Welding, as the most common method of metal joining, is widely used across various industries. However, their failure due to compromised integrity and insufficient strength during operation remains a persistent headache for many procurement and supply chain managers. In such cases, investment castings may offer a satisfactory solution. So, under what circumstances do weldments fail? And when are investment castings a superior choice? This article aims to provide the answers.
What Is a Weldment and Why Is It Commonly Used
Weldments join two or more metal parts through welding to form an integrated structural assembly. Exceptional structural strength and manufacturing flexibility represent the most prominent advantages of welded components. Requiring no mold preparation, welded components offer outstanding cost-effectiveness for simple structural parts. Furthermore, weldments enable the combination of different materials to optimize performance in specific areas of a part. For instance, they can meet your requirements for localized corrosion resistance or wear resistance by substituting materials.
Why Are Your Weldments Failing
Although weldments perform well in simple structural parts, the inherent nature of the process limits their use in more complex structures and performance. These limitations constitute the primary causes of welded component failure. Below are common scenarios that may lead to failure in your welded components.
Embrittlement in the Heat-affected Zone
Since welding inherently involves remelting localized areas of the part, the material on both sides of the weld grain coarsens under high temperatures. This reduces the toughness and creates uneven hardness in the affected zone. Even if the weld itself possesses sufficient strength, the adjacent heat-affected zone—the weakest structural area in the entire assembly—is highly susceptible to brittle fracture under external impact or loading.
Stress Concentration Leading to Fatigue Cracks
Welding creates irregular surface profiles on weldments. The weld toe (the critical point where weld metal meets the base material) and inherent micro-undercuts on the weld surface act as natural stress concentration points. Under cyclic loading, these locations rapidly initiate fatigue cracks, causing sudden component failure at stresses far below the material’s yield strength.
Residual Stress Causing Deformation
Welding rapidly heats and cools localized areas, leaving significant internal stresses within the welded parts. Even when fixtures forcibly maintain shape during welding, these stresses may release upon fixture removal, causing unpredictable warping or distortion that prevents subsequent assembly. Alternatively, gradual deformation during service can lead to weldment failure.
Accumulation of Assembly Tolerances
Welded assemblies combine multiple components. Each part has its own tolerances, and tolerances inevitably accumulate during assembly and welding. To ensure precision in the final welded part, extensive secondary precision machining is required. This poses significant challenges for part consistency in mass production, potentially rendering identical replacement parts unusable due to differing tolerances.
Hidden Internal Defects
Unlike casting’s single-step forming and solidification process, each weld in a weldment carries the risk of defects such as porosity or slag inclusions. These defects require 100% radiographic testing (RT) or ultrasonic testing (UT) for detection; otherwise, they may cause sudden failure of the weldments.
Reduced Corrosion Resistance
Since the chemical composition of weld metal typically differs from the base material, welds are more susceptible to electrochemical reactions in humid or corrosive environments. This accelerates material corrosion in the weld zone (known as weld corrosion), ultimately leading to weldment failure.
What is Investment Casting
Investment casting, also known as lost-wax casting, is a process that involves pouring molten metal into an expendable ceramic mold. Once the metal solidifies and cools, the desired casting is obtained. This method is typically used for producing complex metal components, making investment castings widely utilized in aerospace, automotive, medical devices, and construction machinery industries.
Key Advantages of Investment Casting
After understanding the common failure causes of welded components, I will briefly outline the advantages of investment castings to provide you with a clearer comparison of their performance.
Monolithic Construction
Investment casting, as an expendable mold casting process, produces castings with the characteristic of being formed in one piece. Even parts with complex geometries and intricate structures can be cast in a single operation through investment casting, thereby reducing the need for subsequent machining.
Design Flexibility
This represents the most prominent advantage of investment casting. Whether it involves intricate internal flow channels, undercuts, thin walls, or other part designs challenging to achieve through welding, investment casting handles them with ease. This provides engineers with significant design freedom, opening up more possibilities for project realization.
Various Materials
The production of investment castings is essentially unrestricted by material type. Whether non-ferrous metals (aluminum, zinc, copper), ferrous metals (stainless steel, cast iron), or even high-temperature alloys, all can be utilized to manufacture components through the investment casting process.
Product Strength
Investment castings form continuous, monolithic metal structures with relatively uniform internal microstructures. This ensures consistent distribution of properties like strength, hardness, and toughness, minimizing failure risks due to localized material weakness during use.
Surface Finish
Castings produced via investment casting achieve surface roughness levels of Ra 1.6–3.2 μm, approaching the surface quality of die-cast components. This significantly reduces the need for subsequent machining, with some precision castings even ready for direct use.
Strict Tolerances
Investment casting enables castings to meet tolerance grades of CT4–CT6, representing a major advantage. This high precision satisfies the dimensional requirements of multiple industries.
Weldments vs Investment Castings
Material Consistency
Weldments inherently exhibit material inconsistencies due to process limitations. Discrepancies in material properties within welds, heat-affected zones (HAZ), and surrounding wood serve as origins for electrochemical corrosion and stress cracking.
In contrast, investment castings demonstrate greater isotropy. Their uniform microstructure results from controlled cooling conditions, yielding more stable performance characteristics.
Product Integrity
Weldments consist of multiple sections joined together. The greater the number of welds, the higher the risk of leakage. This places stringent demands on welder skill, particularly in pressure-bearing applications.
The advantage of monolithic construction means investment castings feature seamless surfaces without parting lines. For components and applications requiring airtightness, such as hydraulic valve bodies and pump housings, investment castings offer a more reliable solution.
Wall Thickness
Weldments face limitations imposed by standard profile specifications and strength requirements (achieved by increasing plate thickness), making it difficult to meet thin-wall or custom wall thickness demands.
Investment castings can achieve wall thicknesses as low as 1.5mm – 2mm and enable variable wall thicknesses. This allows for smooth transitions from thick-walled to thin-walled sections, distributing material according to functional needs.
Tolerance
Each new component added during welding introduces additional tolerance variations. Through cumulative processes (including cutting, bending, and welding distortion), tolerances often reach ±1.0mm or higher. Secondary machining is typically required post-welding to achieve target tolerances.
Investment castings achieve “close-to-net-shape” precision, with typical tolerances at CT4-CT6 levels. Many investment castings are ready for use without secondary machining.
Surface Finish
Welded joints typically exhibit rough surfaces with fishscale patterns, spatter, and undercut marks. These imperfections not only compromise aesthetics but also trap contaminants or corrosive agents. To enhance appearance and performance, welds are usually manually ground flush with the base material.
Investment castings exhibit typical surface roughness of Ra 3.2–6.3 μm, with some achieving Ra 1.6 μm. Their smooth appearance facilitates electroplating and polishing processes.
Production Efficiency & Consistency
Producing 100 weldments takes nearly 100 times longer than manufacturing a single welded part. Repetitive processes offer no efficiency gains for your production. Furthermore, since weldment quality heavily depends on the welder’s skill and experience, the quality of 100 weldments may vary significantly.
The initial mold development stage in investment casting is the most time-consuming. Once the mold is developed, production efficiency increases exponentially. Since all castings in a batch are produced from the same mold, product consistency is extremely high, which is crucial for mass production.
Material Utilization
Welding falls under subtractive/assembly manufacturing. It involves cutting sheet metal to achieve target component shapes, generating substantial scrap. Consequently, material utilization in welding often falls below 70%.
In contrast, investment casting is an additive/forming manufacturing process. It involves pouring precisely measured molten metal, which solidifies to form the casting. This not only minimizes material waste but also allows the riser system to be remelted and reused, achieving exceptionally high material utilization. This is particularly significant for expensive casting materials like nickel-based or cobalt-based alloys.
Cost
Welded parts require no tooling costs and have a low manufacturing barrier, but their unit price is high. This is because the welding process involves substantial hidden costs. These include fixture design, skilled labor costs, wire and gas consumption, assembly labor, and inspection fees. Therefore, we recommend welded components for small-batch orders.
Investment casting requires substantial mold investment upfront. However, as production volume increases, the mold cost is amortized, reducing per-unit cost and improving economic viability. Choosing investment castings can also help eliminate some assembly and machining expenses.
The table below provides a clearer understanding of the differences between weldments and investment castings.
| Feature | Weldment | Investment Casting |
| Structure | Multi-piece joined | Monolithic / One-piece |
| Strength Type | Anisotropic, HAZ weakness | Isotropic |
| Design Freedom | Limited to standard shapes | Almost unlimited |
| Tolerance | Low | Precise |
| Surface Finish | Rough (Ra > 12.5μm) | Smooth (Ra 3.2 – 6.3μm) |
| Wall Thickness | Limited by the stock plate thickness | Thin & Variable (down to 2mm) |
| Production Efficiency & Consistency | Challenaging | High |
| Material Utilization | Low | High |
| Lead Time | Short (Weeks) | Medium (4-8 Weeks for Tooling) |
| Tooling Cost | Low / None | Medium / High |
| Unit Price | High | Low (High Volume) |
When Should You Replace Weldments with Investment Castings?
If you’re currently hesitating between investment castings and weldments for your project, the following information may assist you. Opting for investment casting is the wiser choice if any of the following scenarios apply.
Order Volume Exceeds Break-even Point
When order quantities exceed 300-500 pieces, the labor costs associated with welding will surpass the cost of investment casting parts (including mold fees) when averaged per unit. At this point, investment casting offers greater cost efficiency. The larger the production volume, the more pronounced the cost advantage per piece for investment castings.
Need for Part Consolidation
If your weldment consists of three or more separate components, we recommend transitioning to investment casting. Cumulative tolerances in welded and assembled parts become difficult to control, whereas investment casting can consolidate these into a single piece, significantly improving dimensional accuracy.
Strict Weight Optimization
Welded assemblies are constrained by standard sheet thicknesses, limiting lightweight design. Investment casting enables variable wall thicknesses—allowing thicker walls only in stress zones while thinning other areas, achieving true structural topology optimization.
Eliminating Failures & Leaks
If your project demands high airtightness, weldments with multiple seams cannot guarantee 100% leak-free performance. Moreover, the heat-affected zones around welds are more susceptible to fracture and corrosion in extreme environments.
Choosing monolithic investment castings completely avoids the impact of welds on part performance—a critical advantage for pressure-bearing and hydraulic components.
High Machining Requirements
If your weldments require extensive machining post-welding to achieve desired precision or geometry, consider investment casting for “close-to-net-shape” production. Even complex structures can be formed in a single process, drastically reducing subsequent machining needs.
Streamlined Supply Chain
Procuring a weldment involves managing multiple sub-components and coordinating various outsourced processes (cutting, welding, surface treatment, etc.). This inevitably increases management costs and risks. Choosing investment castings requires engagement with only one supplier, significantly easing procurement pressure.
Total Cost of Ownership Reduction
Weldment production incurs substantial machining, labor, and inspection costs. Investment casting eliminates these processes and reduces scrap rates, substantially lowering the total cost of ownership. This benefit is particularly pronounced for complex structural components.
Main Application of Weldments and Investment Castings
Application of Weldments
Weldments play a pivotal role in the manufacturing of large-scale, structural, and low-volume parts. When your target product is massive in size yet structurally simple, welded components are your preferred choice.
Infrastructure & Construction: Bridge components, building structural supports, etc.
Heavy Machinery Bases: Machine tool beds, large mining equipment chassis, generator set frames, etc.
Custom Equipment: Specialized tooling fixtures, non-standard automated equipment mounts, etc.
Simple Structure Parts: Enclosures, brackets, etc., fabricated directly from standard plates, tubes, etc.
Application of Investment Castings
Investment casting, with its exceptional precision and casting capabilities, is often regarded as an outstanding solution for components in precision, complex, and high-performance applications. When your product features intricate internal flow channels or materials that are difficult to machine, casting is the only viable solution.
Automotive Casting: Exhaust manifolds, engine blocks, rocker arms, etc.
Marine Casting: Propellers, anchors, engine components, etc.
Aerospace Casting: Turbine blades, gas turbine components, etc.
Fluid Handling: Complex-shaped pump castings, pump housing castings, valve castings, valve body castings, impellers, etc.
Conclusion
Simply put, if your target product is large, square, or rectangular and made from standard sheet metal, choose welding. However, if you require complex, precision components, investment casting is the ideal choice. As a foundry with 27 years of investment casting expertise, Pingheng Machinery offers comprehensive one-stop casting services. We operate our own foundry and machining facility, capable of conducting airtightness testing on your products to ensure casting quality. Please feel free to contact us with any related inquiries. We look forward to working with you.
FAQ
Q1. What is welding, and how does welding work?
Welding is a manufacturing process that joins two or more metal components together using high temperatures, often with the aid of filler material. It is indispensable for structural fabrication across various industries, from manufacturing to construction. Welding methods are diverse and include MIG/MAG welding, TIG welding, and arc welding.
Q2. What is the investment casting process?
Investment casting, also known as lost wax casting, involves the following key steps: creating a wax pattern, assembling the pattern into a tree, coating the surface with a slurry to form a ceramic shell, melting the wax pattern, and pouring molten metal into the cavity. After cooling, the shell is broken to retrieve the casting.
Q3. What is the difference between welding and casting?
The fundamental difference between the two lies in their manufacturing methods. Welding connects separate components to form an integrated structure, while casting involves melting metal and forming it into a single, unified piece in one continuous process.
Q4. Is investment casting stronger than a weldment?
Generally speaking, investment castings exhibit superior strength compared to welded components. This is because investment casting produces isotropic castings, meaning the material properties remain consistent in all directions. In contrast, welded components experience stress concentration at the weld seam and heat-affected zone (HAZ), areas that typically fail first under cyclic loading.
Q5. What materials can be investment cast vs. welded?
Compared to welding, investment casting offers a broader range of materials that can be cast. Welding is constrained by material properties and can only be performed between metals with compatible chemical compositions. Investment casting, however, can utilize nearly any metal for casting, including high-temperature metals that are difficult to weld and specialty stainless steels.
Q6. How long does it take to develop a casting from a weldment design?
Traditional molds are produced for casting based on welding mold design drawings, with mold opening times typically ranging from four to six weeks. However, you can opt for rapid prototyping casting to print the mold. This reduces model preparation time to approximately one week, accelerating order fulfillment efficiency.
Q7. Why do weldments distort?
During welding, localized temperature increases cause morphological changes in the welded joint or surrounding material. This occurs because metal expansion generates temperature gradients and residual stresses. However, investment castings exhibit greater dimensional and geometric stability due to their more uniform cooling.
Q8. Can investment casting replace a multi-part weldment?
Of course, this is also a direction many project managers are considering. Redesigning welded assemblies composed of multiple independent parts into a single investment casting part can eliminate issues such as assembly errors and welding defects, resulting in components with greater structural integrity.
