating, post-heating and post-weld heat treatment, etc. The welding method has a great influence on the weldability, mainly in two aspects: heat source characteristics and protection conditions.

Different welding methods have very different heat sources in terms of power, energy density, maximum heating temperature, etc. Metals welded under different heat sources will show different welding properties.

For example, the power of electroslag welding is very high, but the energy density is very low, and the maximum heating temperature is not high. The heating is slow during welding, and the high temperature residence time is long, resulting in coarse grains in the heat-affected zone and a significant reduction in impact toughness, which requires normalizing treatment. improve. In contrast, electron beam welding, laser welding and other methods have low power, but high energy density and rapid heating. The high temperature residence time is short, the heat affected zone is very narrow, and there is no danger of grain growth.

Adjusting the welding process parameters and adopting other process measures such as preheating, postheating, multi-layer welding and controlling interlayer temperature can adjust and control the welding thermal cycle, thereby changing the weldability of the metal. If measures such as preheating before welding or heat treatment after welding are taken, it is entirely possible to obtain welded joints without crack defects that meet performance requirements.

Structural factors

It mainly refers to the design form of the welded structure and welded joints, such as the impact of factors such as structural shape, size, thickness, joint groove form, weld layout and its cross-sectional shape on weldability. Its influence is mainly reflected in the transfer of heat and the state of force.

Different plate thicknesses, different joint forms or groove shapes have different heat transfer speed directions and rates, which will affect the crystallization direction and grain growth of the molten pool. The structural switch, plate thickness and weld arrangement determine the stiffness and restraint of the joint, which affects the stress state of the joint.

Poor crystal morphology, severe stress concentration and excessive welding stress are the basic conditions for the formation of welding cracks. In the design, reducing joint stiffness, reducing cross welds, and reducing various factors causing stress concentration are all important measures to improve weldability.

Conditions of Use

Service conditions refer to the operating temperature, load conditions and working medium during the service period of the welded structure. These working environments and operating conditions require welded structures to have corresponding performance. For example, welded structures working at low temperatures must have brittle fracture resistance; structures working at high temperatures must have creep resistance; structures working under alternating loads must have good fatigue resistance; structures working in acid, alkali or salt media The welded container should have high corrosion resistance and so on. In short, the more severe the usage conditions, the higher the quality requirements for welded joints, and the harder it is to ensure the weldability of the material.

The weldability of steel mainly depends on the carbon content. As the carbon content increases, the weldability gradually becomes worse. Among them, low carbon steel has the best weldability.

This article discusses the weldability comparison of aluminum alloys and steel materials. If you would like to learn more, please contact a PROTO MFG sales representative.

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